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Author Topic: Sensor size and DoF  (Read 48766 times)
howiesmith
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« Reply #100 on: October 16, 2006, 09:13:57 AM »
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peripatrtic,

"[Y]ou can't have a DOF number without a COF number, and the COF number is one decided on by you or the lens manufacturer, whomever you trust the most."  Emphasis added

To "lens manufacturer," I would also add Wiki definition, peripatetic, or Ray.  I still trust myself.  Again, if you first assume DoF is format dependant, it is easy to prove it.

Try tis out.  I plan an image with my 4x5 camera with 320mm lens.  I set the CoC at d/1500.  The image plan calls for a 3x5 format, so CoC is 0.0039".  My plan is to make a 6x10 print for my portfolio book.

I do that.  My wife sees the print and likes the image, so suggests it would make a nice 3x5 post card.  So I print up a few.  I mail one to the editor of my next book, who wants to include it as a 12x20.  My neighbor sees the book and wants a 48x80 print for his wall.  Do all these prints have the same DoF?  If CoC=0.0039 is correct, I would say yse, until I look at each print.

I assume Ray's example was meant to be more sarcastic than instructive.  I serious doubt the iamge shown was taken with a 4x5 camera with a 6x7 back.  My hunch is some sort of digital that Ray owns, maybe a Canon, with a zoom lens.  I doubt Ray even know what focal length the lens was set at, the focus distance or the f/stop.  The focus distance and f/stop were probably set by his Canon.  ANyway, if the planning were actually done, the plan would alos include a print size, giving enlargement from a 4x5.  But alas, poor Ray used a 6x7 instead with his 4x5 plan and got the sorry image shown in his example.  If Ray were to continue with the plan and make a print of the 6x7 negative enlarged the same degree as the planned 4x5, he would find the DoF on his print was as planned.  I think that would prove DoF is not format dependant.  Two different formats treated the same produced the same results.  Whether the planned image will fit into the selected format is format, lens focal length and focus distance dependant, and that part of the planning has nothing to do with DoF planning.  I would guess that Ray does very little to no image planning and that may be why he does not know what is involved.  (He has claimed before that he doesn't have time for planning because he is too busy running around trying to capture the moment.)
« Last Edit: October 17, 2006, 09:31:53 PM by howiesmith » Logged
Craig Arnold
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« Reply #101 on: October 16, 2006, 09:42:45 AM »
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peripatrtic,

I do that.  My wife sees the print and likes the image, so suggests it would make a nice 3x5 post card.  So I print up a few.  I mail one to the editor of my next book, who wants to include it as a 12x20.  My neighbor sees the book and wants a 48x80 print for his wall. 

OK good example.

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Do all these prints have the same DoF?  If CoC=0.0039 is correct, I would say yse, until I look at each print.

If one were to say yes, then that person would clearly not understand what CoC means; of course they don't have the same DoF.

Start with the definition again.

Integral to the concept is the print size, and how much enlargement of the negative is required. The standard used is a 30cm print and viewing distance of 50cm. Both are integral. DoF changes if you move back a few paces too.

But it is crucial that 30cm size and 50cm viewing distance are a baseline. It doesn't really matter what the baseline is set to, but without one we are just chasing our tails.

You can define the baseline print size and viewing distance wherever you like, but without them you have no point of reference and the formulae become useless.

In practice of course you (and most everyone here) has a very good grasp of how to achieve the required DoF for their photographs. And considering the equipment you use I have little doubt that you are a great deal better at it than most small-format users (digital or not). You simply have to be in order to get the results you require.

However I think your practical knowledge has mislead you into thinking you have the theoretical concepts down pat. And you are simply mistaken. CoC and DoF simply do not mean what you thought they did.

It all comes down to how you answer the question: When you crop a shot do you change the DoF?

Your answer: "No of course not". Correct answer: "Yes, obviously."

Because inherent in the notion of that CoC from which DoF is calculated is that you are viewing a 30cm print from 50cm distance. So by cropping it is assumed that you mean crop and enlarge to a 30cm print viewed from 50cm.

It isn't even meaningful to compare DoF with prints that differ in size and viewing distance.
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howiesmith
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« Reply #102 on: October 16, 2006, 10:06:21 AM »
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"If one were to say yes, then that person would clearly not understand what CoC means; of course they don't have the same DoF."  They all have the same format and CoC don't they?  Only the degree of enlargement changed.  It is the degree of enlargement that matters here, not format.

"Integral to the concept is the print size, and how much enlargement of the negative is required. The standard used is a 30cm print and viewing distance of 50cm. Both are integral. DoF changes if you move back a few paces too."

You are assuming a 30cm print viewed at 50cm.  You don't need to do that.  DoF can be taylored to any print size and/or viewing distance.

Think of the CoC on the negative as a subject.  How much can you enlarge that subject before it starts to look like a disc instead of a point on your print?  Your print, not just an arbitrary 30cm print.  At any viewing distance you select, not just an arbitrary 50 cm.

"But it is crucial that 30cm size and 50cm viewing distance are a baseline. It doesn't really matter what the baseline is set to, but without one we are just chasing our tails."

It is not only not crucial, it isn't even necessary.  I can select any CoC I want, any print size I want and any viewing distance I want.  To quote someone else, "I don't need no stinking [standard]."  And I don't even want one.

