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Author Topic: Newbe question 35mm vs MF and dof  (Read 20170 times)
Sheldon N
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« Reply #20 on: December 31, 2010, 04:02:26 PM »
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No, this is all wrong.  You cannot arbitrarily change the definition of DOF, just as I can't arbitrarily decide to change the definition of a circle.  DOF already has a universally accepted definition that can be found in most textbooks.

To prevent an unhealthy and possibly drawn out thread, I will cut to the heart of the issue.  What you are describing as "DOF" is known as "viewing resolution".  Viewing resolution is a perceptual quantity that indeed depends on print size, viewing distance, pixel size and sensor size.  However, DOF does not depend on any of these things, except the sensor size as it relates to the object size as measured at the sensor.

My only point in this thread is to please not confuse these terms, especially to someone new here.  There is already so much confusion on the internet with regard to the understanding of DOF.


DOF as a fundamental calculation requires the choice of a numerical CoC. What number you choose for a CoC is rooted in print size, viewing distance and the degree of enlargement required from the native sensor/format size.  To say that DOF does not depend on those things is incorrect, because you cannot calculate DOF without making underlying assumptions as part of your choice of a CoC.

Because the mathematical calculation of DOF requires a CoC which requires underlying assumptions about the nature of the viewing conditions, at it's heart DOF is a perceptual measurement.  You can't say that there is some underlying true DOF and that there is a separate "viewing resolution" since essentially they are one in the same.
« Last Edit: December 31, 2010, 04:04:10 PM by Sheldon N » Logged

David Klepacki
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« Reply #21 on: December 31, 2010, 04:11:55 PM »
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DOF as a fundamental calculation requires the choice of a numerical CoC. What number you choose for a CoC is rooted in print size, viewing distance and the degree of enlargement required from the native sensor/format size.  To say that DOF does not depend on those things is incorrect, because you cannot calculate DOF without making underlying assumptions as part of your choice of a CoC.

Because the mathematical calculation of DOF requires a CoC which requires underlying assumptions about the nature of the viewing conditions, at it's heart DOF is a perceptual measurement.  You can't say that there is some underlying true DOF and that there is a separate "viewing resolution" since essentially they are one in the same.

Viewing Resolution and DOF are NOT the same.

Yes, DOF requires knowing a CoC.  Yes, print resolution also requires knowing a CoC.  However, these CoC values are not necessarily the same.  The CoC of my camera will depend on the pixel size of its sensor, while the CoC of my print will depend on the printer ink drop size and its spreading ability onto whatever substrate I choose to print on.  However, saying that the DOF in my photographs are somehow different due to the CoC of my printer ink droplets is ludicrous.   
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Sheldon N
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« Reply #22 on: December 31, 2010, 05:16:53 PM »
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Viewing Resolution and DOF are NOT the same.

Yes, DOF requires knowing a CoC.  Yes, print resolution also requires knowing a CoC.  However, these CoC values are not necessarily the same.  The CoC of my camera will depend on the pixel size of its sensor, while the CoC of my print will depend on the printer ink drop size and its spreading ability onto whatever substrate I choose to print on.  However, saying that the DOF in my photographs are somehow different due to the CoC of my printer ink droplets is ludicrous.  

Circle of Confusion is generally defined as as the largest blur spot that will still be perceived by the human eye as a point. We use this CoC number to extrapolate DOF based upon what amount of optical defocus is permissible in a given image for a given print size and viewing condition, to where it will still appear acceptably "sharp".

CoC is based upon a set of environmental factors... What size of print are you viewing? What distance will you be viewing it at? What is the underlying visual acuity of the viewer's eyesight?  These factors determine the physical size of a blur spot that will be perceived as being a point on the final print. Once you know that physical size of that blur disc, you can extrapolate it back to the negative/sensor by looking at the degree of enlargement from the sensor to the final print.

The underlying point that I believe you are missing is that the entire concept of DOF and CoC is rooted in your perception of the image in a given set of viewing conditions. Change any of those viewing conditions (print size, viewing distance, whether you're wearing your glasses or not) and you change the CoC because you now perceive a different size blur disk as being a point source. That change in CoC then changes your DOF.

