In article <4kpm0p$c5t@rebecca.albany.edu>, dr8192@CNSVAX.ALBANY.EDU says...

>In article <4kp0v4$8bp@newsstand.cit.cornell.edu>, d_ruether@hotmail.com >(Bob Neuman) writes: [......]

>>I am often down around f32-45 (working aperture, with losses due to
>>extension, converters, etc. taken into account), and still have good
>>sharpness in the images (relatively speaking!). But if you are setting
>>f32 on the 105mm macro lens, you are actually using nearer to f64 at
>>life size (smaller at larger magnification) and the results are
>>probably not very sharp. [......]

> Generally quite true. But do not be extra
> concerned that your marked f:32 is now about
> f:64 as far as loss of sharpness at smallest
> apertures.
>
> The loss is due to diffraction at the edge of
> the aperture. The amount of loss is related
> to *absolute* aperture size, not to relative
> aperture [aka f:stop]. Whatever may be the
> diffraction losses at 25ft at f:32, they are
> the same at life-size where the marked f:32
> is about f:50.
[f50 for a "zoom" type macro, f64 for a simple lens - rest deleted]

Hmmm, if the amount of resolution loss were dependent upon
absolute aperture size (and not relative aperture),
diffraction limiting of resolution should occur at very
different apertures for very different focal-length lenses
in a given format (like 15mm and 300mm for 35mm), but in
reality, diffraction effects set in at about the same
relative aperture for good lenses for a particular format
(about f8-11 for 35mm) regardless of focal-length. Also,
in your example, the film has no way (in terms of
diffraction-limited resolution, ignoring focus-distance
related problems) of "knowing" if you have a 105mm lens
focused at 25' at f32, a 105mm lens focused at 1:1 with
its marked aperture at f16 (f32 working aperture, assuming
a simple lens), or a 210mm lens focused at 25 feet at f32,
but if you change the aperture on the 1:1 focused 105mm to
f32 (f64 working aperture), there sure would be a
drop in resolution! Methinks it is relative aperture -
how about it, DJ?
Hope This Helps