You say I can start with any standard.  If I can do that, I can just skip the standard and go on.  How do you make a 50cm print to be viewed at 60cm?
« Last Edit: October 16, 2006, 10:09:12 AM by howiesmith » Logged
howiesmith
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« Reply #103 on: October 16, 2006, 01:22:47 PM »
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peripatrtic,

The reason I do not need a standard is all the work that goes into making a standard as a place to start is the same as I do already.  Essentially, I create a new standard from a non-format specific base for my own use.  That standard happens to be the print size and DoF I want.  No adjustments required.  If I used a standard, I would have to do the same or perhaps even more work, to adjust the standard to what I need.  So I figure , why bother?  Just start from the basics that I understand and go to my specific, making whatever assumption I want (and know are being made).

If I wanted to make a 30cm print to be viewed from 50cm, I can work that out without using to camera format - just the degree of enlargement from the negative to my print.  How much do I want to enlarge the CoC on the negative to get the desired CoC on the print.

I don't think this is wrong and I know I don't need to know or use the camera's format.  Camera format could be useful if I knew it, wanted to make an uncropped (full frame) print.  I then know the degree of enlargement of the negative is equal to the degree of enlargement of the film's CoC or subject image.  Note that the degree of enlargement of the full frame (format) negative is not usually the same as the degree of enlargement of the subject image. They are equal when and only when no cropping is being done.  This is a very specific case and not the general rule.
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Craig Arnold
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« Reply #104 on: October 17, 2006, 02:18:50 AM »
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That's cool.

As I said, you obviously know what you need to do to get the results you want.

But when you say depth-of-field, you are not using the standard definitions, hence all the confusion.

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They all have the same format and CoC don't they? Only the degree of enlargement changed. It is the degree of enlargement that matters here, not format.

If you change the enlargement and keep the viewing distance the same you have changed the CoC. That is how CoC is defined.

Your "Howie Format Independent" understanding of the concept is different from the accepted definitions. That's fine if it works for you, but you need to understand that most people will work with the standard definitions, and I don't think it's really fair to berate them for that.

Once we all know that you mean something slightly different and understand what it is then we can understand what you mean.  

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You say I can start with any standard. If I can do that, I can just skip the standard and go on. How do you make a 50cm print to be viewed at 60cm?

I believe what they did in the first place at Leica or Zeiss or somewhere (in fact I think the Wiki article includes a copy of some of the early research) is sit down a bunch of people and ask them whether pictures  enlarged to different degrees were sharp. So to create another baseline you either interpolate (perhaps there are some standard formulae) or you experiment by sitting down a bunch of people and asking them, or you experiment with yourself as the viewer and gather up experience over a number of years. You then either lay the results down formally or use your neural net to allow you to estimate it pretty accurately without any formalisation.

The latter method is very effective for producing individual results, but not much use for teaching or passing on that knowledge to other people in the industry.

At any rate you are determined to call your horse a donkey, and it's clear both will get you where you want to go.
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howiesmith
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« Reply #105 on: October 17, 2006, 09:07:38 AM »
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If you insist that DoF is format dependant becasue Wiki told you it is, well then, it is, for you anyway.  Wiki is your undisputed source.  You seem to trust it more than yourself.  I guess that before Wiki, there was no DoF.

I use as my source a book by Ansel Adams.  He provided a write up on DoF using CoC that wasn't bounded by camera format or a 30cm print.  But he wrote the book before digital cameras and Wiki came along.  He was wrong too and died not even knowing it.  Too bad.
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Ray
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« Reply #106 on: October 17, 2006, 11:46:48 AM »
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I use as my source a book by Ansel Adams.  He provided a write up on DoF using CoC that wasn't bounded by camera format or a 30cm print.  But he wrote the book before digital cameras and Wiki came along.  He was wrong too and died not even knowing it.  Too bad.
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Well that explains it, Howie. You should have told us sooner that you are not referring to DoF in a picture, or on a print, which as you know must be a specified format and size. I understand the principle that you could measure the size of a particular object within a composition, in the viewfinder (although that's rather difficult with an SLR), and determine what CoC is required if that object is to appear sharp when enlarged, say 8x on a print. You could then measure another object further away, which is smaller and doesn't require the same resolution of fine detail to appear sharp, and then measure the size of an even smaller object even further away, or a small object in the foreground. By choosing a combination of lens focal length, distance to main subject and f stop, you could determine that all the essential ingredients in your composition are going to appear as sharp as your eye can see (or not, whatever the plan is) when enlarged by a certain degree and viewed from a certain distance.

All this might be possible without reference to a precise format before the shot is taken. You have a composition within an imaginary format of at least a minimum size to include the ingredients of the composition. Your calculations cannot be unattached to a format if you want to take the picture and make the print. If they were you would have no guarantee that the image would not turn out like my example of the dancer at Angkor Wat.

Of course I didn't really borrow a 4x5 camera. The shot was taken with a 5D and 50/1.4 lens at f2.8. I cropped it with a 6x7 aspect ratio to demonstrate the sort of thing (and worse) that could happen when DoF is calculated without reference to format. In that example, if I'd been aware that the camera I was using was really a 6x7cm format despite the viewfinder being 4x5, I would have needed to step back a few paces to fit the ingredients of the composition within the 6x7 format and increase the aperture a stop or so up to maintain that same DoF.

