Basically DOF depends on COC, magnification, and aperture (not f-stop). If you decide to use the same size final image, say an 8x10, then COC and overall magnification become constants. So the DOF depends entirely on the apperture. D=f/N where f = focal length, N = f-stop, and D = diameter of the aperture. Notice that f and N are used only to determine D, they in themselves have nothing to do with DOF.
From the above it is obvious that the smaller format will have greater DOF simply because the aperture at a given f-stop is smaller.
For what it is worth, subject-distance, focal-length, and enlargement-factor determine magnification. These factors are why those numbers are involved in the formulas you usually see. COC if you are talking about an 8x10, or 8x12, print is a constant. Why someone would want to know the DOF on the negative is beyond me unless they are only making contact prints.
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Jens Bladt wrote:
Some of you people are very knowledgeable when it comes to optical science. So, I would like to ask you this:
On the internet there is an ongoing discussion about this subeject. Some say, that smaller formats have greater DOF (Photonet). They say that in order to get comparable images, I must use shorter focal length to go with the smaller format, thus achieving greater DOF. They are using circle of confusion (COF) theories to support their point of view.
I (and Photozone) say, that smaller formats only show a part of the image, captured by a specific focal lenght. If I shoot the same scene twice with the same camera, same lens (focal length( and same aperture and focus point, you will get identical images on let's say APS and 35mm film - that is for the part, that is covered by the smaller format (e.i. APS). I say that the DOF of these two identical images - is exactly the same. I say that focal length, aperure and focal distance determins the DOF.
IMO COF theories are somewhat subjective, because the point to where a point looks like a disc, depends on the degree of enlargement. I think that the smaller image, captured by a shorter focal length needs more enlargement, thus less appearing less sharp.
What is right and wrong here?
Jens Bladt mailto:[EMAIL PROTECTED] http://hjem.get2net.dk/bladt
-- graywolf http://graywolfphoto.com
"You might as well accept people as they are, you are not going to be able to change them anyway."
