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Re: [Rollei] Define "Contrast" and "Sharpness"

At 10:38 AM 02/15/2002 +0100, you wrote:
>yes, a tautology is something tautological!
>Id rather say:
>contrast means measuring how dark(-grey) black subjects/areas) will be
>and how light(grey) white areas/subjects will be reproduced, (good
contrast lense will
>reproduce black as black, not dark-/medium-grey, and white as white not
>and sharpness/resolution refers to the finest pattern reproduceable
disregarding the fact,
>that black has turned into 49.5%reflection and white has turned into 50.5%
>the finer the pattern got.
>The rest in fact is best seen in examples as in Zeiss's propaganda pamphlets.
>Dirk-Roger Schmitt wrote:
>> o.k.
>> very easy.
>> You just need a graph of contrast vs spatial frequency or vs. spatial
>> wavelength for each lens to discuss differences.
>> That is the only way.
>> At 23:49 14.02.2002 -0800, you wrote:
>> >Okay all you optics nerds. Here yer chance to show some of us lesser
>> >scientific underlings a thing or two. My question is this-
>> >    What exactly is contrast,  and sharpness. Explain how they relate to
>> > each other in the context of  a discussion of a lens - (example - the
>> > differenc ebetween a id 50's Tessar, and a F series Type 4 Planar) -
>> >Roberto
>> >
  The spacial frequency or modulation transfer curves are important in
showing the differenc. A high contrast lens has good edge contrast, a very
similar effect to what is called acutance in film. A high resolution lens
may have lower edge contrast. Visually, the high contrast lens will look
sharper even though it will have lower resolution. 
  The high contrast lens will typically have an MTF curve which is fairly
flat up to some value of resolution and falls off quickly above that. The
high resolution lens may start falling off sooner but will fall off at a
lower rate so that, at some high value of resolution, it has greater
contrast than the high contrast lens.
  The energy distribution of the lens in the form of a strehl ratio, or
what in German is Light Mountain, shows the effect also. The high
resolution lens will have a narrow main peak but many side peaks. The high
contrast lens is apodized so that the main peak is somewhat wider but there
are no side peaks. This is probably a better measure of the effect than the
MTF curve. 
  The physical cause is the balance of diffraction and aberrations in the
lens. The spherical aberration curve with respect to image height can be
varied by the designer. It affects the maximum blur spot size. The
adjustment for minimum blur spot may result in zonal spherical. 
  Supposedly Zeiss and Leitz had different opinions as to what was the best
characteristic for pictorial photographic lenses. Also supposedly, Japanese
designers tend to compromise toward higher edge contrast and German
designers go for higher resolution. One would have to measure a number of
lenses to tell if any of this is true. 
  This is not new stuff. Kingslake illustrates it in a book on lenses he
wrote in 1948. _Lenses in Photography_ either edition, Rudolf Kingslake. 
  Generally, the better corrected a lens the better its contrast whatever
the compromise between contrast and resolution. 
- ----
Richard Knoppow
Los Angeles, CA, USA