There's absolutely no point in measuring objects and subjects within your composition if you don't know that they will fit on the piece of film in the camera.

The fact that it is difficult to not know the format of your camera and difficult to be uncertain that your composition in the viewfinder will fit on the sensor, has no bearing on the matter. Whether you know your camera's format through habit without thinking about it, or whether the manufacturer has designed a fool proof system that does not allow you to measure objects that will not fit on the sensor or film, does not detract from the absolute necessity of knowing  the camera format (or your own modified format) in order to make appropriate adjustments to lens and subject distance for DoF purposes on a specified print size.
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AJSJones
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« Reply #107 on: October 17, 2006, 12:52:39 PM »
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.... the absolute necessity of knowing  the camera format (or your own modified format) in order to make appropriate adjustments to lens and subject distance for DoF purposes on a specified print size.
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Peripatetic, The definition of DoF that all agree on is " the range of distances in front of and behind the focus plane that appear sharp (or in focus)".  THEN, convention takes over to calculate the DoF for (arbitrarily decided) print size and viewing distance.  Once people adopt the convention, they can talk to each other without having to go through the "print size and viewing distance" discussion and assumptions.  The 30 cm print and 50cm viewing distance is NOT part of the above definition , it is a convention and it applies only to those conditions. Those who know what variables affect their specific outcome will work based on that convention to  e.g. increase or decrease their aperture, because they plan to deviate from those conditions.  It's a guide they know how to use.  If I make a 50 cm print and view it from 30 cm , am I doing something wrong or just different?  The DoF will not be the same as the 30 cm print viewed from 50 cm, so I need to adjust the acquisition parameters to achieve my goal.  What DoF is is defined above.  How you calculate it depends on the situation.  The 30cm print viewed at 50cm is just one example and the DoF tables/scales are correct BUT only for that situation.

Ray, Is the format you speak of the ones adopted by convention/availability (645, 6x7 4x5 etc) or the actual size of the image on the ground glass that ends up being used in the print?  You have acknowledged that Howie might have a "modified" format and may choose to specify his print size differently.  But with the "phenomenological" definition above (not specified to format/print size, but just appearance of sharpness) I think Howie is doing the right thing to achieve his goal of having some things appear in focus and some things out of focus in his print based on the (artistically but arbitrarily selected dimensions of the) image he captured.  

In his example, if he captures a 3x4 image on a 4x5 or an 8x10 sheet of film and plans to make a 15x20 print from it and view it from 50 cm, what dimensions does he use in the calculations?  3x4, 4x5 or 8x10?Huh?

How do you deal with DoF when you crop heavily???
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howiesmith
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« Reply #108 on: October 17, 2006, 06:32:07 PM »
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Peripatetic, The definition of DoF that all agree on is " the range of distances in front of and behind the focus plane that appear sharp (or in focus)". THEN, convention takes over to calculate the DoF for (arbitrarily decided) print size and viewing distance. Once people adopt the convention, they can talk to each other without having to go through the "print size and viewing distance" discussion and assumptions. The 30 cm print and 50cm viewing distance is NOT part of the above definition , it is a convention and it applies only to those conditions. Those who know what variables affect their specific outcome will work based on that convention to e.g. increase or decrease their aperture, because they plan to deviate from those conditions. It's a guide they know how to use. If I make a 50 cm print and view it from 30 cm , am I doing something wrong or just different? The DoF will not be the same as the 30 cm print viewed from 50 cm, so I need to adjust the acquisition parameters to achieve my goal. What DoF is is defined above. How you calculate it depends on the situation. The 30cm print viewed at 50cm is just one example and the DoF tables/scales are correct BUT only for that situation.

Ray, Is the format you speak of the ones adopted by convention/availability (645, 6x7 4x5 etc) or the actual size of the image on the ground glass that ends up being used in the print? You have acknowledged that Howie might have a "modified" format and may choose to specify his print size differently. But with the "phenomenological" definition above (not specified to format/print size, but just appearance of sharpness) I think Howie is doing the right thing to achieve his goal of having some things appear in focus and some things out of focus in his print based on the (artistically but arbitrarily selected dimensions of the) image he captured.

In his example, if he captures a 3x4 image on a 4x5 or an 8x10 sheet of film and plans to make a 15x20 print from it and view it from 50 cm, what dimensions does he use in the calculations? 3x4, 4x5 or 8x10?Huh?

How do you deal with DoF when you crop heavily???
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I think that clarifies my thoughts.

Actually it is very easy for me to measure the size of the image on the ground glass.  I use a ruler and a pair of dividers.  I put the points of the dividers on the top and bottom of what I want to measure, then transfer to the ruler.  On my 4x5, the ground glass is right there on the back of the camera, easy to reach.  On my 6x6 (an SLR), the ground glass is right there on the top.  I need to have the prism and the waist level magnifier removed to reach it easily, but that is how I use it.

