What is the formula for caluclating the depth of field for a given lens focused at a given distance at a given aperture? I don't mean the hyperfocal distance. I need to be able to figure d-o-f for macro work as well as for work at more normal subject distances. I now I've seen this somewhere, maybe in of of Adams' books, but I haven't been able to find it. I also recall encountering a web page feature somewhere that would calcucate d-o-f hwne you plugged in lens, aperture and distance data but I can't find that either. Thanks.

-- Dennis Walsh (dewalsh@ksbe.edu), July 15, 1998

Answers

where:

h=hyperfocal U=object distance

Near limit (Dn) = hxU/h+U Far limit (Df) = hxU/h-U DOF = Df-Dn

--Stroebel, "View Camera Technique" pp. 137-138.

Some URLs related to DOF:

http://www.worldphoto.com/depthof.htm

http://www.dof.pcraft.com/dof.cgi

Hope it helps.

-- Gordon Vickrey (krmhlz@earthlink.net), July 16, 1998.

In the quoted source, Stroebel is a bit naughty in not mentioning that these formulae are approximations. They are increasingly inaccurate for closer distances. Dennis's question was for macro, where a more suitable equation (also approximate) is:

T = 2.C.N.(1+m)/(m*m)

where: T is the total DoF, C is the diameter of the circle of confusion, N is the f-number, and m is the magnification. The DoF can be assumed to be symmetrical, in other words you have T/2 in focus in front of the sharp point, and T/2 behind it. [Focal Encyclopedia.]

Accurate formulae for distances to near and far points, suitable for any distances, are:

Df = (v - C.N').f / (v - C.N' - f)

Dn = (v + C.N').f / (v + C.N' - f)

where v is the image distance, N' is the effective aperture and f is the focal length.

-- Alan Gibson (gibson.al@mail.dec.com), July 20, 1998.