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Author Topic: shallow DOF limits of current MF systems  (Read 20078 times)
howiesmith
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« Reply #20 on: October 27, 2006, 09:50:00 AM »
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Many medium format cameras have built in leaf shutters which limits the maxiumum aperture of the lens.  That is why there is an 80mm f2.8 but only a 150 f4.]
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Is there some reason a leaf shutter could not be made larger if needed to make a faster MF lens?  Maybe lens size is limiting shutter size instead of the othr way around.  No use making a really big shutter for the same "small" lenses.

Just curious.
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« Reply #21 on: October 27, 2006, 10:03:34 AM »
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Is there some reason a leaf shutter could not be made larger if needed to make a faster MF lens? .
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A copal 3 shutter is big and has a max speed of 125

Copal 0/1 are smaller and go up to 500 I think

One would assume that the physics of 'hurling the gate shut' gets harder as size increases - not impossible just more costly

I think copal know a thing or two about shutters - if it was easy theyd do it

----------

I used to have a nikkor 80.14 was Massive compared to the 80 1.8 - when it got nicked I replaced it with the lense that went in my pocket - the 1.8

Its all about percived market demand by the manufacturers for silly glass - manufacturers already sharing a small slice of cake?
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howiesmith
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« Reply #22 on: October 27, 2006, 10:39:45 AM »
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A copal 3 shutter is big and has a max speed of 125

Copal 0/1 are smaller and go up to 500 I think

One would assume that the physics of 'hurling the gate shut' gets harder as size increases - not impossible just more costly

I think copal know a thing or two about shutters - if it was easy theyd do it

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Sounds like an interesting challenge that could be solved for a price.   But what price for the few they might sell?  A Hasselblad 80/2.8 goes for $2150 at B and H these days.  An 80/1.4 would be fairly low demand and very high priced (comparitvely), making demand even lower and price yet higher.

I would think that a fast MF lens would require a fast shutter to use the increased lens speed.  Sounds to me like an economics problem not worth solving.

I agree Copal knows shutters.  I think if it were economical (not necessarily easy) Copal could solve the problem.  Like going to the moon.  It can be done, but at what price and is it really worth it?

Thanks for the answer.
« Last Edit: October 27, 2006, 10:43:50 AM by howiesmith » Logged
BJL
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« Reply #23 on: October 27, 2006, 10:40:45 AM »
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Thanks for many interesting responses. Two related ideas particularly appeal. Firstly:
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2) The POTENTIAL for shallower depth of field, when print sizes larger than the limits of 35mm are used.
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and relatedly, the idea that perceived DOF is related to the difference in sharpness between completely in-focus elements of the image and other parts: increasing the sharpness of the main in-focus elements increases the perception of other parts being OOF, especially if the greater sharpness invites closer viewing of the print.

In other words, the use of medium format rather than a smaller format can lead to some combination of larger prints, closer print viewing, or more careful print scrutiny, all making OOF effects more noticeable and so decreasing what I will call "perceived DOF".


Indeed, medium format is typically used for the sake of higher resolution, larger prints and prints that receive more careful viewing, so standard DOF calculations based on equal sized prints might be misleading. And standard DOF charts and CoC values are based on viewing rather small 5"x7" prints!.

So for larger, higher resolution formats, another approach to DOF comparisons could be this:
compare prints of equal l/mm resolution on the print, which in digital terms means roughly equal PPI printing, and then equal viewing distance since the equal print resolution allows that.

In that viewing comparison, the DOF formulas should be used with a CoC value related to pixel pitch, not directly to format. Current 35mm and MF digital offerings offer similar pixel pitches and so can perhaps be compared roughly with the same CoC, and the same is roughly true with film. If so, DOF scales rough proportional to aperture ratio and inversely with square of focal length.

So for example, 80mm f/2.8 in MF matches about 50mm f/1.1 in 35mm. Or to compare 24x36mm to 36x48mm digital, the focal length factor is about 1.4, so the equivalent f-stop for equal DOF changes by a factor of two: f/2.8 in Hassleblad-Imacon/Fujifilm's "48mm format" matches f/1.4 in 35mm format, and the brightest current AF MF lens, the Fujinon HC 110mm f/2.2 for the H system, matches f/1.1 in 35mm format.