The ability for my camera to capture an image on the film as intended has no relationship to DoF.  Yes, it is very possible to have a poor composition with perfect DoF.  I have accomplished that many times.  It's called a reshoot.  DoF and composition are truely unrelated.  I certainly don't need to know anything my composition to calculate DoF.  If I need to change the lens or move my camera to get the right composition, then I also need to redo the DoF calculation.  I have changed the image design.  The lens' focal length and/or focus distance have changed.  I think that is pretty fundamental and should be obvious to even the most casual reader, regardless of how they do DoF.
« Last Edit: October 17, 2006, 07:44:15 PM by howiesmith » Logged
Ray
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« Reply #109 on: October 17, 2006, 11:01:38 PM »
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In his example, if he captures a 3x4 image on a 4x5 or an 8x10 sheet of film and plans to make a 15x20 print from it and view it from 50 cm, what dimensions does he use in the calculations?  3x4, 4x5 or 8x10?Huh?

How do you deal with DoF when you crop heavily???
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Andy,
If Howard captures a 3x4 image on his 4x5 viewfinder of his 4x5 camera (or captures a 4x5 image with the intention of later cropping it to 3x4), then for DoF purposes, whatever the subjective variables of viewing distance and eyesight capability, the results will be exactly the same as though he had used a 3x4 format camera without cropping, using the same lens, same distance, same f stop. He knows the format because he has measured it. It is disingenuous of Howard to claim that the format has no bearing on his DoF calculations.

Howard seems to be claiming that he hasn't measured the format but just the size of an object, or objects, within an imaginary or planned format. Whether the format is is in the planning stage, or undecided at the time the shot is taken and created later in post processing, the format is the reference point for DoF purposes.

How do you deal with DoF when you crop heavily? You've just effectively converted your camera into a very small format camera using probably a rather low resolution lens in relation to what's available for small formats.

If the lens, film or sensor are of the same quality, you simply get the DoF characteristics that would apply using a dedicated small format camera with the same lens and settings. For example, if I use a 1Ds2 with a 50mm lens at f8, then decide later to crop the image to the same size as I would have got using a D60 with the same lens at the same f stop from the same position, I get the same image in all respects.

The point here is that for DoF purposes it is the format of the image that is actually used in the process of making a print that has a direct bearing on the final DoF perception.
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Ray
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« Reply #110 on: October 17, 2006, 11:51:14 PM »
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On the matter of cropping heavily, the DoF formulas will give you only very approximate results because they don't take into consideration such factors as lens quality and film or sensor resolution, ie. system resolution. These factors play no part when cropping moderately as in the case of the 1Ds2 image cropped to the size of a D60 image, but do play a significant part with extreme cropping.

For example, one of Howie's analogies is an 8x10 view camera with standard 300mm lens but using a 35mm format back. The formulas tell you that DoF will be the same as using a standard 35mm camera with 300mm lens at the same f stop and same distance to a central object within the composition. I doubt whether any 35mm lens would have a sufficiently small aperture setting that would be required to get equal DoF in these 2 situations. In the aperture ranges that are typically used with a 35mm format 300mm lens, the lens will be very much sharper than a standard 300mm lens for 8x10 format. The 35mm image from the dedicated 35mm camera will thus have less DoF, at the same f stop, to the extent that the lens is sharper.

I would expect the differences to be even greater in my example of a Canon 30D image cropped to the same size as a G7 P&S camera which uses not only a sharper lens than the DSLR equivalent (EF-S 10-22) but also a much higher resolving sensor. But I'm repeating myself.  
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Craig Arnold
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« Reply #111 on: October 18, 2006, 02:25:10 AM »
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Peripatetic, The definition of DoF that all agree on is " the range of distances in front of and behind the focus plane that appear sharp (or in focus)". THEN, convention takes over to calculate the DoF for (arbitrarily decided) print size and viewing distance. Once people adopt the convention, they can talk to each other without having to go through the "print size and viewing distance" discussion and assumptions. The 30 cm print and 50cm viewing distance is NOT part of the above definition , it is a convention and it applies only to those conditions. Those who know what variables affect their specific outcome will work based on that convention to e.g. increase or decrease their aperture, because they plan to deviate from those conditions. It's a guide they know how to use. If I make a 50 cm print and view it from 30 cm , am I doing something wrong or just different? The DoF will not be the same as the 30 cm print viewed from 50 cm, so I need to adjust the acquisition parameters to achieve my goal. What DoF is is defined above. How you calculate it depends on the situation. The 30cm print viewed at 50cm is just one example and the DoF tables/scales are correct BUT only for that situation.

Quite so, and eloquently put. But in order to calculate some useful values for any given format, one must (arbitrarily) choose a print size and viewing distance. Without that it becomes impossible to gather the evidence (what actually does look sharp to people) and apply the formulae.

Of course, given sufficient input, our neural nets are quite adept at adjusting for all the variables simultaneously, and ending up with the required results.

The problem with the "experience" approach is that it's very difficult to impart that knowledge except by saying "you will learn eventually if you practice enough". It can also be extremely difficult to disentangle the actual variables taken into account unconsciously.

I think the point that Ray is making so well is that Howie is in fact taking format into account no matter how vigorously he denies it.

By analogy, most people who can hit a baseball quite well wouldn't have the vaguest idea of the mathematics behind what they are doing. They might well strenously deny that they are taking account of Newton's Law of Universal Gravitation. But they are; they have an approximation of the formulae in their heads that they have built up by hitting the ball (and living on the planet) so many times.

