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Re: [OM] Theoretical lens resolution

Subject: Re: [OM] Theoretical lens resolution
From: "Mark Hammons" <astair@xxxxxxxxxxxxx>
Date: Sat, 26 Sep 1998 20:25:27 -0500
>>>Mark writes:
>>>
>>
>>>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.
>>
>>
>>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.   Look at some of
>>the numbers in the web pages linked to Lee Hawkins web site.  Some
>>lenses, wide open, are down around 50 lp/mm.  This doesn't seem to
>>make sense.
>>
>>
>>Mark H.
>>
>Thank you, Mark. And in the light of the usual published resolution by
>Fuji, for Velvia for instance,  of 160 lines per millimeter, it makes even
>less sense.
>
>Winsor
>
>Winsor Crosby


With the aid of John Petrush's post I think I might have this figured out.
The SYSTEM resolution (lets say in the case of slides ) is a function
of both the lens and film resolution.  If you have film resolution of
around 100 lp/mm but have a diffraction limited lens of around 500 lp/mm
(or whatever) then the resulting image will have a resolution of around
98 lp/mm or something like that.  Thus you are limited by the
coarser resolution component, be it lens or film, which of course makes
sense.

However, when we read lens tests where lp/mm numbers vary from one
lens the next or with the same lens at different apertures, the fact that it
differs CANT be attributed to the film.  Since most lens test to be less than
100lp/mm and since there are films (Velvia, at least) that have significantly
higher resolutions then we ought to be able to test lenses with the
film resolutin limit not detracting MUCH from the lenses "True" or
"Aerial" resolution.

What this means is -- as per my original post is that typicall 35mm optics
COULD theoretically be much, much better in resolution.  I believe that
one of the sharpest Zuikos is the 50mm F1.8, which if I recall is about
90 lp/mm at F5.6, or something ( too lazy to go look it up).  This is certainly
very good with respect with other lenses but it could theoretically be up
toward 220 lp/mm if it were diffraction limited.  But then, we would again
need to have film up around that area too to make use of this resolution.
By the way, do we have published figures on the resolutions of various
films?

As to some other comments made by other posters:

1) Yes, my instructor did show us how diffraction limits related to F numbers
and yes I did realize that at say F16 the diffraction limit will be 1/8th of
what it
is at F2.

2) I do realize that CONTRAST is an important consideration -- both with the
test film AND with the test target AND with the illumination of the test target.
What I don't know is what the standard lines/mm test parameters are for
lighting and refletance values of the test target -- How "black" are the black
lines and how "WHITE" are the white lines -- are they Zone 0 and Zone X as
per Ansel Adam's zone system?

3)  The class I took also finally explained to me a term which I had never
fully understood before -- "Modulation Transfer Function" or MTF.  The best
thing to really do is to measure the MTF of an optical system, as it gives you
a result of system response on the "Y" axis versus spatial frequency on the
"X" axis.  By the way, my instructor put up a slide that was used to test an
optical
system and with it you actually could see the MTF response of your eyes
( that is your own personal optical system ) by looking at what part of the
test image had a distinct "wavy" pattern and what portion was solid
and imagining a curve deliniating these 2 areas -- VERY NEAT!


Mark Hammons

P.S.  -- I can back from Oblivion because it was SOOOOO Boring.  My wife tells
me that somebody wanted my equipment -- Naughty - Naughty.

P.S.S -- Have to 'fess up about something else -- I don't have a wife.  Its a
drag
in a lot of ways but at least it makes the decision to buy toys less difficult!!




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