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Old Sep 2, 2010, 2:10 AM   #11
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Okay. So.

We know that focus on most lenses is a PLANE. Lens makers try to make that plane flat, but sometimes it has curvature. But that plane is the only place where the lens is in perfect focus. And even that's not really correct, because we think of "in focus" as being able to produce a resolution greater than 50lp/mm at the film or sensor plane or something like that.

But here's the point. DOF is subjective. It's what happens in the viewers mind when they look at an image and some of it looks sharp and some of it doesn't. Technically we should always call it "apparent depth of field". DOF is apparent. It's not objective or universal.

Large format photographers, fine artists if you like, people who frequently hang their work for exhibitions, are accustomed to selecting an appropriate CoC, before the print, and often before they shoot. Think about it for a second. At 25cm/12" a "spot" or "blur circle" about 0.2mm in diameter will appear sharp to most people. If the distance is greater the spot can get bigger. If your eyesight is very acute the spot needs to be smaller.

Consider the CoC for a motorway advertising billboard - it's about 15mm right?
If you know that you are going to be hanging a print in a gallery where the closest viewing distance is going to be 6ft/2m and you want a particular region of your photograph to be sharp, and you know the resolution of your camera, lens, enlarger or printer you can make decisions about the size of your prints for example or if you already have decided that, then you can decide what aperture you need to use, or what focal length of lens, or whether you need to use tilt and shift to get the Scheimphlug effect going, etc.

So the focal plane of the image is fixed when we trip the shutter, DOF is subjective. So I disagree therefore with TCav and Brian.

I agree with Mark and John and Greg.

1. Yes, it changes. This is known as zooming. When you zoom out by walking away from a print on your wall or in a gallery it looks sharper. When you walk up close sometimes it looks all blurry. In an image where there is substantial "3D-ness" it's really simple to see stuff go in and out of sharpness.

2. No. Because the viewing conditions haven't changed. You have cropped away half the print, but you also cropped away half the negative at the same time.

3. Yes. Crop camera! You have changed the degree of enlargement from the film or digital negative.

4. Yes! Or maybe No! I'm not sure. Obviously it depends on whether you are short or long sighted, but of course I meant that when you take off your reading glasses your visual acuity at 25cm decreases. And therefore the image is less sharp at the same distance. Maybe this is one is a trick question. What if you can hardly see at all?

So now that we've thought about what CoC is, and know the factors which affect it. We can look at the variables in the DOF calculation:

http://dofmaster.com/equations.html

1. Your chosen CoC.
2. The distance to the subject.
3. The focal length of the lens.
4. The physical aperture - i.e. the size of the hole in the front that lets in the light.

And from these variables we can calculate the near and far distance of apparent sharpness. Because we know that the focus is only at a plane, right? DOF is an illusion that the focus extends in front of and behind that plane because when a blur circle is small enough it looks like a point to us.

We know how these work right? Well, rules of thumb yes. But these are not simple linear equations, they are quadratic. In particular people tend to overlook the importance of the distance to the subject. And because we generally don't have marked anywhere the aperture of the shutter, we use the f-ratio instead, which confuses things.

But we all know that:

1. For a fixed and chosen CoC.
2. Focussing further away gives bigger apparent DOF (unless you pass the hyperfocal distance of course, at which point you then start to decrease it).
3. Longer focal lengths give smaller apparent DOF.
4. Bigger apertures give smaller apparent DOF.

Let's wait for a round of comment and discussion and then move onto the question of the complications of digital photography, and the dreaded issue of using a lens of a particular focal length with different sized sensors.
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Old Sep 2, 2010, 2:47 AM   #12
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I must say Im with Tcav on this once this shoot is taken all the points are fixed, you enlarge it move it further away what you cant do is change the dof it is fixed on that picture. The only thing that changes is your perspective of that picture. In other words the way you look at it. But the depth of field is totally fixed.
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Old Sep 2, 2010, 2:49 AM   #13
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Quote:
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I must say Im with Tcav on this once this shoot is taken all the points are fixed, you enlarge it move it further away what you cant do is change the dof it is fixed on that picture. The only thing that changes is your perspective of that picture. In other words the way you look at it. But the depth of field is totally fixed.
This is where the circle of confusion comes in to play.
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Old Sep 2, 2010, 5:55 AM   #14
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This is where the circle of confusion comes in to play.
exactly - you have to remember, it's about how a person PERCEIVES something to be sharp or not. And remember, it's not like anyone is saying you're going to dramatically change what is considered in and out of focus by cropping/enlarging. But enlarging or shrinking a photo or changing viewing distance can make borderline cases appear in focus or out of focus where they didn't used to be. You must remember - it is all about PERCEPTION by the human eye and brain.
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Old Sep 2, 2010, 6:35 AM   #15
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The Circle of Confusion is for predicting how an image will be perceived. The Depth of Field is carved in stone once you press the shutter button. As we've seen here before, the DoF can be made larger by using sharper lenses and higher resolution image sensors, neither of which have anything to do with how the CoC or the DoF are predicted.