In fact I have a challenge for Howie - can you produce some tables for us that would allow Ray and I to borrow or rent a large format setup for a weekend and go out and have a good chance of getting the required DoF with a couple of lenses and variety of apertures, and at different print sizes and viewing distances. You may not make any reference to the format/crop.
« Last Edit: October 18, 2006, 02:44:43 AM by peripatetic » Logged

howiesmith
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« Reply #112 on: October 18, 2006, 08:55:29 AM »
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In fact I have a challenge for Howie - can you produce some tables for us that would allow Ray and I to borrow or rent a large format setup for a weekend and go out and have a good chance of getting the required DoF with a couple of lenses and variety of apertures, and at different print sizes and viewing distances. You may not make any reference to the format/crop.

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No, I cannot produce the tables you seek as proof because each photo is different.  Simply apply the DoF calculations for each photo at a time.

Tables imply that you have decided on a set of fixed variables (say a print size) and the table then lists some numbers as you change on variable.  "DoF tables" are possible only because the user has already arbitarily decided on a certain set of fixed values for teh finished print.  It is impossibe to make a two dimensional table that accounts for changing more than two variables.  So, the makers of tablrs fix several variables, arbitarily, so they can make a table that shows the effect of changing two variables.

In fact, your tables show only decrete values, like f/stop.  The table shows only he common, marked on the lens values.  If I want f/9.2, I interpolate.

Lens makers used to put markings on their lenses to show DoF.  These were mechanical DoF calcuators.  How would you do that for any print size, plus any print viewing distance, plus any viewer conditions, as well as any focus distance, and f/stop.  So, they assumed all prints were one certain fixed size, viewed from one certain fixed distance, one CoC fit every condition, and none of their negative would be cropped away.  These assumptions had to be made in order to account for all those variables in a "calculator" that was capable of only two inputs.  But then the photographer was stuck with a "calculator" on the lens barrel that has only focus distance and f/stop inputs for that focal length lens. Problems are:

1. these assumption just aren't usually (never?) aren't true.  I have no prints designed to be 30cm prints and viewed from 50cm and aren't from a cropped negative.  How many do you have?

2. Users forgot about how DoF really works and starting thinking the calculators were right for everything.  

3.  Users began to think the calcualtors worked for every situation.  They ignored the assumtions built into the calcuators.

4.  Finally, the calcuator was on-line.  Has to be true.

On-line calculators usually have more than two inputs.  But to produce a table using these calcuators, the user would have to fix all but two (the horizonal and verticle table vaules) in order to produde the third value to put in the tables.  You just make a two dimensional (variable, if you will) table with more than two dimensions (variables).   If you fix a CoC, then all the CoC pages are reduced to one.  One page is all that is needed for one variable.  Still too many pages (dimensions).  So fix the enlargement size.  Still too many pages (dimensions).  Fix the format so enlargement can be reduced to one page.  How about fix the camera.  One camera, one page, and enlargement means only one thing.  Can't have more than one full frame enlargement factor for one print size.  And the beat goes on.

To make the original aswer to your question more accurate, not a table, but books.  You won't carry a programmed pocket calculator, and a ruler.  Would you lug around a set of books?  No.  You would keep throwing away books by assuming your print will fix that variable.  Don't need books for all those possible print sizes.  You don't have a 400mm lens, so you don't need the 400mm books.  But a 70-200 zoom has plenty of focal lengths (books).  All those CoC values.  Too many.  Assume one size fits all.  I think you get the idea.

The answer is no, tables don't work for multidimensional problems.  DoF, like it or not, is a multidimensional problem.  DoF can be simplified by making a lot of assumptions that replace variables with constances.  But then you have to live within those assumptions.

If I had only one lens, then focal length would not be a variable for me.  If that one lens were a mirror lens, then f/stop goes away.  This is getting easier.  Why not fixed focus.  Only at the hyperfocal distance.  But you would argue that focal length, f/stop and focus distance are essential.  Why?  Because you own a zoom lens with an adjustable iris that you you (or your camera) can focus and you want to make those adjustments.  Well, I can make different sized prints.  I can crop.  So I don't limit myself to 30cm prints from full frame negatives.  (I can slice, dice and make julianne fries but that is another topic and some humor.)
« Last Edit: October 18, 2006, 08:59:06 AM by howiesmith » Logged
AJSJones
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« Reply #113 on: October 18, 2006, 02:32:09 PM »
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Peripatetic, I think we've been in agreement - I use DoF calculators and scales (if the lens has one) and use a reasonable CoC, and find the "convention" is extremely useful for disucssion and education.  I was just saying Howie's approach is different is all - interesting to note here that in Japanese the same word is used for "wrong" and "different" (chigau).

Ray,

"The point here is that for DoF purposes it is the format of the image that is actually used in the process of making a print that has a direct bearing on the final DoF perception. "

  I think this is what Howie has been saying all along.  This now becomes clear to me that the word format can mean different things in different contexts or conventions.  If you ask most photographers "What format do you shoot?" they would answer 35 FF, 645 6x7 8x10 etc and the common understanding is that those are the dimensions of the image acquisition area.  In the sentence I quote, you are using the word to refer to the actual image used - therein lies almost all this discussion which seems to revolve around a simple semantic issue here - if you define "format" that way, I suspect Howie would agree...
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Craig Arnold
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« Reply #114 on: October 18, 2006, 02:48:05 PM »
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Howie you make some very good points. And of course it would be totally unreasonable to expect you to produce all those books just for our edification.

