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Old Sep 2, 2010, 8:46 AM   #31
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By definition, perception is part of it. If I perceive something as in focus and then step forward and now perceive it as not in focus, DOF has changed.
No. The Circle of Confusion has changed. That's all.

The following image is of a painting hanging on my living room wall. The image is slightly out of focus. The painting is flat, and therefore, every portion of the painting is equally outside the Depth of Field. If you are seated 10 feet from your monitor, it's hard to tell that this image is out of focus. But as you get closer and closer to your monitor, the circle of confusion decreases, and the blur in the image becomes more and more obvious. Surely you're not going to assert that, since you moved closer to your monitor, the depth of field of my photograph has changed.
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Old Sep 2, 2010, 9:25 AM   #32
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tclune - I submit you mistake what the DOF calculations do. They are not calculating a scientific fact. It isn't like calculating velocity. They are attempting to model a subjective phenomenon using some assumptions. If you read through the articles all over the place you'll see they all make some assumptions. Depth of field, by it's definition is about perception. [depth of field (DOF) is the portion of a scene that appears acceptably sharp in the image.]
No, you completely missed my point. You do not -- you CANNOT -- provide a mathematical equation for what you are presenting as "the definition" of DOF. The talk about "assumptions" is just a red herring -- ALL equations of the natural world have underlying assumptions.

Let us look at how this discussion was originally framed by Peripatetic:
"1. People find it much easier to get the practical results they want than to understand the math. This is because it's fairly easy to reduce the equations to a number of useful rules of thumb.

2. It is devilishly difficult to use a natural language like English to accurately explain even a mildly complicated mathematical equation."

A reasonable person would come to the conclusion that the relevant definition of DOF for this discussion was the mathematical equation. Alternatively, you can say that Peripatetic was willfully misleading everyone, But there doesn't seem to be any evidence for that.

And this gets right to the heart of what I was saying originally -- TCav answered the question that was asked, Mark answered a different question that is of interest in deciding whether we want to take the mathematical equation seriously or not. When we switch the question to touchy-feely notions of DOF, we are switching the very nature of the conversation. The mathematical equations implement a particular model of the world, and that model is not subject to the psychological variations of individuals. That is both why we want to use them and why they may fail to capture everything we might want to say about focus.
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Old Sep 2, 2010, 9:26 AM   #33
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TCAV - I submit we will never agree because I believe we are using two fundamentally different definitions of what DOF is. It is my belief you are using a definition espoused on the DOF master page:
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Depth of Field: The zone, or range of distances, within a scene that will record as sharp
Whereas I am using the definition from other sources:
Quote:
depth of field (DOF) is the portion of a scene that appears acceptably sharp in the image
Two completely different definitions. Without agreement on the definition of DOF we will never agree. But we don't have to agree. People can experiment for themselves - even using the images you initially provided and determine which definition of DOF they prefer to use.

For example, using the definition I am using we take this from the wikipedia article:
Quote:
Cropping an image and enlarging to the same size final image as an uncropped image taken under the same conditions is equivalent to using a smaller format under the same conditions, so the cropped image has less DOF.
Cropping the image equates to reducing DOF. Which is exactly what your first and 2nd image demonstrate. Because the definition I am using is about how an image is perceived by a viewer.

Your definition ignores perception of the image, mine includes it. In this theoretical discussion we're having, people can decide for themselves which definition of DOF they choose to accept. In practical terms it doesn't really matter. In practical terms, it's all about RELATIVE DOF. If I change this variable at shot time what happens. In both definitions the same thing happens. After the shot is taken and edited, perception is everything - a viewer looks at the photo and judges for themselves whether something is acceptably sharp or not. Whether you say that perception is included in the definition of DOF or not doesn't matter for terms of practical photography.
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Old Sep 2, 2010, 10:15 AM   #34
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It's called "Circle of Confusion" for a reason. As you get closer or further away from my photograph, your level of confusion about my photograph varies. It's your level of confusion that changes, not my photograph. The CoC is useful for predicting the Depth of Field, but once the photograph is taken, the CoC is only useful for predicting perceptions of the photograph based on the visual accuity of the observer and the distance from the observer to the photograph. You're trying to retrofit the CoC to a formula that is no longer applicable. Changing the CoC of a photograph changes the level of confusion an observer has about a photograph. It does not change the photograph.

Once you take the photograph, the entire scene, including the in-focus and out-of-focus objects are rendered on a two-dimensional medium. When you look at that rendering, the circle of confusion applies to the entire rendering, such that the entire rendering gets sharper or more blurred as you move closer or further away. That is, the sharp portions of the rendering get blurred too, and to the same degree that the blurred portions get (more) blurred.

You're confusing your perception of blur with Depth of Field. If you were actually talking about DoF, as you moved further away, the DoF would change. That is, objects that were further away from the focus distance would get more blurred, but objects at the focus distance would stay sharp. That's not what's happening. Everything, sharp or blurred, gets blurred as you move further away. That's not Depth of Field.