DOF is perceptual, not absolute.
« Last Edit: December 31, 2010, 05:18:46 PM by Sheldon N » Logged

David Klepacki
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« Reply #23 on: December 31, 2010, 05:35:17 PM »
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Circle of Confusion is generally defined as as the largest blur spot that will still be perceived by the human eye as a point. We use this CoC number to extrapolate DOF based upon what amount of optical defocus is permissible in a given image for a given print size and viewing condition, to where it will still appear acceptably "sharp".

CoC is based upon a set of environmental factors... What size of print are you viewing? What distance will you be viewing it at? What is the underlying visual acuity of the viewer's eyesight?  These factors determine the physical size of a blur spot that will be perceived as being a point on the final print. Once you know that physical size of that blur disc, you can extrapolate it back to the negative/sensor by looking at the degree of enlargement from the sensor to the final print.

The underlying point that I believe you are missing is that the entire concept of DOF and CoC is rooted in your perception of the image in a given set of viewing conditions. Change any of those viewing conditions (print size, viewing distance, whether you're wearing your glasses or not) and you change the CoC because you now perceive a different size blur disk as being a point source. That change in CoC then changes your DOF.

DOF is perceptual, not absolute.

I do not disagree that CoC can be based on a perceptual interpretation of sharpness.  My only point is that you cannot mix different CoC values when talking about DOF.   The CoC relevant to a print is different from the CoC of a digital image.   For example, the DOF of my captured images are solely determined by how much I stop down my lens of a given focal length with a given sensor.  The DOF of my captured images do not suddenly and magically change from printer to printer just because the size of the inkjet droplet changes, which is what determines the CoC of the final print.   While the CoC of my camera sensor influences the DOF of my image, it is the CoC of my printers that influence their actual viewing resolutions.  And yes, both CoC can be perceptually determined.  However, the bottom line is that the DOF of my captured images never change; only their viewing resolutions can change based on the size and nature of the final print.
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eronald
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« Reply #24 on: December 31, 2010, 05:57:49 PM »
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I regularly use F1.4 on my D3x with an 85mm and find the focus to be perfectly ok.
My old Canon 1Ds could also focus the 85/1.2 wide open quite well.

Edmund

Hi,

I didn't do the math but I guess about f/1.7. DoF will be same on a D4X regardless of MP, albeit you may get a bit more demanding. Anyway the difference between 30 MP and 24.5 MP is quite ignorable, it's about 10% on linear scale.

You can assume something like one stop difference.

Focusing is quite critical, BTW, and you cannot really rely on AF for pinpoint focus, but neither can you rely on your eyes. Live view is probably the best way to achieve dead on focus.

Best regards
Erik

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Sheldon N
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« Reply #25 on: December 31, 2010, 06:03:12 PM »
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I do not disagree that CoC can be based on a perceptual interpretation of sharpness.  My only point is that you cannot mix different CoC values when talking about DOF.   The CoC relevant to a print is different from the CoC of a digital image.   For example, the DOF of my captured images are solely determined by how much I stop down my lens of a given focal length with a given sensor.  The DOF of my captured images do not suddenly and magically change from printer to printer just because the size of the inkjet droplet changes, which is what determines the CoC of the final print.   While the CoC of my camera sensor influences the DOF of my image, it is the CoC of my printers that influence their actual viewing resolutions.  And yes, both CoC can be perceptually determined.  However, the bottom line is that the DOF of my captured images never change; only their viewing resolutions can change based on the size and nature of the final print.


DOF cannot exist without visually viewing the image. DOF is defined as those portions of the image in front of and behind the plane of focus that appear acceptably sharp. You can't have anything appear to be acceptably sharp without actually looking at it.

If you want to state that your standard for what the true DOF of your images is what is in the captured file as it comes out of the camera, that is totally fine. However you have to understand that this requires the additional parameters of saying that you will view them on your specific monitor, at 100% magnification, with your desk chair sitting 24" from the screen. Roll your chair back a couple feet further away from your monitor and you will be able to perceive in less detail a blur disc from a point - and that is a change in CoC, and therefore a change in DOF. 