That f/1.4 to f/1.1 equivalent DOF is probably shallow enough for most purposes.


P. S. As to the disappearance of those even larger f/2 and f/1.9 apertures, leaving only one current AF MF lens faster than f/2.8: the motivation for that extra aperture size might have been mostly more shutter speed rather than less DOF, and that is somewhat less of an issue with the higher ISO speeds of digital, even MF digital sensors with their lesser emphasis on high ISO speeds.
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howiesmith
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« Reply #24 on: October 27, 2006, 10:56:45 AM »
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BJL, DoF is perceived - it is the OoF areas that are perceived as acceptable in focus.  No need for changing the definition.  Diferent people will perceive the same print differently.

I think the standard way of determining DoF still works just fine for larger, higher resolution camers.  Just scrap the "standard" that says all prints are small and viewed form a certain distance.  Using the same methods, DoF can be determined for large prints viewed at close range.  It requires a smaler CoC, not some "standard" value.

Smaller CoC at the sensor allows both greater enlargment and closer viewing.  That also makes DoF non-format dependant - which it always has been.
« Last Edit: October 27, 2006, 11:29:06 AM by howiesmith » Logged
Morgan_Moore
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« Reply #25 on: October 27, 2006, 11:06:03 AM »
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Indeed, medium format is typically used for the sake of higher resolution
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Not any more buddy

It is used for shallow DOF that pings the subject off the page

and because canon WA a are a bit dodgy

I wager that 80% of commercial MF shooters rarely have a client that needs more than 16mp

This argument is rather bakced up by the fact that a P25 with a bigger sensor still outprices the higer res smaller sensor options

SMM
« Last Edit: October 27, 2006, 11:10:14 AM by Morgan_Moore » Logged

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« Reply #26 on: October 27, 2006, 01:33:10 PM »
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Not any more buddy

It is used for shallow DOF that pings the subject off the page
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Perhaps you should reread my original post: by standard DOF reckoning (comparing equal sized prints from the same distance), medium format lenses cannot give DOF as low as one can get from fast 35mm format lenses, due to fact that MF lenses have significantly higher minimum f-stops.

Are you stuck in the endless fallacy of assuming that equal f-stops always are (or even always can be) used with different formats and different focal lengths, ignoring the fact that minimum f-stops available actually tend to increase with focal length (beyond normal) and with format for which the lens is designed?

As to current MF back resolutions, the minimum I know of is 17MP, not less than anything in smaller formats, most are at least 22MP, and the new wave is 30MP to 40MP, well beyond 35mm format pixel counts.

Of course there are other reasons for some to get a MD digital back even if it were to only match 35mm digital for pixel count: backward compatibility with a good and expensive collection of lenses, and access to some MF lenses for which there is no match is any smaller format system from Canon, Nikon etc. But the weakness of the Canon alternatives in particular is not that they do not offer extremes of low DOF. More likely the weakness is in resolution, which is part of my point.
« Last Edit: October 27, 2006, 01:57:55 PM by BJL » Logged
BJL
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« Reply #27 on: October 27, 2006, 01:47:51 PM »
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BJL, DoF is perceived ...  No need for changing the definition. ... Diferent people will perceive the same print differently.
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I know that DOF is perceived: that is why I used the phrase "perceived DOF" to emphasize that one must take account of differences in viewing conditions and even in how careful the image gets scrutinized.

The only definition I am putting aside is the standard one underlying standard DOF tables and DOF marking on lenses, which is based in part on the assumption of a particular print size (about 5"x7") and a particualr viewing distance (about 10"?) and a particular degree of visual acuity in the viewer. This is the definition that you also reject, since by using CoC values in proportion to format size, it suggests that DOF depends directly on format!

Finally, nothing I am saying is related to differences between different viewers: I am interested in how changing the camera, lens choices, focal lengths etc. effects the DOF that is perceived by the same viewer.

