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Author Topic: Sensor size and DoF  (Read 50988 times)
howiesmith
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« Reply #140 on: October 20, 2006, 04:28:34 PM »
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Howard, your analogy is ridiculously irrelevant. It turns on asserting a falsehood. (At least, I take it that you intend it to be clearly false that pigs can fly at 100 mph. If instead pigs can do that, there is no point to your analogy, and the answer is two hours, as surely as my answer is that DOF at equal aperture ratio increases.)

Where is the false assertion or assumption in my argument?

All I see in my argument is th statement that I am considering the quite real case of photographs taken with the larger format camera composed to fill most or all of the frame with the desired subject matter, so that little or no cropping is needed. This happens far more often than pigs fly at 100mph without mechanical assistance.
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howiesmith
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« Reply #141 on: October 20, 2006, 04:50:16 PM »
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All I see in my argument is th statement that I am considering the quite real case of photographs taken with the larger format camera composed to fill most or all of the frame with the desired subject matter, so that little or no cropping is needed. This happens far more often than pigs fly at 100mph without mechanical assistance.

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And all I have been trying to say is it is an unnecessary given or assumption that the photo fill the frame, even if that is the way most photographers work.  All that you are saying by making that requirement is the film format and the image of the CoC are enlarged the same (true) and that the full photo will now fill the print (not necessarily true).

And I think you are saying the full format will never be larger than the piece of paper it is being printed on (clearly wrong).  I can certainly try to ptint a 2x enlargement of a 4x5 format film (8x10) on a 5x7 piece of paper.  I won't all of the stuff on the piece of film, but what I do get will be exactly the same as if I used an 8x10 piece of paper and the DoF will be exactly (not nearly) the same.  SAme as if I printed the film on an 8x10 piece of paper and used my paper trimmer to make a 5x7.  What is left is exactly the same as it was , but cause it is what it was before.  That is very simple and should be easy to follow.  

I do not have to make a full frame enlargement.  

And yes, it is possible that the full photo intended is too big to fit on the full frame.  Try to tell me you have never wished you had a little more of that scene.   But that does not keep me from making the same print I had planned from the captured part with the same DoF.

My example about the pigs was intended only as an example.  I do not really think pigs fly.  But if the given for the problem is pigs do fly, then the aswer will reflect that pigs do fly.  You said it would take the pig 2 hours to fly 200 miles.  It requires no trueth or faith about flying pigs.
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howiesmith
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« Reply #142 on: October 20, 2006, 05:29:31 PM »
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BJL,

I have a negative with a man's image on it.  The man's image is 1" high.  

1)  How many times must I enlarge the negative to make a print of the man where the man is 2" high?

2)  What format is the film?

Of course the answer to 1) is 2x.

The answer to 2) has to be 1xsomething because all images fill the frame.  I say wrong.  Mightn't it be possible to have a 1" high image of a man on a 645?  Or a 6x6?  Or a 4x5?  I think the only requirement is the film be at least 1" high (not only 1" high). And would the 2" high print of the man look exactly the same regardless of whether you printed a 645, 6x6 or 4x5 or even a 1xsomething?
« Last Edit: October 20, 2006, 05:31:57 PM by howiesmith » Logged
Ray
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« Reply #143 on: October 20, 2006, 06:28:30 PM »
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I say wrong.  Mightn't it be possible to have a 1" high image of a man on a 645?  Or a 6x6?  Or a 4x5?  I think the only requirement is the film be at least 1" high (not only 1" high). And would the 2" high print of the man look exactly the same regardless of whether you printed a 645, 6x6 or 4x5 or even a 1xsomething?
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I thought we'd got beyond the notion that format only refers to camera format. Cropping the image bound by the camera's format is common practice even amongst users of small cameras. To make a picture, you have to make a crop of the sene in front of you. The maximum size of that crop, with a given focal length, is the format of the camera.

It's perfectly true that a 1" high figure enlarged 2x will be 2" whatever the original format of the camera, provided the format is at least 1" high or wide. But doesn't DoF refer to the relative sharpness of different elements within a composition? Are we talking here about 'cut-outs' with no background?
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Ray
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« Reply #144 on: October 21, 2006, 10:13:56 PM »
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To expand upon my previous post, the problem with many of Howard's examples attempting to support his assertion that format is unattached to DoF, is that essentially they are all saying that A=A, or if A=B then B=A.

There's no great insight here. If I cut a print in half, then of course that half is the same as the half before it was cut, but that half is clearly not the same as the uncut whole.