DOF does not exist as a theoretical absolute, it only exists when you view an image. Viewing an image always requires a set viewing conditions, and those all affect your ability to perceive a blur dot from a point in an image. That means that there is always an underlying inferred CoC involved. A lot of photographers do what you do, choose to view their images at 100% at a viewing distance that lets them see ALL the possible detail in the digital file - this makes sense. However, it's just the functional equivalent of choosing a really stringent CoC, as if you were to print all your images really large and view them really close. It's no more or less the "true" DOF of an image than any other viewing condition.
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David Klepacki
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« Reply #26 on: December 31, 2010, 06:35:27 PM »
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OK, I think you are in agreement with what I have said.  Indeed, there involves a CoC in every aspect of viewing. This means that there is an optical CoC associated with your eyes/lens, a pixel based CoC associated with the sensor that captures the image, and the ink nozzle based CoC of the printer.  This is exactly what I have been trying to point out.   DOF is strictly an optical term associated with the first one, as it involves a 3D context of distance.  Once you introduce film or sensor, then the use of DOF is no longer correct as you are referring to the 2D rendering of a 3D scene.  Same with the printer.  In both of these 2D contexts, there is no distance before/after any plane of focus and the concept of DOF has no meaning.  Rather, it is the concept of resolution, whether it be the capture resolution of the sensor or the viewing resolution of the print that is now meaningful. 

All I am trying to say is that DOF ( a 3D concept) does not depend on the print (a 2D concept) in any way.  It is actually the viewing resolution that is determined by the size and nature of the print.
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Sheldon N
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« Reply #27 on: December 31, 2010, 07:09:20 PM »
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OK, I think you are in agreement with what I have said.  Indeed, there involves a CoC in every aspect of viewing. This means that there is an optical CoC associated with your eyes/lens, a pixel based CoC associated with the sensor that captures the image, and the ink nozzle based CoC of the printer.  This is exactly what I have been trying to point out.   DOF is strictly an optical term associated with the first one, as it involves a 3D context of distance.  Once you introduce film or sensor, then the use of DOF is no longer correct as you are referring to the 2D rendering of a 3D scene.  Same with the printer.  In both of these 2D contexts, there is no distance before/after any plane of focus and the concept of DOF has no meaning.  Rather, it is the concept of resolution, whether it be the capture resolution of the sensor or the viewing resolution of the print that is now meaningful.  

All I am trying to say is that DOF ( a 3D concept) does not depend on the print (a 2D concept) in any way.  It is actually the viewing resolution that is determined by the size and nature of the print.


I think I'm tracking with what you're saying, but not entirely sure I agree.

DOF is expressed in a way that refers back to the original three dimensional scene. When we say that there is 1 inch or 1 foot of DOF in the image, we speak with three dimensional language. That's because the DOF math refers back to the optics, focus distance, focal length, etc that all tell us about the original three dimensional scene. However, the thing that we are calculating is where to place the arbitrary planes that represents in-focus vs out-of-focus (sharp vs. unsharp) in front of and behind the plane of focus when we view a print.  The basis for that calculation is rooted in our viewing of a 2 dimensional output, and the variables for that calculation all fluctuate based on viewing conditions (CoC).


Depth of Field is not an optical property, it is those portions of an image (print/monitor/etc) in front of and behind the plane of focus that appear acceptably sharp, expressed in a distance measurement referring back to the original scene. DOF absolutely depends on the print and the act of viewing the 2D image, it's just that the answer is expressed as a three dimensional measurement of the subject as it existed in the real world at the time you took the photo.  Without the print or monitor, DOF does not exist.  
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Nick Rains
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« Reply #28 on: January 01, 2011, 03:23:58 AM »
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Sheldon is absolutely correct, and it's fair to say that many people get a bit baffled by DOF discussions.

DOF is an illusion, it's merely a region of acceptable sharpness, whats actually acceptable is very subjective.
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Nick Rains
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« Reply #29 on: January 01, 2011, 05:31:08 AM »
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Thanks to all!

Although more complicated than I expected, I think I learned quite a bit.

It made me realize that dof should probably not be a main concern when I decide to either go for MF or continue with Nikon. 

Christopher
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jsch
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« Reply #30 on: January 01, 2011, 06:00:13 AM »
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Thanks to all!

Although more complicated than I expected, I think I learned quite a bit.

It made me realize that dof should probably not be a main concern when I decide to either go for MF or continue with Nikon.  

Christopher

Hi Christopher,

if I understand your question right, the answer is not that complicated:

To find the corresponding f-stops of two formats (i.e. where the impression in terms of DOF is identical/similar) there is a very simple approximative formula*.

