I've never actually read anything on that subject that I can recall but
I strongly suspect that DOF is increased to the effective rather than
marked aperture. I based that statement on the geometrical nature of
DOF which is based on the angle of convergence to the focus point and
the diameter of the light beam as it crosses the image plane. If the
diameter of the beam at that point is less than or equal to the CoC the
point is in focus. With an extended image distance I assumed that the
angles of light beams would be smaller and DOF enhanced (at least over
the marked aperture)
But then I went to Wiki looking for some support for my supposition and
see that it's far more complicated than I thought. For macro work it
says that DOF is better thought of in terms of magnification and focal
ratio which then makes DOF (almost) independent of focal length. I'll
have to study up on that. There's too much math there for my feeble
mind to tackle right now. :-)
From checking the Oly bellows manual yesterday I recall that it gives
DOF for the macro lenses at various magnifications. Since they do give
their assumed CoC it would be interesting to compare the numbers in the
bellows table with those from a conventional DoF calculator and see how
much difference there is.
Chuck Norcutt
On 12/16/2010 12:30 PM, Jan Steinman wrote:
>> From: Chuck Norcutt<chucknorcutt@xxxxxxxxxxxxxxxx>
>>
>> ... exposure adjustments for magnification when shooting macro...
>> The effective aperture is reduced and the image becomes dimmer.
>
> Ah, but does the resulting "effective aperture" effect
> depth-of-field, or is DOF still strictly the result of reproduction
> factor and true focal ratio (as opposed to light transmission)?
>
> I suspect the answer is "no, DOF is independent of extension light
> fall-off" just as slapping a neutral density filter on the end
> doesn't increase DOF.
>
> I've often wondered briefly about that, but I've been too lazy to
> fully research it and puzzle it through.
>
> ---------------- An American is a person who demonstrates against a
> new power plant, then goes home and flips on all the lights, turns up
> the air conditioner, puts a tape in the stereo, opens the
> refrigerator door, plugs in the coffee maker and sits down to see if
> the television cameras caught him protesting. -- Wendell Trogdon ::::
> Jan Steinman, EcoReality Co-op ::::
>
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