Note: The term "DOF" in the following list is an abbreviation of "depth of field".
Do I use the ACTUAL FOCAL LENGTH or the 35MM EQUIVALENT FOCAL LENGTH for depth of field calculations?
Always use the ACTUAL focal length when calculating depth of field.
Usually, the actual focal length is printed on the front of a lens' barrel. Sometimes the length is printed on a digital camera body. The actual focal lengths of many digital camera lenses are listed here.
DO NOT use the 35mm equivalent focal length for depth of field calculations.
Be sure to use the appropriate circle of confusion for your camera.
What is the circle of confusion for my lens?
See the "Circles of Confusion For Digital Cameras" page.
There is also a circle of confusion calculator on the "Circles of Confusion For Digital Cameras" page for cameras not listed there.
Suggested circles of confusion for other formats are:
Circle of Confusion
What is the meaning of circle of confusion?
Circle of confusion is difficult to explain in non-technical terms. You shouldn't worry about it if you're just beginning to learn about depth of field. The circles of confusion listed on this page are reasonable values that you can use to get reasonable estimates of depth of field.
Note that the circle of confusion is subjective. Thus, some people use 0.025 mm as the circle of confusion for 35mm format, while others will use 0.030 mm. There isn't a "correct" value. The values of circle of confusion on this website are calculated using, approximately, the circle of confusion that Canon uses for calculating the depth of field tables for their lenses (Canon Camera Museum - Len Details).
See Anthony Collins' article "Aperture and Depth of Field - how it works" for a somewhat technical explanation of circle of confusion.
How accurate are depth of field calculations?
DOF calculations are remarkably good estimates of the subjective depth of field in photographs. However, you shouldn't expect to get highly accurate results from the calculations.
To ensure that you get enough depth of field in a photograph, stop down an extra stop. For example, calculate the depth of field at f/11, but set the f-stop on the lens to f/16.
Use a tripod whenever possible to prevent camera shake. John Shaw says that "depth of field refers to the section of a photograph, from the nearest to the furthermost points from the camera, which appears to be in sharp focus." Camera shake reduces that zone of sharp focus.
What is hyperfocal distance?
See the Hyperfocal Distance Guide.
Is the 'subject distance' measured from the front of the lens or from the film plane?
This question is relevant only for close-up and macro photography. For other photography, any error caused by measuring subject distance from the wrong location is neglible.
The depth of field equations are derived from the "thin lens" equation, which assumes a single lens element with no thickness. And, subject distance is measured from the thin lens. So, technically, subject distance is measured from the front of a lens.
However, that doesn't apply directly to a photographic lens. These lenses have many elements, and the front of a lens isn't necessarily the location that subject distance is measured from. The actual location is something called the "front nodal point" of the lens. The location of the front nodal point isn't documented
by lens manufacturers, nor is it easy to find experimentally.
I measure subject distance from the front of the lens. I believe that the nodal point would actually be somewhere between the front and rear elements. But, by assuming it is at the front of the lens, I get a conservative estimate of the depth of field from the calculator. In other words, the depth of field calculation shows less depth of field than will actually be seen in the photo.
© 2003 Don Fleming. All rights reserved.