Mark writes:
One of the things we talked about on our last day (today)
was the diffraction limited resolution of lenses. He gave
us an equation and we worked through an example with
a 50mm F2 lens. It turns out that the resolution limit
DUE TO DIFFRACTION is on the order of about
500 line pairs per millimeter. Thats right -- 500!!!!
===============================================
at f/2, the theoretical limit is even higher than 500. When some
simplifying approximations are made to the diffraction equation,
we can get a good approximation to the diffraction limit
of a lens at a given f-stop to be about 1600/f where f is the f-stop.
some people use 1500/f and others 1800/f. but the point is the limit is
much lower at, say, f/16, than at f/2
=================================================
He then had as part of his presentation "typical" values
of various lenses. For 35mm lenses, the typical values
given were 50 to 100 lp/mm. He mentioned that there
ARE lenses in this focal length that go up to around 500 lp/mm
================================================
50-100 lp/mm is typical for _on-film_ resolution. Aerial resolution,
which could be measured by focusing the lens on a resolution target
and looking at the back of hte lens with a microscope and focusing
the microscoe on the aerial image produced by the lens, is always
much higher. Today's optics for 35mm have aerial resolution
well past 100 lp/mm. It is film that limits them to 50-100 lp/mm,
not diffraction (up to f/11). Even if you had a lens with a
500 lp/mm aerial resolution, it probably would only resolve around
150 lp/mm on film, if even that.
The diffraction limits are limits on the aerial resolution, not the
on-film resolution.
one reason teleconvertors are so useful is that the aerial resolution
is so much higher than on-film resolution. A 2x teleconvertor just magnifies
the middle half of the image to fill the frame-- that is, it takes
a 12mm x 18mm rectangular cropping in the middle of the aerial image
of the prime lens, and magnifies it 2x to produce a 24x36 image on-film.
You could of course just enlarge a cropped portion of the negative,
but this not only enlarges film grain, it also cuts the resolution to
half the on-film resolution. By enlarging a cropped portion of the
aerial image, you might be enlarging an image with 200-300 lp/mm,
which is typical of 35mm optics to get an aerial image that resolves
about 100-150lp/mm which is still enough for good on-film resolution.
The limits to resolution in 35mm is film, whereas in large format it is
diffraction because of depth of field requirements, so large format
lenses typically are the best they can be. If film were, say, 100x
fine grained and 100x sharper, then there would be no reason to have
medium format or large format, or even 35mm. We would have very lightweight
cameras with a very small format and super sharp lenses that rendered
super sharp images on small format so wide apertures could be used
without depth of field limitations, and we wouldn't need tripods
as often due to the faster speed.
j. albert
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