To paraphrase something you have often said, it is best compare by varying only one factor (or as few as possible) while holding others constant, so when comparing cameras and lens systems of different  formats, hold the viewer constant, so as to eliminate effects of differences in visual acuity or tolerance for image fuzziness.
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howiesmith
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« Reply #28 on: October 27, 2006, 01:54:37 PM »
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Perhaps you should reread my original post: by standard DOF reckoning (comparing equal sized prints from the same distance), medium format lenses cannot give DOF as low as one can get from fast 35mm format lenses, due to fact that MF lenses have higher minimum f-stops.

Are you stuck in the endless fallacy of assuming that equal f-stops always are (or even always can be) used with different formats and different focal lengths, ignoring the fact that minimum f-stops available actually tend to increase with focal length (beyond normal) and with format for which the lens is designed?
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Just because a print design may require a focal length lens or f/stop that I do not have or is not available, does not make the design invalid or wrong.  I just can't achieved it.  No reason to throw out science or methods because a design calls for a 200mm f/1.4 and I don't have a 200mm f/1.4 or it doesn't exist for my camera.

I need another image, not new methods.  I just can't take the photo I had in mind.  Methods don't care what I have in my camera bag.
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howiesmith
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« Reply #29 on: October 27, 2006, 02:01:37 PM »
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I know that DOF is perceived: that is why I used the phrase "perceived DOF" to emphasize that one must take account of differences in viewing conditions and even in how careful the image gets scrutinized.

The only definition I am putting aside is the standard one underlying standard DOF tables and DOF marking on lenses, which is based in part on the assumption of a particular print size (about 5"x7") and a particualr viewing distance (about 10"?) and a particular degree of visual acuity in the viewer. This is the definition that you also reject, since by using CoC values in proportion to format size, it suggests that DOF depends directly on format!

Finally, nothing I am saying is related to differences between different viewers: I am interested in how changing the camera, lens choices, focal lengths etc. effects the DOF that is perceived by the same viewer.

To paraphrase something you have often said, it is best compare by varying only one factor (or as few as possible) while holding others constant, so when comparing cameras and lens systems of different  formats, hold the viewer constant, so as to eliminate effects of differences in visual acuity or tolerance for image fuzziness.
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So we agree?

How about when comparing camera systems of different formats, also holding the lens focal length and f/stop constant (unchanged) when looking for the effect of format changes?
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BJL
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« Reply #30 on: October 27, 2006, 03:32:15 PM »
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How about when comparing camera systems of different formats, also holding the lens focal length and f/stop constant (unchanged) ...
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Howard,
    that is one comparison that can be done of course, and you have done it many times. But as you know well, it involves changing the FOV of the full image recorded by the camera, and that is a rather uncommon situation. Most of us are far more interested in what happens when we hold the (uncropped) FOV constant.

It is just a matter of which quantities ones wants to hold constant, which depends on the photographic purpose for which the comparison is being done. My purpose is usually to record a given FOV using as much as possible of the available film or sensor area, so I usually increase focal length when I use a larger format.
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« Reply #31 on: October 27, 2006, 03:41:40 PM »
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But the weakness of the Canon alternatives in particular is not that they do not offer extremes of low DOF. More likely the weakness is in resolution,[a href=\"index.php?act=findpost&pid=82547\"][{POST_SNAPBACK}][/a]

I disagree with this - it is fine to disagree - each chooses a system suitable to thier needs

My hypothesis on the popularity of the Mf look is that for example with fashion a really wide aperture is not suitable becuase say for a half length the clothes and the models face cannot both be sharp with a lense like an 80 (mf645)

A small enough aperture must be selected to produce acceptable sharpness over this required range - eyes to logo on breast in this case

The desire MAY  then to be have the bacground OOF - a larger chip will make this occur and is then for some highly desirable

My observation (that may be totally wrong) of glossy fashion and food is that very wide apertures are rarely used but the look (of the background) is created by the large format of the recording device - often still 67 or 54

ROll on a 16mp 67 chip - I would choose it over a P30 - others wouldnt

I am still struggling with why I cant the right result with  the razor focus with say my 300 2.8 (35mm) - basically the focus is a nearer to infinity so doesnt drop away and the background pulled in which creates an unnatural perspective even with the right DOF

