How to determine DOF

Started Mar 18, 2013 | Questions thread
wfektar
Contributing MemberPosts: 689
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Re: How to determine DOF
In reply to Draek, Mar 19, 2013

Draek wrote:

wfektar wrote:

Anyway, I figured I'd look at DOF. Intuitively you'd expect DOF to follow the same trend as exposure and that the CV would have shallower DOF at f/2 than the OM at f/1.8 (hence the possibility of bias in the top image). In fact, the CV should have the same DOF at f/2.3 or so as the OM wide open.

Actually, no. The difference you're seeing is due to T-stops, which aren't due to the *aperture* being less than nominal, but losing part of the light on internal reflections and etc. You could think of them as a built-in ND filter that acts only when near wide-open; DOF stays the same, but your shutter speed decreases relative to what you would've expected.

Differential T-stop could conceivably answer the exposure issue, at least for the CV (though that remains to be demonstrated -- a digital camera is the wrong tool for the job here). But I've had the OM since OM1 and OM4 days and it certainly didn't behave that way on film. Conceivably the lens is getting older and losing transmission (though that's not obvious) but that would affect exposure throughout the f-number range.

IOW: how do you measure DOF, without painstaking micrometric measurements on Airy discs and distances to precisions I probably can't replicate? That seems the only real way to deal with OOF rendering differences.

You shoot a ruler and count the distance between points that look "sharp" to you. No, it's hardly scientific, but then neither is it the concept of DOF itself.

True that. But it could be rigorous from an Airy disc dimension perspective. And it certainly seems that DOF is shallower for the lens that's stopped down a bit more.

None of this is particularly relevant photographically, of course, it's more in the way of figuring out what end's up.

Also, how do you measure actual f-numbers? Focal length I can manage (bellows plus target the size of the sensor, unless someone has a better way) -- is the entrance pupil measurement just a matter of taking calipers and measuring the image of the aperture?

Yup but, as I explained above, I doubt you'll see much difference anyways; the differences you're seeing are not due to the aperture itself.

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