Well, I can't do what I hoped. Turns out I was wrong about the bellows
manual giving DOF figures for the various macro lenses. It does have
tables with distances and magnification but no DOF numbers. So then I
looked at the lens data. The 80mm macro has no DOF numbers at all, the
50/3.5 macro had DOF numbers but they're only given to the nearest
centimeter. Below 1/4 meter camera to subject distance the DOF numbers
in centimeters are the same since DOF is less than a centimeter. Not
much help in the Oly manuals. And what else is new. :-)
Chuck Norcutt
On 12/16/2010 2:49 PM, Chuck Norcutt wrote:
> 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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