I own a 50 1.2 and know that (close in) the DOF is actually a bit useless exept for creating what some what call a bit of a joke effect I rarelly shoot it open although I love the birghtnes of view

My 50 1.2 cant do what my 80/2.8 blad can do - give a sharpish subject with a natural relationship with its background - both in terms of size or OOFness

For my magazine work my SLRN has very adequate resolution but just looks wrong or not as right as my blad
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« Reply #32 on: October 27, 2006, 05:03:11 PM »
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A small enough aperture must be selected to produce acceptable sharpness over this required range - eyes to logo on breast in this case

The desire MAY  then to be have the bacground OOF - a larger chip will make this occur
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Are you saying that with the same DOF, a larger format gives stronger OOF effects in the background? I do not see it. Instead, the relationship between DOF and background OOF effects depends only on distance from the subject.

So yes, changing from 50mm to 80mm in the same format, increasing ones distance from the subject 60% to keep the same FOV on the main subject, and keeping the same f-stop, the DOF stays the same, but the more distant background appears larger and blurrier, but that goes with change of position and thus of perspective.

If instead one changes from 50mm to 80mm staying at the same place, using a larger format so you get the same FOV, and increase aperture ratio (about one stop) to get the same DOF on the subject, then the more distant background appears the same size with the same amount of OOF blur.


Contrary to some comments in earlier posts, the relationship between DOF (OOF blur for subjects just far enough from the plane of exact focus to be visibly OOF) and stronger far background OOF effects, depends only on camera position relative to the subject and background.
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« Reply #33 on: October 27, 2006, 05:24:12 PM »
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Sam Morgan Moore Cornwall
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howiesmith
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« Reply #34 on: October 27, 2006, 05:57:43 PM »
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Howard,
    that is one comparison that can be done of course, and you have done it many times. But as you know well, it involves changing the FOV of the full image recorded by the camera, and that is a rather uncommon situation. Most of us are far more interested in what happens when we hold the (uncropped) FOV constant.

It is just a matter of which quantities ones wants to hold constant, which depends on the photographic purpose for which the comparison is being done. My purpose is usually to record a given FOV using as much as possible of the available film or sensor area, so I usually increase focal length when I use a larger format.
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OK, I prefer to keep the degree of enlargement constant, and you prefer something else.  FoV is a consideration in my selection of a lens and format, but has nothing to do with my calculation of DoF.  Just optics and some personal choices for CoC.

As you know, FoV is dependant on format, focal length and focus distance.  To keep FoV constant while changing format requires changing another variable to conpensate.  Change format, change FoV.  To get FoV back, change focal length or focus distance.  Then to get DoF back, change f/stop.  What ever.  We just do not agree and likely never will.
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BJL
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« Reply #35 on: October 27, 2006, 07:17:33 PM »
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Howard, surely there are some facts on which we can agree.

First, DOF is not determined by format; it depends on numerous choices, and so DOF comparisons can only be made for specific combinations of choices.
With equal subject distance, image size on the print, and viewing distance (and equal viewer, with equal visual acuity etc.!),
1. If one chooses the same focal length and aperture, ones get the same DOF with any format.
2. If one chooses the same FOV and aperture ratio, one gets more DOF with a larger format.
And my personal favorite, not so much part of this discussion:
3. If one chooses the same aperture (meaning aperture size, measured by effective aperture diameter), one gets the same DOF.

Here by equal DOF, I mean that at every corresponding point on the prints, the circle of confusion (the disk into which the light from a single point of the subject is blurred by OOF effects) is the same size, so everything from DOF to extreme OOF effects in the far background are equal.

NOTE: do not confuse this more basic meaning of "circle of confusion" with its derivative use for a threshold value of the CoC diameter, occurring in some DOF formulas: my conclusions do not depend on any such choices of "CoC" value.

Do you disagree with any of these three claims? If not, then we have no significant disagreement on fact.
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« Reply #36 on: October 27, 2006, 10:38:43 PM »
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Are you saying that with the same DOF, a larger format gives stronger OOF effects in the background? I do not see it. Instead, the relationship between DOF and background OOF effects depends only on distance from the subject.

