"M Barnes" wrote:
Gisle Hannemyr wrote:
M Barnes writes:

Anyway, isn't the circle of confusion a characteristic
of the lens, not the sensor?

No - it is proportional to sensor size.

Hm. Okay, I think I get it. How is the CoC computed
for lenses then? And for film body/lens combinations?

It's computed for a given print size assuming a given viewing distance and a
viewer with typical visual acuity. DOF is a perceptual phenomenon in the
viewer.

Also, I believe the OP was asking if it's required to adjust the "f"
in your formula due to the _apparent_ focal length adjustments
required by fractional-frame sensor form factors,

I don't think that was what the OP asked - but if it was, then the
answer is "no". "Adjusting" the focal length to 35 mm equivalent
will result in a wrong answer.

But the crop factor enters in to the formula. You need to use the
correct CoC for the 300D. Which - has it happens - shrink
compared to 35 mm full frame with an amount equal to the crop
factor: 0.025/1.6 = 0.016.

Is it "as it happens" or is it "as a function of?"

I'm not trying to be snarky here, honestly. I have a D100
(that I haven't used all that much yet) body to replace my
N2020, and usehyperfocal focusing a lot in landscape shots
to keep the foreground sharp.

I'm used to using the lens set I have in a certain way,
and am wondering if I need to adjust for the D100,
which of course has the "crop factor" you speak of.

To make a long story short, you do have to adjust. A 24mm lens will have
less DOF when used as a 36mm lens on a D100 than when used as a 24mm lens on
an F100, but will have more DOF when used as a 36mm lens on a D100 than a
35mm lens on an F100.

The best thing to do is to test. For each focal length, for each f stop,
shoot a series of images of a scene with both near and far detail at various
distance settings on the lens starting at infinity. For each focal length/f
stop combination, record the closest distance that you find the detail at
infinity to remain acceptable.

Since it's digital, all it costs is your time. And even then, not much of
that since you don't have to scan.

David J. Littleboy
Tokyo, Japan