As it happens I often do have a programmable calculator with me. (How sad is that?!!)

But at any rate, let me reduce the variables and hope you can still meet my challenge. This should boil down to one single line from one of those books.

The rental shop up the road tells me that they will have a 150mm lens available for use on one of their large format backs this weekend.

I have a particular scene in mind at Kew gardens. It will be a vertical shot of a tree, it has an interesting twisted trunk, and the leaves are turning colour just at the moment. The top will be approximately 30m away from me, and I would like to get the top of the tree sharp and as much of the tree in sharp focus as possible, I would like to know in advance though from what distance I may expect the trunk above me to be sharp for composition purposes and to adjust the angle from which I shoot the tree and tilt the camera.

As it happens this shot is one of a series which I have been printing to 16"x20" and I have a frame waiting for this last shot to complete the series.

It will be my first use of large format so I don't want to get into shift and tilt, I feel that is all too complicated and will have to wait until after a thread on Scheimflug.

One thing that the camera shop cannot tell me at the moment is whether they will have an 8x10 or a 4x5 back available, because another customer has priority and he will only decide on the day. I will have to take what is left over.

Of course I realise that being stuck with the 150mm lens I will simply have to take whatever angle of view I get on the day and that would differ between the two backs. However I'm confident that I shall nevertheless be able to compose the scene to my satisfaction.

As I do not have a darkroom, I shall be handing the negatives to the same shopowner, but he tells me he is not going to crop for me he will print a 16x20 from either the 4x5 or 8x10. I am fairly sure the aspect ratio will work out either way.

Perhaps you could help me with what aperture and focal distance I should set for this shot. If I understand you, it will make no difference what back I am using. I would also be interested in knowing how you choose the appropriate CoC for this shot, which of course will be completely unrelated to the back I happen to be using on the day.
« Last Edit: October 18, 2006, 02:56:19 PM by peripatetic » Logged

howiesmith
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« Reply #115 on: October 18, 2006, 03:57:49 PM »
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Howie you make some very good points. And of course it would be totally unreasonable to expect you to produce all those books just for our edification.

As it happens I often do have a programmable calculator with me. (How sad is that?!!)

But at any rate, let me reduce the variables and hope you can still meet my challenge. This should boil down to one single line from one of those books.

The rental shop up the road tells me that they will have a 150mm lens available for use on one of their large format backs this weekend.

I have a particular scene in mind at Kew gardens. It will be a vertical shot of a tree, it has an interesting twisted trunk, and the leaves are turning colour just at the moment. The top will be approximately 30m away from me, and I would like to get the top of the tree sharp and as much of the tree in sharp focus as possible, I would like to know in advance though from what distance I may expect the trunk above me to be sharp for composition purposes and to adjust the angle from which I shoot the tree and tilt the camera.

As it happens this shot is one of a series which I have been printing to 16"x20" and I have a frame waiting for this last shot to complete the series.

It will be my first use of large format so I don't want to get into shift and tilt, I feel that is all too complicated and will have to wait until after a thread on Scheimflug.

One thing that the camera shop cannot tell me at the moment is whether they will have an 8x10 or a 4x5 back available, because another customer has priority and he will only decide on the day. I will have to take what is left over.

Of course I realise that being stuck with the 150mm lens I will simply have to take whatever angle of view I get on the day and that would differ between the two backs. However I'm confident that I shall nevertheless be able to compose the scene to my satisfaction.

As I do not have a darkroom, I shall be handing the negatives to the same shopowner, but he tells me he is not going to crop for me he will print a 16x20 from either the 4x5 or 8x10. I am fairly sure the aspect ratio will work out either way.

Perhaps you could help me with what aperture and focal distance I should set for this shot. If I understand you, it will make no difference what back I am using. I would also be interested in knowing how you choose the appropriate CoC for this shot, which of course will be completely unrelated to the back I happen to be using on the day.
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What do you use your programmable calculator for?  Is it programmed with the cook book DoF calcs or something else?

How about instead of saying you have reduced the variables, we say you have selected a few.  The degrees of freedom of this system have not changed.

4x5 or 8x10 makes no difference for DoF but you have selected a full frame enlargement of the negative.  No.  You can't do that.  You want to change the degree of enlargement in mid stream.  I don't think that's fair even in your frame of reference (all I need to know is format) if you can't decide between a post card or a bill board.  Maybe a 30cm print?

The format size determines how much of the tree is on the negative, and that has nothing to do with DoF.  If I trim an 8x10 neagtive to a 4x5, do I change anything on the negative?  Of course not.  If while you are waiting to take the photo, the tree grows another 10 meters, you still get the same portion of the tree as before, just maybe not all of it from 30m and up.  A 150mm lens forms the same sized image on any format.  If a 30m tree just fills the frame, you will still get 30m if the tree grows to 40 m.  At the risk of being pendantic, a 150mm is a 150mm lens is a 150mm lens.  The two images you have in mind, one 4x5 and one 8x10, will differ in enlargement by a factor of two if you are planning to make full frame enlargements and that does change DoF.