...

If instead one changes from 50mm to 80mm staying at the same place, using a larger format so you get the same FOV, and increase aperture ratio (about one stop) to get the same DOF on the subject, then the more distant background appears the same size with the same amount of OOF blur.

Contrary to some comments in earlier posts, the relationship between DOF (OOF blur for subjects just far enough from the plane of exact focus to be visibly OOF) and stronger far background OOF effects, depends only on camera position relative to the subject and background.
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I think this is the point I still have some confusion on. My earlier thinking was that your position above was correct. To try and sum my earlier thoughts up -

If you equalize the field of view, subject distance, background distance, and print size - then shoot the same scene with smaller and larger formats, increasing the focal length and using a smaller maximum aperture as the format gets larger (increasing the aperture to the degree that calculated depth of field at the plane of focus remains relatively constant) - the resultant background blur should also remain constant.

But Sam's comments made me rethink this, and my own experience with MF and LF compared to 35mm made me question the above as well.

So, my own little test was to set up a fixed subject and background, and focus and view the same scene with three different systems (sorry, didn't go as far as I should have and make similar sized prints). I viewed the scene with a 50mm f/1.4 on 35mm, and 80mm f/2.8 on 6x6, and a 150mm f/5.6 on 4x5. Each have very similar fields of view and calculated depth of field, but to my eyes the larger formats defocused the background to a greater degree. I also saw what I was talking about earlier - the increased perceived separation of subject from background with the larger formats.

Now I know one big influence is that the size of the viewfinder or ground glass increases with larger formats, and that can sway my perception. However, even though I mentally tried to take this into consideration, I still saw more background blur with the larger formats. That is why I agreed with Sam on his point.

Not being a mathmatician I can't say whether the above conforms with optical theory. I did glance at an interesting website that tries to address the question but couldn't quite make sense of all the factors involved. Here's the website if you want to take a peek -

[a href=\"http://www.bobatkins.com/photography/technical/bokeh_background_blur.html]http://www.bobatkins.com/photography/techn...round_blur.html[/url]

I suppose one way to try to solve this would be to make prints and view the results for myself. I'm not particularly enthused about doing this, but I am curious about how to reconcile the two positions.

I still think that even if optical theory proves the first position right, Medium Format does exactly what Sam says it does - provides a sharper subject with adequate depth of field plus a nicely blurred background. My thought is that it may do this not because of the quantity of background blur but instead because of the reasons I outlined earlier (larger format = less lp/mm demand on the lens = better MTF performance at wider apertures = sharper subject with more pleasing background blur).

Sam is absolutely right - there is no way that a Canon 50mm f/1.4 or Nikkor 50mm f/1.2 on 35mm looks as good wide open as a Zeiss 80mm f/2.8 on MF.
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« Reply #37 on: October 28, 2006, 07:40:40 AM »
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Howard, surely there are some facts on which we can agree.

First, DOF is not determined by format; it depends on numerous choices, and so DOF comparisons can only be made for specific combinations of choices.
With equal subject distance, image size on the print, and viewing distance (and equal viewer, with equal visual acuity etc.!),
1. If one chooses the same focal length and aperture, ones get the same DOF with any format.
2. If one chooses the same FOV and aperture ratio, one gets more DOF with a larger format.
And my personal favorite, not so much part of this discussion:
3. If one chooses the same aperture (meaning aperture size, measured by effective aperture diameter), one gets the same DOF.

Here by equal DOF, I mean that at every corresponding point on the prints, the circle of confusion (the disk into which the light from a single point of the subject is blurred by OOF effects) is the same size, so everything from DOF to extreme OOF effects in the far background are equal.

NOTE: do not confuse this more basic meaning of "circle of confusion" with its derivative use for a threshold value of the CoC diameter, occurring in some DOF formulas: my conclusions do not depend on any such choices of "CoC" value.

Do you disagree with any of these three claims? If not, then we have no significant disagreement on fact.
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I certainly agree with 1.