Remember that the DoF is measured in feet (or meters) in front of and behind the focus distance. Whatever that DoF is, and however accurate any DoF predictions might be, it is permanently a part of the image. There's nothing you can do to increase or decrease those linear measurements in post-processing. You can alter how they are perceived, but those alterations have consequences that extend beyound what you think of as the DoF. If you enlarge the image, you decrease the sharpness of the image in its entirety, not just that portion of the image that is within the DoF. Yes, it increases the size of the CoC, and that helps determine how the sharpness of the image will be perceived, but you can't then use that larger CoC to recalculate the DoF in a photograph that has already been taken. Changing the CoC after the fact, does not change the DoF of an preexisting photograph.
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Old Sep 2, 2010, 7:05 AM   #16
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Old Sep 2, 2010, 7:50 AM   #17
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Actually TCAV, the DOF calculations are based on some ASSUMPTIONS as the article Craig listed points out. There are assumptions about image size and viewing distance. What the DOF calculator is doing is saying:
The image created, if viewed at X size at Y distance by a person with normal visual acuity will be roughly ZZZZ. That doesn't change the fact that image size and viewing distance affect DOF. Because DOF is a perception thing. So, if you take the image and are not viewing it from the distance the formulas ASSUME or at the size the formulas ASSUME the perceived depth-of-field will not be what the formulas predict.

People can test this for themselves - stage a photo where a 2nd object is behind your subject just beyond the calculated DOF amount. Print the photo at 8x10. Look close and the second object will look 'slightly out of focus'. Now move back another 5 feet or so - your eyes can't pick out the fuzziness as well and the lines look more defined - suddenly said object APPEARS to be in focus. Obviously this doesn't work with an object well beyond the DOF limits - but with objects around the edge of DOF it's easy to see this happen. Depth of field is a perception. The forumulas used in it's prediction are based on assumptions.

If you read the article again you'll see they clearly talk about DOF of original image and final image and the fact they can be different. If you enlarge the original image and don't change viewing distance, the DOF has changed.
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Old Sep 2, 2010, 7:53 AM   #18
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As it happens I went to the Photographers' Gallery last night to see the Sally Mann exhibition they have on. (Very highly recommended btw! for those of you who can get to London.)

With some of the portraits, and certainly some of the landscapes, from across the room the whole image looked in focus to me, but standing at arms' length clearly some of the background appeared OOF.

It seems to me that the apparent depth-of-field was changing as the CoC changed due to my changing distance from the image.

TCav, can you explain why I am mistaken here? I'm pretty sure I wasn't imagining it. You can do this experiment yourself.

[Although of course not all images are amenable. Just run the numbers in the DOF calculators and you can see that the CoC for some combinations can change a lot without affecting the apparent DOF. ]
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Old Sep 2, 2010, 7:54 AM   #19
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Quote:
Originally Posted by peripatetic View Post
But here's the point. DOF is subjective. It's what happens in the viewers mind when they look at an image and some of it looks sharp and some of it doesn't. Technically we should always call it "apparent depth of field". DOF is apparent. It's not objective or universal.
...
So I disagree therefore with TCav and Brian.

I agree with Mark and John and Greg.
...
So now that we've thought about what CoC is, and know the factors which affect it. We can look at the variables in the DOF calculation:

http://dofmaster.com/equations.html

1. Your chosen CoC.
2. The distance to the subject.
3. The focal length of the lens.
4. The physical aperture - i.e. the size of the hole in the front that lets in the light.

And from these variables we can calculate the near and far distance of apparent sharpness. Because we know that the focus is only at a plane, right? DOF is an illusion that the focus extends in front of and behind that plane because when a blur circle is small enough it looks like a point to us.

We know how these work right? Well, rules of thumb yes. But these are not simple linear equations, they are quadratic. In particular people tend to overlook the importance of the distance to the subject.
...
There is a fundamental confusion in your approach here. You cannot have a mathematical equation that calculates subjective responses. What mathematics can do is calculate the predictions of a model of subjective responses. A wonderfully complex example of this is mp3 compression of music. There is a very elaborate model of our ability to hear various frequencies in the presence or absence of other frequencies, and at various amplitudes for same. The mathematics of mp3 compression implements that model, not our actual perception of music. Anyone who has listened to an mp3 of a piece of music and recognized that it has lost something is aware of the difference.

So what? Well, if you take the DOF equations as calculating DOF, then there is no psychological aspect to the determination at all. You may define DOF in terms of change in MTF or what-have-you, but the DOF is an objective reality. If your hope is that it models something subjective, then you may worry about how well DOF tracks our preceived sense of "sharp" or "out-of-focus," but that is simply not what the equation does. The meta-question of whether your DOF calculations track your expectations has absolutely nothing to do with crunching the numbers -- it is all about deciding whether or not you are interested in using the equation.

ISTM that TCav has answered the question you asked -- how does DOF change with the given situations. And Mark et. al. have answered the question you wish you had asked -- does the equation properly track what you care about when you talk of DOF.
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Old Sep 2, 2010, 8:02 AM   #20
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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.
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the depth of field (DOF) is the portion of a scene that appears acceptably sharp in the image.
As you move closer or further away from an image what appears acceptably sharp changes. As you enlarge or shrink an image, what appears acceptably sharp changes. DOF does indeed change. The calculations are an approximation based on assumed viewing size and distance and viewer's visual acuity. Or put another way - you and I could both view the same image and the DOF can be different for both of us. As in Craig's example - you stand 5 feet from the image I'll stand 20 feet away - your perception of what is acceptably sharp will differ from mine for images where subjects are "on the bubble" of DOF.
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