Misinformation about depth of field is rife. The problem is that the formulas, while not terribly complex, can't be summarized in simple rubrics using words. Almost all such statements are right in some circumstances and wrong in others.

I encountered this recently, and I did an analysis, which I think is correct, but I hope someone will check my calculations.

It is well known that for a fixed format and a relatively close subject, depth of field doesn't depend much on focal length, but mainly on the scale of reproduction. (Without the qualifications, this statement is definitely false.) But what happens if you fix the subject size and the image size in the final print, but vary the format. I come up with the conclusion that you get less depth of field (again with relatively close subjects) with a larger format, where it is presumed the larger format is enlarged less for a final print.

Here is the formula I came up with, and perhaps someone can check it.

Let N be the f-number, C the coc in the final print, E the degree of enlargement necessary to produce that print, and M_p the ratio of subject size to size of the image in the final print. Then I get

NC(E + M_p)/(M_p)^2

for the front and rear depth of field. C, N, and M_p are assumed fixed. So if E is smaller, you get less depth of field.

## Bookmarks