Hyperfocal and Zone Focusing with APS-C Digital SLR · Заметки

This is a re-post on my message to Z.G.C. regarding use of DoF scale on prime lenses for manual focusing on APS-C DSLR. The discussion was about focusing M42 lenses on Canon EOS 300D (6mp), but of course the same logic can be applied to any other camera.

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...Standard DOF scale is based on assumption of CoC<=1/1800 of 35mm frame diagonal, if I remember correctly. This is based on idea that normal human eye can't distinguish smaller CoC when looking at print from normal viewing distance (far enough). This doesn't take into account that digital images are often viewed on computer screens with huge enlargement, or that you might want to crop a little part of frame for printing, and etc.

For APS-C camera you have to remember that your frame is 1.6 times smaller (and thus has to be enlarged 1.6 times more to get the same size print), so you need 1.6 times smaller CoC to achieve the same DoF. Half this number if you want pixel-per-pixel sharpness, since you have 6mp sensor and diagonal is 3692 pixel widths, about twice more than 1800. Now some calculations via fcalc:

1) DoF for f=50mm, aperture=f/1.8, focus distance=2.5m
Standard DoF for 35mm film: 2.36-2.65 (29 cm)
Standard DoF for APS: 2.39-2.62 (22 cm)
For "per pixel sharpness" on 6mp APS/C (e.g. 300D), first we need to calculate CoC which would give it to us. Frame width is 22.7mm or 3072 pixels => acceptable CoC=0.007389.
According to fcalc, "perfectly sharp" DoF would be: 2.46-2.53 (6.5 cm)
Now we further close aperture to achieve results same DoF as 35mm or get per-pixel-sharpness in the same field:
APS-C: f/2.4 (~1 stop)
Per pixel: f/8 (~3 stops)

2) Hyperfocal distance for f=50mm, f/8
Standard CoC, 35mm film: 9.47m
Standard CoC, APS: 12.5m
300D, with CoC for per pixel sharpness: 42m
Now we take 300D and further close aperture to achieve hyperfocal distance close to what we get on 35mm film, or to get per-pixel sharpness in the same field:
Standard CoC, APS-C: f/11 (~1 stop)
Per pixels sharpness: f/32 (~3 stops)

So, you can use DoF scale on your lens, but close it by 1 stop more to achieve the same result as you would get with 35mm film or by 3 stops more to get close to best possible sharpness...