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Old Sep 1, 2010, 3:20 AM   #1
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Default DOF - spin off from Superzoom v DSLR.

Oh goody, a DOF thread. We haven't had one of those for a while... :-)

There are number of psychological factors to grasp first:

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. Even when people do understand the equations (unusual) they can argue over the best way to express them endlessly.

3. People generally don't like to realise or admit that they haven't understood the subject fully, even though most photographers can achieve the results they want without fully grasping the concepts involved. Instead of being pleased and happy to be corrected and thereby improve one's knowledge, most people get angry and defensive and prefer to cling to erroneous beliefs. Self-esteem somehow attaches itself readily to DOF. [I find this the hardest thing to understand about DOF.]

4. As John points out, with multi-variate equations it is possible (actually quite easy) to change one variable to compensate for changes in another variable and think you have demonstrated something that you actually haven't.

The best place to start to try to understand DOF is to understand CoC first.

Here's a little DOF quiz for y'all:

You have an 8x12 print in your hands. You are viewing it from 12 inches away.

1. You move the picture to 24 inches away, at arms' length. Have you changed the DOF? If so, have you made the DOF larger or smaller?

2. You cut the picture in half and throw away the one half. Have you changed the DOF? If so, have you made the DOF larger or smaller?

3. You cut the picture in half and enlarge the remaining half so that you now have an 8x12 print again. Have you changed the DOF? If so, have you made the DOF larger or smaller?

4. You take off your reading glasses. Have you changed the DOF? If so, have you made the DOF larger or smaller?

If you can get these right then it's reasonable to move on to the next stage, which is to start considering focal length, f-stop and distance to plane of focus.

If you are sure you got the answer to 2 & 3 right, then think about the corollary - what about the size of the negative? Let's leave digital sensors out of the picture for the moment, for they introduce additional complexities, assume we are using film.
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Old Sep 1, 2010, 5:29 AM   #2
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Interesting.

The concept of a "Circle of Confusion" pressumes a conventional viewing distance and a conventional visual accuity. Just because someone isn't conventional, doesn't mean the image has changed. Two people can be looking at the same image and their perceptions of it may be very different, but the image itself hasn't changed. Therefore, my response to questions 1, 2, and 4 would be no.

But it's too early in the morning for me to ponder the details of question 3 just yet.

And as to the questions as they apply to negatives, the circle of confusion is proportional to the image size, both positive and negative, and therefore, my responses for negatives would be the same as for prints.
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Old Sep 1, 2010, 9:38 AM   #3
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My first reaction without really thinking too much, is to say:

1. No
2. No
3. Yes, and smaller.
4. No

Could be wrong.


.......Or actually, I take it back. I think it's "no" to all of them, because it's not a dynamic system where you are changing the parameters, it's the SAME photo, without any change other than the viewpoint.

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Old Sep 1, 2010, 10:17 AM   #4
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For reference:

http://en.wikipedia.org/wiki/Circle_of_confusion

In particular:

Quote:
In photography, the circle of confusion diameter limit (“CoC”) for the final image is often defined as the largest blur spot that will still be perceived by the human eye as a point.
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Old Sep 1, 2010, 12:17 PM   #5
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Quote:
Originally Posted by peripatetic View Post
Oh goody, a DOF thread. We haven't had one of those for a while... :-)

There are number of psychological factors to grasp first:

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. Even when people do understand the equations (unusual) they can argue over the best way to express them endlessly.

3. People generally don't like to realise or admit that they haven't understood the subject fully, even though most photographers can achieve the results they want without fully grasping the concepts involved. Instead of being pleased and happy to be corrected and thereby improve one's knowledge, most people get angry and defensive and prefer to cling to erroneous beliefs. Self-esteem somehow attaches itself readily to DOF. [I find this the hardest thing to understand about DOF.]

4. As John points out, with multi-variate equations it is possible (actually quite easy) to change one variable to compensate for changes in another variable and think you have demonstrated something that you actually haven't.

The best place to start to try to understand DOF is to understand CoC first.

