[newarts]

Here's a practical, easy to remember estimate of DOF for a Pentax Digital (APS-C) camera :

DOF% is field depth as a percentage of field width

DOF% ~ 10*F-stop*Field_width_meters

Example: a photo of a cat with a total field width of 1 meter at f/4 will have a DOF of about 40% of a meter. If you move back or zoom out so the field width is 2 meters, the DOF at f/4 would be 80% of 2 meters.
Dave

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FINE PRINT:

This is approximate of course but "good enough" for guideline estimates when the distance is more than a few focal lengths. It is based on a circle of confusion of about 0.03mm on the displayed image and standard optics theory where DOF=2CN(1+1/m)/m where m is magnification.

Field width is used rather than distance and focal length because they are mathematically related & using width reduces the number of separate things needed for the calculation.