With 2., I also agree.  Changing format does change the cropped FoV, if I keep the degree of enlargment unchanged.  And yes, the FoV can be changed (preserved) if I also chose to change focal length or crop.  It is the change in focal length changes the DoF.  This seems like a very subtle point but important - it is the change in focal length that changes DoF not the change in FoV.  If I changed to a larger format with the same focal length and degree of enlargement, I can retain FoV with a paper trimmer on the final print.  No need to change focal length, degree of enlargement, or DoF.  Merely paper size to get a larger photo on the paper.  Now, if I just start with the smallest format and work my way up, this all works out.

And 3. is also true, same as above but changing f/stop to compensate for the change in DoF due to dhanging focal length.

I will not disagree that the way most (many? some?) people take photos is to find about the right spot for the tripod (focus distance) then select a focal length to fill the frame (zoom?) and then an f/stop.  f/stop might even be selected by the camera or the photographer's notion of the "sharpest."  Distance is usually then selected by the autofocus camera.

This last point could be important.  From a DoF consideration, it is not neceaasry that any part of the subject be at the focus distance.  A case would be some foreground elements at the edge of the Grand Canyon.  Some stuff a few feet away, some stuff at essentially infinity.  Auto focus picks something but maybe the DoF design would call for a focus distance of 100 feet.  Does the usual auto focus camera user ever give this a thought?

If suddenly the photographer decided to change formats, then a new focal length would be selected to fill the new frame.  New DoF.  While true, the cause/effect relationships are not changed.   I also believe that few photographers are aware of all the changes they are making when they zoom, let the camera focus and select f/stop, the effects of those changes, and, perhaps most important, why those changes have those effects.  (No time to futz with all that stuff.  I gotta capture the moment.  I'll fix all that stuff later when I edit this moment in PS.)  That is why some (many? most?) still think DoF is format dependant.
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« Reply #38 on: October 28, 2006, 07:56:39 AM »
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Howard, surely there are some facts on which we can agree.

First, DOF is not determined by format; it depends on numerous choices, and so DOF comparisons can only be made for specific combinations of choices.
With equal subject distance, image size on the print, and viewing distance (and equal viewer, with equal visual acuity etc.!),
1. If one chooses the same focal length and aperture, ones get the same DOF with any format.
[a href=\"index.php?act=findpost&pid=82606\"][{POST_SNAPBACK}][/a]


I'm not so sure of 1, in the sense that I'm not so sure that two lenses of different design set to the same aperture would have the same depth of field even when mounted on the same camera and focused to the same distance. Sorry, but this is not obvious.

If someone wants to point us to a decent lens simuator, with a library of lens designs and target objects we could do some (simulated) experiments.

However, it would also be nice to see what the wide-angle shooters here think - is depth offield with a retrofocus lens on an SLR body the same in practice as that of a pseudo-symmetrical wide on a view camera ?

Edmund
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howiesmith
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« Reply #39 on: October 28, 2006, 08:20:00 AM »
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I'm not so sure of 1, in the sense that I'm not so sure that two lenses of different design set to the same aperture would have the same depth of field even when mounted on the same camera and focused to the same distance. Sorry, but this is not obvious.

If someone wants to point us to a decent lens simuator, with a library of lens designs and target objects we could do some (simulated) experiments.

However, it would also be nice to see what the wide-angle shooters here think - is depth offield with a retrofocus lens on an SLR body the same in practice as that of a pseudo-symmetrical wide on a view camera ?

Edmund
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The example you gave is changing more than focal length - "two lenses of different design."

All other things being equal, a fuzzy image will have more DoF simply because it becomes more difficult to perceive what is in and out of focus.  Opposite is true for sharper images.  Pin hole camers have large DoF for at least two reasons - very small f/sop and poor resolution (fuzzy).

True, the calcualted DoF would be the same, but the actual (applied) DoF could vary.  Because I use the same lenses (even when changing lenses, I keep using the same ones over and over), I take these "real" variations into account when planning prints by adjusting CoC.

DoF, while something that can be calcuated, usually requires some fine tuning.
« Last Edit: October 28, 2006, 08:23:44 AM by howiesmith » Logged
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