Originally posted by MossyRocks I usually assume that 1.5 to 2 stops will produce good enough results. It doesn't hurt that the lenses I do that with are either a wide or fisheye so you could go 3 less on the gauge than the actual f-stop on the lens and still have the vast majority of the scene in perfect focus.
A reference I read quoted 1 stop as being necessary, which kind of made sense to me as the APS-C sensor is about half the size of a full-frame 35mm negative.
Which brings me to a sudden realisation !! The Industar-69 I recently acquired came from a half-frame camera, so the DOF markings should be good for an APS-C sensor and I don't need to compensate
I should point out here that this lens isn't used on a Pentax but on a mirrorless body, and needed some mechanical modification to achieve infinity focus.