If you deside the tree will be so many inches high on the negative (and this a very reasonable assumption given you are going to use a 150mm for either, you are in good shape.  Format independant.  Otherwise, I'm afraid you must decide just how much you will be enlarging the portion of the tree on the negative.  You can't just deside to make a print bigger in stream.  

I know you recognize that the tree will be the same "size" on both negatives, you will just see more of it with the 8x10 back.  It is the lens focal length and focus distance that determines the tree's projected image size.  Once you select that, the focused image size is fixed.

For simplicity sake, I will assume the focus distance is 30m.  You haven't provided enough information to determine the focus distance is the camera isn't pointed directly at the top of the tree.

How much DoF do you want.  If you want all you can get, just stop down to the smallest aperture (largest f/number) you got.  We will assume the tree will be about in the middle of the DoF range, and that you don't care that half the DoF is in front of the tree and half is behind.  (30m is much much greater than 150mm.)

I will not continue.  I think you may get the idea.  You can't leave the planning of  important details like the degree of enlargement till later.  Or you will need to plan on location when certain values are available.  If you say Im' going to make a 16x20 print from either format, but the print will be the same (crop the 8x10), then you can go on.

I am also assuming that you are aware that for a 150mm lens and a subject 30m away, the focused image size is fixed and can be calculated, regardless of the film format.  That's just optics.  Ha nothing to do with the size of the screen that image is being focused on (format) or DoF.  Again, at the risk of being pendantic, a 150mm is a 150mm lens is a 150mm lens.

Let me challenge you.  You decide on the 4x5.  You have the 150mm lens.  But instead of a 30cm print, you want to make a 50cm print.  And indead of viwing it from 50cm, you want to hang it on the wall and view it from 100cm.  And you just got new glasses, so you see better, and wnat the CoC on the print to be 2/3 of the standard.  And to make life simple, let's further say the focus distance is 30m.  And you decide on f/19.3.  What is your cookbook DoF?

Like I am with Ray, I'm beginning to think you are not willing to see this or change your believe.  I feel the conversation should end with a simple let's just agree to disagree.
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EricV
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« Reply #116 on: October 18, 2006, 06:57:47 PM »
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Quote from: peripatetic,Oct 18 2006, 12:48 PM
I have a particular scene in mind at Kew gardens. It will be a vertical shot of a tree, it has an interesting twisted trunk, and the leaves are turning colour just at the moment. The top will be approximately 30m away from me, and I would like to get the top of the tree sharp and as much of the tree in sharp focus as possible, I would like to know in advance though from what distance I may expect the trunk above me to be sharp for composition purposes and to adjust the angle from which I shoot the tree and tilt the camera.

----------------------------------------------------------------------------------------------

This would be an interesting challenge, which might even lead to something useful, but only if you define the problem unambiguously.  Otherwise everyone is going to interpret your requirements differently and disagree on the conclusion.  At one point, you imply you are shooting from a predetermined location (30m from top of tree, image as part of existing series), but then you imply you are willing to adjust the distance and change the perspective depending on format (angle from which I shoot the tree, printer will print full negative).  You can't do both!  

Let's say you decide the perspective is correct only when shot from a particular location, so that is now fixed.  Hopefully your 150mm lens is sufficient to capture as much of the tree as you want, whether you use a 4x5 back or an 8x10 back or anything else.  Then Howie would argue correctly that the format does not matter.  Of course you cannot get the print you want by printing the full negative in both cases.  Rather, in both cases, you must crop to use the same area of film, containing the part of the tree you are interested in.  Then the final print requires the same enlargement, and I think everyone will agree that the DoF is the same in both cases, because for all intents and purpose they are the same case.  Now that every other variable is fixed, you can consult tables or calculators of depth of field and hyperfocal distance to choose the focus point and f/stop which give you the depth of field you require.

Let's say instead you decide that perspective is unimportant and you want to fill the negative with the tree for both formats.  Since you are stuck with a single lens, you must move closer when using the 8x10, making it harder to achieve the desired depth of focus.  Now your printer can print both negatives full frame, so the 8x10 will be enlarged half as much as the 8x10 to make the final print, making it easier to achieve the desired depth of focus.  You will have to take all of these factors into account to determine the f/stop needed in both cases.  This time you will claim correctly that format does matter, and Howie will complain that you are changing the ground rules.

Let's take one final interesting case.  Suppose you have a 150mm lens for the 4x5 and a 300mm lens for the 8x10.  Now we can make many more vairables constant.  You can shoot from the same location and make essentially the same print in both cases.  This time again format does matter (since magnification is different), but so does lens focal length.  I will leave problem this as an exercise.  At least it is unambiguos and has a single correct answer.
« Last Edit: October 18, 2006, 07:39:11 PM by EricV » Logged
BJL
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« Reply #117 on: October 18, 2006, 07:35:12 PM »
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We seem to be back to the old Howard Smith doctrine, which is effectively that the same viewer can arbitrarily choose a different maximum allowable CoC value for judging somethig to be in-focus on different displayed images (prints, on-screen images, etc.) , even when comparing two equal sized prints side-by-side. This renders DOF judgements purely personal and capricious, not subject to any comparisons at all. I can change the DOF on the same print by changing my maximum allowable CoC choice!


I propose this instead: viewer of prints can choose their own personal maximum CoC crietera, and so have different standards for the degree of sharpness needed to deem something "in-focus", but ...