(diagonal of format 1)/(diagonal of format 1) = (f-stop of format 1)/(f-stop of format 2)

example:
(diagonal 35mmSLR):(diagonal 8x10'') = 43,3 : 300 = 1:7
=> f-stop 35mmSLR = (f-stop 8x10'') : 7 = 5,6 : 7 = 0,8
That means if you use an f-stop of 5,6 on a 8x10 inch camera you have to use an f-stop of about 0,8 with a 35mmSLR to get a similar DOF impression.

see also: http://www.luminous-landscape.com/forum/index.php?topic=48840.msg403994#msg403994

Hope that helps,
Johannes
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* The angle of view should be similar/identical.
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David Klepacki
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« Reply #31 on: January 01, 2011, 10:18:52 AM »
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DOF absolutely depends on the print and the act of viewing the 2D image, it's just that the answer is expressed as a three dimensional measurement of the subject as it existed in the real world at the time you took the photo.  Without the print or monitor, DOF does not exist. 

Sheldon, This is just not true.  DOF exists, irrespective of whether or not a print or monitor exists.  It is entirely an optical phenomena, and only an observer needs to exist.

Sheldon is absolutely correct, and it's fair to say that many people get a bit baffled by DOF discussions.

DOF is an illusion, it's merely a region of acceptable sharpness, whats actually acceptable is very subjective.

Nick, The fact that DOF is subjective is not what is under debate here.  Sheldon (and Doug) insist that DOF depends on the print sze, which is simply not true.  In fact, if we refer to the actual documentation by Phase One for their 645 DF camera (attached here for convenience), Phase One explicitly points out that they provide a DOF Preview Button.  They do not call it a stop-down button or even an approximate DOF preview button, but refer to it as a DOF Preview button.  Furthermore, they explicitly state what the DOF depends on in the text, and it does not depend in any way on the print.

So, you can take one of three positions here.  First, either the Phase One engineers have no clue about DOF, and their DOF Preview button on their 645DF is actually bogus, since you believe that DOF depends on the print.  Second, the Phase One engineers have developed incredible technology for their 645 DF camera that somehow lets you see DOF based on your printing size by pushing a button on the camera, i.e., before you actually decide what size print you intend to make.  Or third, that the Phase One engineers actually know what they are doing and that DOF is actually an optical phenomena that is determined by an observer independently of print size.

I choose the third option, but I respect your right to believe in either of the other alternatives.
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BartvanderWolf
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« Reply #32 on: January 01, 2011, 10:57:42 AM »
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DOF exists, irrespective of whether or not a print or monitor exists.  It is entirely an optical phenomena, and only an observer needs to exist.

David,

Depth in DOF is a dimension, it requires a definition of its boundary. The COC is that boundary.
 
Cheers,
Bart
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« Reply #33 on: January 01, 2011, 11:07:07 AM »
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Hi!

Point 1 is right. But there is a real theory behind it, based on human vision.

You may check this: http://www.betterlight.com/downloads/whitePaper/depth_of_field.pdf

The normal DoF tables are based on smallish prints viewed at 25 cm distance and taking the angular resolution of the eye into account. Assuming that viewing distance is proportional to the size of the print we would not include viewing distance or print size in the DoF formula. But we do have a tendency to look at large print at close distance, and in that case the old DoF formulas don't hold.

So, if you make smallish prints, like 5x7 inch and have normal vision the DoF scales would be just fine. If you print larger and looking at the prints from close the DoF scales on the lenses are far to optimistic.

Best regards
Erik


So, you can take one of three positions here.  First, either the Phase One engineers have no clue about DOF, and their DOF Preview button on their 645DF is actually bogus, since you believe that DOF depends on the print.  Second, the Phase One engineers have developed incredible technology for their 645 DF camera that somehow lets you see DOF based on your printing size by pushing a button on the camera, i.e., before you actually decide what size print you intend to make.  Or third, that the Phase One engineers actually know what they are doing and that DOF is actually an optical phenomena that is determined by an observer independently of print size.

I choose the third option, but I respect your right to believe in either of the other alternatives.

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Sheldon N
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« Reply #34 on: January 01, 2011, 01:20:50 PM »
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Sheldon, This is just not true.  DOF exists, irrespective of whether or not a print or monitor exists.  It is entirely an optical phenomena, and only an observer needs to exist.

