The format size determines how much of the tree is on the negative, and that has nothing to do with DoF. If I trim an 8x10 neagtive to a 4x5, do I change anything on the negative? Of course not. [a href=\"index.php?act=findpost&pid=81072\"][{POST_SNAPBACK}][/a]
This is the sort of reasoning that I find bizarre. It's almost like saying, 'if I chop off an arm or a couple of feet, do I change your appearance'.
If you trim the negative before making a print, you have definitely changed the negative. You've removed some of it. If you make 2 prints of equal size, one from the trimmed negative and one from the full negative, you are quite likey going to have prints with different DoFs. Even if you make different size prints by enlarging the elements that are common to both images by the same degree, you can still have the perception of different DoFs, even though the DoF of the elements that are common to both prints is the same.
For example, if the full shot of the tree contains some buttresses (above ground roots) which are out-of-focus because I made a mistake with my DoF calculations or adjustments, I might decide to crop that part of the negative. (We all know that stuff in the foreground that's OoF is often not acceptable.) By cropping away the 'mistakes', I create a different composition, perhaps as good, perhaps not as good, but one which now has the
correct or acceptable DoF.
On the other hand, the reason for cropping might not be to correct a mistake. If I have a fixed format camera, a prime lens with a fixed aperture and not much room to change my distance to the tree, I might well make the choice to sacrifice part of the negative, or use a smaller format (a film holder with a 6x7cm piece of film, if I have one).
To put it another way, from the perspective of the viewer of a print, DoF is correct, satisfacory, acceptable or not, as the case may be,
only in relation to the format used. Changing format does not change the hypothetical range of distances where elements will be OoF by the specified degree in accordance with the rules of DoF
but it does make the settings of f stop, distance to subject and choice of focal length either appropriate or not appropriate for the composition.
It's my view that the confusion in this thread results from a failure to distinguish clearly between the perception of DoF on the finished print and the hypothetical, imaginary DoF described by the mathematics, which of course must remain imaginary. I don't believe I have ever captured infinity on sensor or film, yet the DoF tables will often tell me that everything is sharp from x feet to infinity.