Mark writes:
===================================
>The diffraction limits are limits on the aerial resolution, not the
>on-film resolution.
I hadn't heard of this before.
If you are limited by the film to be about 100 lp/mm resolution when
in fact most 35mm lenses are far above this, then why are ANY lenses
quoted in having less than 100 lp/mm resolution.
===================================
That's because of the way multiple systems interact. If you have a lens
focused perfectly on the film without vibrations or other interference,
so you are only seeing the effects of lens aerial resolution and
film resolution, the combined resolution on-film is approximated
by a Gaussian error accumulation, or:
1/R = 1/Rf + 1/Rl
where R = resulting on-film resolution
Rf = on-film resolution
Rl = aerial resolution of the lens
note that this is an approximation only, but is representative of what
is going on.
So, if a lens has aerial resolution of 300 lp/mm, and a film can resolve
up to 100 lp/mm you might see a combined on-film resolution when the
two are used together of about:
R = 1/(1/300 + 1/100) = 75 lp/mm on-film
But if the lens only resolved 200 lp/mm in its aerial image, then the
on-film resolution would be approximately:
R = 1/(1/200 + 1/100) = 67 lp/mm
Keep in mind this is just a reasonable approximation and also assumes a
perfect world of no shutter vibration, perfectly accurate focus etc.
Of course, the real world isn't quite that friendly, and this will reduce
the on-film resolution a bit more. It is quite tough to get 100 lp/mm
on film because there isn't much head room in the maximum resolution
of the film for the other errors to accumulate in without lowering
the numbers, hence we see 50-100 lp/mm as typical on-film resolutions.
Most films are up in the 125-160 lp/mm range for high contrast subject
matter, and down in the 50-80 lp/mm for low contrast subject matter,
by the way.
Too see why it isn't worth the extra money normally to build a lens with
an aerial resolution of 500 lp/mm, let's do the calculation for that assuming
a film capable of 100 lp/mm:
R = 1/(1/500 + 1/100) = 83 lp/mm.
So, (and again this is just a back of the envelope approximation, but it
is a good rule of thumb or guideline) going from a 300 lp/mm aerial
resolution to 500 lp/mm aerial resolution the on-film resolution
might only jump from 75 lp/mm to 83 lp/mm, about a 10 0ncrease
(ie you can enlarge 10% more) but the price of the lens might go
up 4x or more, hence would not be a very profitable venture.
hope that helps.
Joseph Albert
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