When two images are displayed the same size and viewed from the same distance away, the value of the CoC on the displayed image that divides in-focus from out of focus for any particular viewer is the same for each image.

Thus, if we use different formats and focal lengths and then compare on the basis of equal sized prints presenting equal sized images of the subject, the DOF is the same when the circles of confusion on the prints are all the same size, and if the circles of confusion on the prints are of different sizes, the DOF is less on the print with larger circles of confusion.

This means that the relevant CoC size at the focal plane must be adjusted in inverse proportion to the degree of enlargement needed to get equal sized prints, and so is adjusted in proportion to focal length. Different people might use different values, but the same person will use "camera focal plane" CoC limits that scale this way when the same FOV is photographed with different focal lengths (i.e. with different formats).

Then the rule for camera settings is simple: for images taken from the same position, focused on the same object at the same distance from the camera, and with the same FOV, the DOF is the same if the ratio of the focal lengths is the same as the ratio of the aperture ratios.

Equivalently, the ratio of focal length to aperture ratio is the same in each case.

Equivalently, the DOF is the same if the effective aperture diameter is the same, because
(aperture ratio) = (focal length)/(effective aperture diameter)
so
(effective aperture diameter) = (focal length)/(aperture ratio)
and rule is that DOF is the same when this ratio is the same.


Basic geometrical camera optics shows that in this scenario of equal effective aperture diameter, equal FOV and equal distance from camera to subject (equal focus distance), the circle of confusion at every point of the image produced with the larger focal length is larger than the one at the same part of the smaller focal length image in the same proportion as the focal length is larger. Thus each CoC is larger in the same proportion as the image size is larger, so when the two images are enlarged to give prints of the same size, each circle of confusion on each print is the same size as the CoC at the same point on the other print.

You can't get more equal OOF focus effects and DOF than that, no matter how much personal choice you allow!
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howiesmith
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« Reply #118 on: October 18, 2006, 08:16:05 PM »
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We seem to be back to the old Howard Smith doctrine, which is effectively that the same viewer can arbitrarily choose a different maximum allowable CoC value for judging somethig to be in-focus on different displayed images (prints, on-screen images, etc.) , even when comparing two equal sized prints side-by-side. This renders DOF judgements purely personal and capricious, not subject to any comparisons at all. I can change the DOF on the same print by changing my maximum allowable CoC choice!

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I have no idea how you came up with this.  

CoC is personal depending on such factors as eyesight and as basic as what I consider in-focus.  The standard does not change between equal prints.  An 8x10 "print" displayed on a computer monitor is deifferent from an 8x10 chrome on a light table is different from an ink print on an 8x10 piece of paper.  Same image same negative, but not the same "print" and different DoFs for each when viewed by the same viewer.  It simply is easier to see some "prints" than others, even when they are the same size, viewed by the same person from the same distance.

An honest viewer can compare two prints and two really equal prints should appear really equal.  Change one thing and all bets may be off because you just are not looking at equal prints anymore.

Since you seem so sure DoF is format ependant, just point to the optical equations that define DoF and put your finger on "format."  That will satisfy me.  Please jyst show me "format" as an input.  

And don't say it is in the CoC.  CoC depends on the degree of enlargementt of a negative to a print.  If you want to assume DoF is format depenfant by insisting all prints are made from full frame enlergements of the negative (no cropping allowed), then, you are right.  But I claim that is an artificial and totally unnecessary restriction.  You just happen to be enlarging the entire mrgative and the CoC equally.
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BJL
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« Reply #119 on: October 18, 2006, 09:46:32 PM »
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Since you seem so sure DoF is format ependant, just point to the optical equations that define DoF and put your finger on "format."  That will satisfy me.  Please jyst show me "format" as an input. 
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Welcome back Howard.

Reread my post and you will not find the words "format dependent", nor any appeal to standard values of the maximum allowable CoC size.

You will find references to images covering the same FOV taken with lenses of different focal lengths. This means that the images formed and recorded at the focal plane of the camera and used to make prints are of different sizes, and that "image size in the focal plane" is perhaps what many people refer to as the format.

If however you wish to avoid that use of the word "format", here is a true statement that should satisfy you. It is based on the basic geometrical optics that determines how large a disk ("circle of confusion") a point is blurred into at the focal plane of the camera.


If one makes equal sized images (prints) of the same scene, with the same FOV and taken from the same position, focused on the same object at the same distance, and using the same effective aperture diameter, the prints will have the same DOF, because indeed at each part of the image, the circle of confusion (the disk into which light from one point is blurred by OOF effects) will be the same size.

Thus, if one image is made using a shorter focal length, the aperture ratio needed to get equal aperture diameter and equal DOF is smaller in proportion to the focal length. Conversely, if the same aperture ratio is used in each case, the shrter focal length gives a smaller effective aperture diameter and so every CoC on the print is smaller in proportion to the focal length, which surely means more DOF.

Forming and recording a smaller image of the same subject (thus using a lens of shorter focal length) and using the same aperture ratio gives more DOF.



The rest of us can interpret "forming and recording a smaller image of the same subject (thus using a lens of shorter focal length)" as "using a smaller format", but you are free to use words otherwise. Using words differently does not change the optical facts stated above.
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