Originally posted by jatrax Most of what I understand came from reading various posts on this forum. For example read post #4 on this thread:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/183231-how-use...cale-lens.html Sadly RioRico does not seem to be with us any longer so I cannot ask him.
I wonder if the difference is only that when DOF scales were put on lenses, printed output was smaller and therefore the acceptable degree of focus was not as critical? With everyone looking at things 1:1 on big monitors maybe the scale is not as relevant anymore? Just thinking out loud. I would really like to understand how to better use those scales on my lenses with APS-C.
Sensor size does not affect DOF directly. A lens stopped down to a particular aperture will always produce the same DOF, no matter what size sensor is used.
There is a pretty good two-page discussion of DOF on the Canon Europe website:
Canon Professional Network - Depth-of-field
The criteria used for DOF scales are somewhat arbitrary. Best approach is to determine through experimentation what you consider acceptable sharpness, and what apertures give you that sharpness at appropriate shooting distances. Compare your own results with the DOF scales on your lenses and develop a personal fudge factor if necessary.
For example, for FF I tend to close down one more stop than recommended by DOF scales. For APS-C, I use the scales as-is.
In this case I think it is much wiser to simply take a bunch of pictures at different apertures and focal lengths and draw your own conclusions than to agonize over theories.