Nick, The fact that DOF is subjective is not what is under debate here.  Sheldon (and Doug) insist that DOF depends on the print sze, which is simply not true.  In fact, if we refer to the actual documentation by Phase One for their 645 DF camera (attached here for convenience), Phase One explicitly points out that they provide a DOF Preview Button.  They do not call it a stop-down button or even an approximate DOF preview button, but refer to it as a DOF Preview button.  Furthermore, they explicitly state what the DOF depends on in the text, and it does not depend in any way on the print.

So, you can take one of three positions here.  First, either the Phase One engineers have no clue about DOF, and their DOF Preview button on their 645DF is actually bogus, since you believe that DOF depends on the print.  Second, the Phase One engineers have developed incredible technology for their 645 DF camera that somehow lets you see DOF based on your printing size by pushing a button on the camera, i.e., before you actually decide what size print you intend to make.  Or third, that the Phase One engineers actually know what they are doing and that DOF is actually an optical phenomena that is determined by an observer independently of print size.

I choose the third option, but I respect your right to believe in either of the other alternatives.


Sigh....  I give up.

I've laid out a clear, comprehensive, and accurate explanation of the issue. Short of typing out large excerpts of Ansel Adams "The Camera" or other photographic texts, I don't know what else to tell you.
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David Klepacki
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« Reply #35 on: January 01, 2011, 01:27:59 PM »
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I've laid out a clear, comprehensive, and accurate explanation of the issue. Short of typing out large excerpts of Ansel Adams "The Camera" or other photographic texts, I don't know what else to tell you.

Unfortunately, what you have said is far from being clear, comprehensive and accurate.  I have tried to show you where your concepts are inaccurate, but it appears I am unable to communicate these to you effectively.

David,

Depth in DOF is a dimension, it requires a definition of its boundary. The COC is that boundary.
 
Cheers,
Bart

Bart,  I agree.  Let me try to again point out what I find preposterous.  According to Sheldon's previous note above he says, "Without the print or monitor, DOF does not exist."  I can't see how any sane person can believe this.  If it were true, how would it even be possible to set proper exposure settings on your camera to get the desired DOF that you wish to capture?  Obviously, you must have some ability to establish DOF prior to actually capturing the image, irrespective of whether or not a printer or monitor exists.

Point 1 is right. But there is a real theory behind it, based on human vision.
You may check this: http://www.betterlight.com/downloads/whitePaper/depth_of_field.pdf

So, those guys at Phase One better get their act together.  What were they thinking when they created a purely digital camera with a DOF button on it?  Smiley

Technically, there is nothing in the above pdf document that is inconsistent with what I have been saying.  When you transfer the concept of DOF, a purely optical phenomena, from your eyes/lens to the digital sensor, you are basically substituting the CoC of the pixel for that of your retinal viewing area.  This creates an effective DOF relative to the digital sensor, and is no different than what photographers have been referring to as DOF for many years with film using film grain instead of pixels to establish the CoC.  Indeed, at this point in the photographic process the DOF is fixed, whether it be film or digital.  I am not disputing that the DOF is drastically different with digital as opposed to film as pointed out by the Betterlight document, since the CoC values are indeed drastically different.

However, what I am disputing is the role of print size here.  Printing involves its own and different CoC, which determines how much resolution can ultimately be seen in the final print in conjunction with a particular substrate, e.g. whether you will be able to see 200 dpi, 300 dpi, 400 dpi, etc. on your printing substrate.  In no way does this printing process affect the DOF of my image that I previously captured.  The printing process only establishes one particular viewing resolution, which will vary with print size.  BUT, the DOF of my captured images do not change just because I print them.  This is the point of my contention.  Doug and Sheldon claim that DOF depends on the print size.  I simply do not find this to be true.

The ONLY case where the DOF is actually determined by the print is when the image capture process and the print process are one and the same, i.e. Polaroid.  The DOF is always fixed at the point of capture.
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Sheldon N
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« Reply #36 on: January 01, 2011, 02:58:49 PM »
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If it were true, how would it even be possible to set proper exposure settings on your camera to get the desired DOF that you wish to capture?  Obviously, you must have some ability to establish DOF prior to actually capturing the image, irrespective of whether or not a printer or monitor exists.

So, those guys at Phase One better get their act together.  What were they thinking when they created a purely digital camera with a DOF button on it?  Smiley


Without debating the issue further, let me just point out that when you look through the eyepiece of an SLR camera (be it Phase One, Canon, Nikon, etc.), what you are actually seeing is the light projected from the scene, through your lens, off a mirror, and onto a small flat piece of frosted glass or plastic called a focusing screen. You are looking at a live 2 dimensional representation of the original scene on that frosted focusing screen, magnified for viewing by the pentaprism and eyepiece.

