AJSJones
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Imagemaster wrote
Take a Mamiya 645 camera and take a photo. Now, without changing the lens, aperture, or lens-to-subject distance, put on a 35mm roll-film holder and take a second photo. Now take your loupe and see if you can find greater or less DOF on the 645 film. Surprise, it is the same regardless of the change in film size.
The DOF was determined by the focal length of the lens, the aperture, and the lens-to-subject distance. I could put a piece of lettuce on the film plane and it would not change the DOF just ahead of that plane. 
One last try and then I'm going to give up (sorta like evolution vs creation )
NO-ONE is saying that the piece of film cut from the 645 image is IN ANY WAY different from the 35mm film in your example above. If you think that's what they're saying, you have not been reading the posts.
[ BTW "Depth of Field" has nothing to do with the areas above or below film plane - there is another term Depth of Focus, but that is not what is under discussion here - Depth of field has to do with the plane of focus on the subject side of the lens]
Your example, however, allows me to ask you How YOU evaluate the DoF on those pieces of film? (Imagine you took a shot of a ruler on the ground in your yard  ) You could look at the film- either one will do since they are identical - on a light box on the other side of the room (not likely that's how you'd do it) or on a light box a foot away from your eye, or with a 3x loupe or a 6x or 10x loupe. Let's say you find something that is *just* slightly less sharp than the object originally focused on - for example the mark for 36 1/4" on the ruler shot , where you focused on 36", is ever-so-slightly fuzzier with the 10x loupe and the 36 1/2" mark is even fuzzier. You will not see this difference in apparent sharpness if you don't use a loupe or if you use the 3x loupe. Perhaps with the 6x you could see it. This is because of the limitations of your eye so IT MATTERS WHICH LOUPE YOU CHOOSE when you try to decide which of the marks on the ruler appears as sharp as the sharpest one. This determines how much the image is magnified - just like we've been saying it matters how big you print and how far away you view it.
It is not until you *look* at the image somehow, that the concept of DoF has any meaning at all. You cannot define DoF without saying how you are going to *look* at the image....
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