>The equation accounts for it perfectly... it's just that you have to
>compute the effective focal ratio. Not hard actually if you refer to
>your macro lens's exposure table. If it says to increase the exposure
>by a factor of two at a given extension then the effective f-stop is
one
>stop smaller than indicated. If it's reading f/8 then it's actually
f/11.
>Chuck Norcutt
Jim
Chuck wanted to keep things nice and easy to demonstrate the principle
and prolly didn't wish to rouse Dr. Diffraction from his
usual aestivation. He usually appears in the depth of cold dark
winters when we have more detailed discussions of
effective apertures and DOF.
Yes it is the effective aperture not the lens setting that determines
dof. A complicating factor that Dr. Diffraction
has discussed in the past to some degree is pupillary magnification.
Effective F-Stop = F-Stop x (1 + Magnification / Pupil Magnification)
Thus
Total DoF = 2NC(1 + M/P) / [ M^2 - (CN/f)^2 ] (thin lens approximation
and of course ignores very complex full wave solutions)
where: N = aperture, C = circle of confusion diameter, M = subject
magnification f = focal length
Assuming a symmetric lens P=1 and thus going from infinity to 1:1, it
is easy---lose 2 stops.
Most telephotos the P is <1 and thus actually "gains" dof at cost of
more light lost. Retrofocus lenses usually have P>1.
I believe the P may change with mag for a given lens as well. Then
there is the focal length shortening that often occurs as mag increases
in many lenses
as well as other floating element shenanigans. Big Foot loses about
1/2 stop less than it should at 1:1 with its internal machinations
compared to just adding extension.
I have never come across any formulas of just how that works and
didn't quite believe John Shaw until I empirically proved it to be
correct.
In any event don't be too afraid of some diffraction if you really need
dof. If you develop a liking to focus stacking, by all means avoid all
diffraction.
Oh, here is an OK link I think is worth a quick look:
http://www.cambridgeincolour.com/tutorials/macro-lenses.htm
IIRC, many Nykon cams actually display the effective aperture--not the
case with any of mine though it really doesn't matter all that much
from a practical
POV.
Uh Oh, I might have awoken Dr. D, Mike
--
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