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Old Sep 2, 2010, 9:08 AM   #21
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I do think tclune has a point though.

Possibly I should have used the word "normative" in some of the places where I have used "subjective".

The main place I was trying to get to is to realise that an appropriate CoC is something you get to choose.

You don't have to take the 5lp/mm at 25cm for normal acuity standard as laid out by Kodak and Zeiss in the 1950s. (Or something like that).

That was just a convenient set of standard parameters they chose to make it easier to draw up DOF tables and put DOF markings on their lenses. It aided with a simplification of the underlying model into some easy-to-understand rules for non-technical photographers.
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Old Sep 2, 2010, 9:09 AM   #22
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Attached are four images. I just created them with my Nikon D90 and my Nikon 85mm f/1.8 lens. They were recorded in Large Fine JPEG files. They are:
  1. f/4.0, reduced for posting here.
  2. 100% crop of the full size version of the first image.
  3. f/11, reduced for posting here.
  4. 100% crop of the full size version of the third image.
Use whatever post processing techniques at your disposal, but give one the depth of field of the other. You can go in either direction.

If you would prefer to work with the original images, PM your e-mail address to me and I'll send them.
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Old Sep 2, 2010, 9:17 AM   #23
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TCAV - I believe this shows exactly what we are talking about. The 1st and 2nd photo came from the same image. In the first photo, the '5' and '6' mark on the ruler appear about the same when I view on my 17" monitor from 4 feet away. When I look at the crop now the "5' is not in focus. The DOF has changed because you cropped the photo.
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Old Sep 2, 2010, 9:19 AM   #24
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oh and when the crop image is displayed and I move back 20 feet the 5 & 6 look the same. depth of field has changed.
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Old Sep 2, 2010, 9:20 AM   #25
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Quote:
Originally Posted by JohnG View Post
TCAV - I believe this shows exactly what we are talking about. The 1st and 2nd photo came from the same image. In the first photo, the '5' and '6' mark on the ruler appear about the same when I view on my 17" monitor from 4 feet away. When I look at the crop now the "5' is not in focus. The DOF has changed because you cropped the photo.
That results from your own private Circle of Confusion, and has nothing to do with the Depth of Field in the photograph.
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Old Sep 2, 2010, 9:22 AM   #26
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At a guess - your camera was focussed maybe 24" from the lens?

With such a close focus distance and an 85mm lens @ f4 you can pretty much change the CoC to crazy levels and not get a very dramatic change.

But in fact simply from looking at the whole image and the crop I think you have demonstrated our point very well. In the 100% crop I'd put the DOF at about 1/3". From the whole picture I'd put it at around 1".

In the second image uncropped, the book title in the backgound is clearly blurred and not within the apparent DOF when I am sitting in front of my monitor, but when I walk back 5 yards it clearly is.

Isn't that what you see?
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Old Sep 2, 2010, 9:24 AM   #27
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Quote:
Originally Posted by TCav View Post
That results from your own private Circle of Confusion, and has nothing to do with the Depth of Field in the photograph.
Let us look again at the definition of depth of field:
Quote:
depth of field (DOF) is the portion of a scene that appears acceptably sharp in the image.
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.
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Old Sep 2, 2010, 9:24 AM   #28
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Whatever. Change one so it has the DoF of the other.
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Old Sep 2, 2010, 9:33 AM   #29
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By the way, the first and second shot also help illustrate the concept of sensor size and it's affect on DOF. Whether the 'crop' happens because the recording media is smaller or because you crop the photo afterward in post processing, you end up with a photo that has an apparent depth of field that is shallower. The larger image appears to have deeper DOF. If DOF were cast in stone in an image at shutter press, you could not crop the image as we see here and something that did appear to be in focus before suddenly appears out of focus. I would suggest that in the original discussion, the test performed by Justin did precisely this - cropped the larger image files to the field-of-view of the crop camera at which point the DOF in all 3 images looked the same. But if you looked at the un-cropped full frame file vs aps-c the DOF in the full frame file would appear greater.
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Old Sep 2, 2010, 9:34 AM   #30
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the dof has not changed its the perspective that has changed. we see what is there if the 2nd shot was enlarged and not cropped we would still have the same picture and the same dof
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