Here's a little DOF quiz for y'all:

You have an 8x12 print in your hands. You are viewing it from 12 inches away.

1. You move the picture to 24 inches away, at arms' length. Have you changed the DOF? If so, have you made the DOF larger or smaller?

2. You cut the picture in half and throw away the one half. Have you changed the DOF? If so, have you made the DOF larger or smaller?

3. You cut the picture in half and enlarge the remaining half so that you now have an 8x12 print again. Have you changed the DOF? If so, have you made the DOF larger or smaller?

4. You take off your reading glasses. Have you changed the DOF? If so, have you made the DOF larger or smaller?

If you can get these right then it's reasonable to move on to the next stage, which is to start considering focal length, f-stop and distance to plane of focus.

If you are sure you got the answer to 2 & 3 right, then think about the corollary - what about the size of the negative? Let's leave digital sensors out of the picture for the moment, for they introduce additional complexities, assume we are using film.
At the risk of getting everything totally wrong here is my understanding of this....... hmmmmm.

1. Yes, the DOF is now wider when viewing further away. Same as a smaller print or what we do when sharing small on the web, the DOF seems deeper (and a way that I can make a slightly OOF photo look good )

2. Nothing is changed.

3. Opposite to one, the DOF is now shallower.

4. Depends if you were needed reading glasses in the first place, if you did and now can't see so well the DOF is shallower (or probably all OOF lol), if you shouldn't be wearing glasses then probably you were amazed to find you can now see and for some reason you are holding a photograph that is in focus

Looking forward to hearing how I did, pm me if you don't want to share anything here as yet...... also if I did really bad I can just delete my post LOL.
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Old Sep 1, 2010, 1:27 PM   #6
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I'll play along - my understandings mimic Mark's. Which is also why I think there was disagreement between the parties in the other thread. Here is my understanding:
Given same distance, focal length, f/stop a full frame and crop camera produce images with different DOF values. If you then crop the full-frame image so it matches the crop-camera image you change the image DOF and the two should now have the same DOF (but only if you crop the full-frame image and thus change it's DOF).

If, however, you adjust FOV by changing either distance or focal length with the full frame camera to match FOV on the crop-camera the two images will end up with different depths-of-field. In this instance you cannot adjust via processing crop to get the depths-of-field to match and the FOV to match.

I'll be interested to hear how this discussion progresses. I will freely admit I am not an engineer so I think this is an interesting discussion and welcome being educated by those here that can explain the "WHY"
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Old Sep 1, 2010, 1:48 PM   #7
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Whoa!

They're all trick questions!

The Depth of Field is the measure of how far in front of, and how far behind the focus distance, objects will be in focus. Once you press the shutter button, there's nothing you can do to change the DoF in an image. Things that were too close to the camera or too far away, such that they were outside the DoF can't be brought into the DoF. Things that were inside the DoF can't be shifted such that they're no longer in the DoF. And there's nothing you can do to change that.

All those formulas are for predicting the DoF, and those formulas are subject to variations in the conditions under which the image is presented. But being further away, or having bad eyesight, or only seeing a portion of the image, enlarged or not, does not affect the actual DoF that was recorded in the image.
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Old Sep 1, 2010, 7:12 PM   #8
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It's a great topic!!!!!
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Old Sep 1, 2010, 9:30 PM   #9
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TCav has got this right - the DOF is determined when the shot is taken. All you do by changing the print size, or viewing distance, is to change the basis for considering the circle of confusion.
Consider a pinhole camera, in which the DOF is for all practical purposes, infinite. You are not going to change that in PP or printing.

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Old Sep 1, 2010, 11:22 PM   #10
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1. Yes, and made it larger

2. No, the same

3. Yes, made it narrower. Crop camera

4. Yes, made it deeper

Time to come clean. My comment "If you blow up the crop to the same size, COC is the exact same." Was dead wrong, but a comment made shortly after "Get cocky and get embarassed." Was dead on.

About the absolute aperture and focal length determining DOF, this is key if perspective is maintained between systems. The focal length is chosen to match FOV, and the same physical aperture diameter maintains the same DOF.

Apologies to all.
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