Essentially, it's an itty bitty little monitor... and all the rules about DOF, visual acuity and CoC can be applied to your ability to see a blur spot as a point on that 2 dimensional focus screen. A DOF preview button (something that's been on most SLRs long before Phase One was in existence) just lets you see the effect of aperture changes on the focusing screen in real time.
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David Klepacki
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« Reply #37 on: January 01, 2011, 03:10:45 PM »
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Without debating the issue further, let me just point out that when you look through the eyepiece of an SLR camera (be it Phase One, Canon, Nikon, etc.), what you are actually seeing is the light projected from the scene, through your lens, off a mirror, and onto a small flat piece of frosted glass or plastic called a focusing screen. You are looking at a live 2 dimensional representation of the original scene on that frosted focusing screen, magnified for viewing by the pentaprism and eyepiece.

Essentially, it's an itty bitty little monitor... and all the rules about DOF, visual acuity and CoC can be applied to your ability to see a blur spot as a point on that 2 dimensional focus screen. A DOF preview button (something that's been on most SLRs long before Phase One was in existence) just lets you see the effect of aperture changes on the focusing screen in real time.

Yes, I agree with this exactly.  My only point here has always been that DOF is fixed at point of capture (whether film or digital), and it does not change thereafter.  So, the claim that DOF depends on print size is just not true.
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« Reply #38 on: January 01, 2011, 04:02:38 PM »
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Yes, I agree with this exactly.  My only point here has always been that DOF is fixed at point of capture (whether film or digital), and it does not change thereafter.  So, the claim that DOF depends on print size is just not true.


If you re-read the Betterlight document I think you will find that it is explained there.

Whilst it is true that the characteristics of the capture are fixed at the time of the exposure, how this subsequently appears to the viewer very much depends on the display size.

Rather than continue to argue, why not try it for yourself? Shoot a pic on a decent res dslr, at say f8 on a medium wide lens set to infinity and then make a 4x6 print from it. It will all look sharp from front to back. The region of acceptable sharpness is vey wide. Now either make a big print or just look at it enlarged on the monitor. What looked sharp on the small print now is revealed to actually be less so.

This is why DOF is display dependent.

I also understand why you are confused. You are trying to distinguish between some sort of absolute camera DOF and the rest of the process. This is a bit of a red herring as the two cannot meaningfully be considered in isolation as one depends very much on the other.
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David Klepacki
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« Reply #39 on: January 01, 2011, 04:56:06 PM »
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If you re-read the Betterlight document I think you will find that it is explained there.

Whilst it is true that the characteristics of the capture are fixed at the time of the exposure, how this subsequently appears to the viewer very much depends on the display size.

Rather than continue to argue, why not try it for yourself? Shoot a pic on a decent res dslr, at say f8 on a medium wide lens set to infinity and then make a 4x6 print from it. It will all look sharp from front to back. The region of acceptable sharpness is vey wide. Now either make a big print or just look at it enlarged on the monitor. What looked sharp on the small print now is revealed to actually be less so.

This is why DOF is display dependent.

I also understand why you are confused. You are trying to distinguish between some sort of absolute camera DOF and the rest of the process. This is a bit of a red herring as the two cannot meaningfully be considered in isolation as one depends very much on the other.

Nick,  I am not at all confused, and have done this many times with prints at a large variety of sizes.  I understand the printing process extremely well.  I am trying to correct misunderstood notions regarding DOF here.  Again, what you are seeing is simply the result of changing the viewing resolution by printing a file either larger or smaller.  It is not the DOF that changes, but the perceived resolution of the entire image. 

The DOF is NOT display dependent.  It is fixed by the capture medium (film or digital sensor).  How you choose subsequently to view this captured image, whether on screen or in print, is limited by the viewing resolution of the particular device/medium that you choose to view it.  So, assuming a fixed viewing distance, of course a smaller print will appear sharper than one that is printed larger, simply because a smaller print has higher resolution as you are mapping a higher density of image pixels onto a given print area.  Conversely, printing larger and larger will yield a less and less sharp image since you are lowering the resolution each time by spreading the image pixels over a larger area.  However, in each case the DOF of your image remains the same as it was originally captured.
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