RolandRB wrote:


No, you can pick a closer target so long as it stays in the same
place. The 135mm lens would be a good choice as it would amplify any
incorrect focussing. Using a 50mm or a 35mm lens from a 35mm camera as
a loupe should be adequate. It would be worth using the same setup
again to check that the focussing coincides.

Pulled out my sums:

Depth of focus = 2 x circle of confusion x aperture

From this, with a 0.06mm circle of confusion we get:

f D o Focus (mm)
2.8 0.34
3.5 0.42
4.0 0.48
5.6 0.67
8.0 0.96
11.0 1.32
16 1.92
22 2.64
32 3.84

Depth of focus does not, surprisingly to me, depend on lens focal length.


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