As BJL has mentioned, if we are comparing the perception of DoF on 2 prints, we should keep the prints which are under comparison the same size to reduce the number of variables. We should also not vary the quality of the viewer's eyesight or the viewing distance to the 2 prints.

Howard has brought up these subjective variables time and again. They are really just a smokescreen.

I shall now attempt a definitive proof that changing format can directly affect the perception of DoF on a print when comparing equal size prints of the same subject and same FoV.

I'll use a range of f stops that are more familiar to users of 35mm, rather than 4x5 format.

I'll use Howard's example of a full height figure. Here's the scene; Howard standing in front of a huge tree with spreading canopy. The background stretches to a theoretical infinity, mountains on the horizon, other trees and tall grass in between. Howard is very close to the tree trunk, in fact leaning back on it. I have a prime lens which, from a convenient position, takes in a good portion of the scene in front of me. No need to crop smaller than the camera's format. I calculate that f4 is sufficient to get all of Howard sharp on a good size print, from the tip of his nose to the patch of ground he is standing on, to the hair at the back of his head, to the tree trunk he is leaning against. For good measure, I use f5.6, just in case I later want to make a large print of a crop.

However, I'm a bit concerned about the close foliage at the top of the frame, which at f5.6 is definitely going to be out-of-focus. The mountains and other trees in the background will also be OoF, so I take another shot at f13 to get everything looking sharp, even in a large print.

I make a 16x24 print of each shot. The one at f5.6, as expected, has a shallow DoF, but the one at f13 appears equally sharp from corner to corner. (I'm actually using the Canon TS-E 45mm which has a larger than usual image circle and produces a reasonably good result from corner to corner.)

I show Howard the 2 prints and he agrees (as anyone would) that the f13 shot has much greater DoF, but Howard doesn't like all the distracting foliage and asks me to make a crop of just him against the tree trunk, which I do, cropping to a panoramic aspect ratio of 3:1, vertically orientated.

I crop both images. I'm not sure why I bothered cropping both images, but Howard told me that cropping or changing format does not change DoF, so I expected one print might still exhibit greater DoF than the other. (Not really   ).

Lo and behold! I find that both prints, still both equal in size, now have identical DoF.

By changing just one variable, the format, I have changed the DoF on the final result, the print. Of course, by cropping I have changed the composition and FoV, but that's what cropping does when you use the same lens. It's unavoidable. The cropped print is also smaller than the uncropped print, but again, that's what cropping does.

Perhaps we could summarise this principle as follows.

If 2 prints of the same scene have equal DoF, cropping to a different format will also result in 2 prints with the same DoF. However, if 2 prints have an unequal DoF, then cropping (ie. changing format) can change that inequality.
« Last Edit: October 21, 2006, 10:59:07 PM by Ray » Logged
BJL
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« Reply #145 on: October 23, 2006, 10:52:25 AM »
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Howard,

   we agree that if you crop or enlarge suitably (in this case to what I am inclined to call a 1" high image format), you can get the same DOF with the same aperture ratio from different size pieces of film or sensors.

My statement is about a different situation, which I believe is far more common:
what happens when one roughly fills the frame with the same subject in each case (and then compare prints on which the image of the subject is the same size, viewed from the same distance). This leads to the use of different focal lengths and to different DOF with equal aperture ratio.

Or as I like saying: once you specify
- aperture size (effective aperture diameter, which is focal length divided by aperture ratio)
- focus distance
- FOV on the final print (after possible cropping)
- print size, and
- print viewing distance
then DOF is not effected by choices like format or focal length.
(But if focal length varies, aperture ratio is varied in proportion, to keep aperture size the same.)


P. S. Format does have some effect in your case, since any format with a frame less that 1" high forces the use of a shorter focal length in order to fit the man's image into the format of the frame. You come close to assuming or declaring a falsehood when you talk of having an image 1" high and yet are trying to argue that this is a format independent situation.


 and thus more DOF at equal apertuer ratio.
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BJL
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« Reply #146 on: October 23, 2006, 11:02:35 AM »
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And all I have been trying to say is it is an unnecessary given or assumption that the photo fill the frame, even if that is the way most photographers work.
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And all I am saying is what happens in that common and interesting situation, without denying for a moment that other approaches are sometime taken.

Why does this bother you, so long as what I say about that situation is true, and I make it explicit that I am referring to that situation, with no hidden assumptions?


I in turn agree that if one uses films of various different formats but then view only the part of the image recorded on a certain sized part of the film (say 1" high or 24, by 36mm), then the DOF is not affected by the format (height and width) of the whole piece of film, only by the format (height and width) of the piece of film on which the viewed image is recorded.
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