By your definition of Depth of Field, moving closer or further away would alter the Depth of Field of a sheet of newsprint or a child's stick figure drawing, and that's absurd.
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Last edited by TCav; Sep 2, 2010 at 10:18 AM.
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Old Sep 2, 2010, 2:32 PM   #35
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DOF is a property of the lens focal length/aperture/and camera format. The use of CoC is a means of determining how rapidly DOF changes with respect to the focal plane. It tells us what the relative change DOF is going to affect the photo.

Your perception of DOF does not alter the characteristics of the lens or the camera.

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Old Sep 2, 2010, 4:08 PM   #36
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Apparent DOF is a function with FOUR inputs.

DOF = f(CoC, Focal Length, F-stop, Focal distance)

CoC = f(Enlargement, Visual Acuity, Viewing Distance)

Enlargement = f(sensor size, print size)

by substitution you can see that:

DOF = f(sensor size, print size, visual acuity, viewing distance, focal length, f-stop, focus distance)

Seven distinct variables. If you change ANY of those variables you change the apparent DOF.

Most importantly, once the sensor or film has captured the image, you still have 3 variables that have yet to be determined, therefore the DOF at that point is indeterminate. [Notice though that it is not necessarily unbounded depending on the values of those variables that have been fixed.]

By convention, if left unstated, those 3 variables are assumed to be:
12" viewing distance, normal vision, 8x10 print size.

Those were reasonable conventions in the old days when 35mm film was the most popular format, but these days they don't apply so well.

But please note those are VARIABLES not constants, and apparent DOF depends in part on their values.
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Old Sep 2, 2010, 4:23 PM   #37
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When we switch the question to touchy-feely notions of DOF, we are switching the very nature of the conversation. The mathematical equations implement a particular model of the world, and that model is not subject to the psychological variations of individuals. That is both why we want to use them and why they may fail to capture everything we might want to say about focus.
This is correct. But notice my original psychological observations refer only to the context in which the discussions are held, they do not refer to the equations. There is nothing "touchy-feely" about the equations.

Visual acuity can be expressed in lp/mm, and doing so removes all subjectivity from the model. It is a physiological and psychological observation to say "for the average person 5lp/mm is what is regarded as sharp". But that is not to say that there is no variation between viewers.

Print size, viewing distance, focal length, focus distance, aperture, sensor size - are all physical measurements.
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Old Sep 2, 2010, 4:30 PM   #38
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TCav,

Apparent DOF is poorly applied to drawings, etc. It is a very specific notion that applies to photography.

Unless you can tell me what the focal length, aperture, focus distance and degree of enlargement of the print was, when applied to a drawing, it's hard to see what point you could be making.

Just because the apparent DOF is PARTIALLY determined by some variables that may be observed when looking at a drawing, doesn't mean that anyone is asserting that it is WHOLLY determined by that subset.

You cannot determine apparent DOF simply by knowing the values of the print size, viewing distance and visual acuity of the observer. No one is saying that you can.

Similarly you cannot determine apparent DOF if you ONLY know the focal length of the lens, but surely you don't assert that it is not a necessary requirement?
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Old Sep 2, 2010, 5:07 PM   #39
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If you move backwards and forwards, how is the change in the blur that you might see when looking at a photograph different from the change in the blur that you might see when looking at a drawing? The Circle of Confusion changes at the same rate for both.

When you move backwards, don't otherwise sharp objects also blur? If a thing that is in focus gets blurrier, and gets blurrier at the same rate as the out of focus objects, doesn't that indicate that what we're experiencing is a change in the Circle of Confusion and not a change in the Depth of Field? If we were experiencing a change in the Depth of Field and not a change in the Circle of Confusion, wouldn't something still be sharp?
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Old Sep 2, 2010, 6:02 PM   #40
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Suppose you have a photograph. (I know that's a stretch for many of you, but humor me. ) Suppose it's a photograph of the edges of pieces of paper. The pieces of paper are evenly spaced. One of the edges is in focus, and the edges that were closer or farther away from the camera are blurrier because they are farther and farther away from the edge that is in focus. Suppose the photograph is printed on a piece of paper, and is hanging on a wall.

As you move farther and farther away from the photograph, doesn't the edge of the piece of paper that's in focus in the photograph, get blurrier?

As you move farther and farther away from the photograph, doesn't the edge of the piece of paper that the photograph is printed on, get blurrier?

Don't they both get blurrier at the same rate?

Clearly, whatever is causing the edge of the piece of paper that the photograph is printed on, to blur, it has nothing to do with Depth of Field, because the edge of the piece of paper that the photograph is printed on isn't itself a photograph, and only photographs have Depth of Field.

If the edge of the piece of paper that the photograph is printed on, and the edge of the piece of paper that's in focus in the photograph, both get blurrier at the same rate, can't we presume that whatever is causing one to blur, is also what is causing the other to blur?

And since we've already established that Depth of Field isn't what's causing the edge of the piece of paper that the photograph is printed on, to blur, then Depth of Field can't be what is causing the edge of the piece of paper that's in focus in the photograph, to blur.

(Whew.)
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