Re: [Leica]Lens contrast

[John McLeod <johnmcleod@worldnet.att.net>]

Erwin -

I have a question about lens contrast.  Let's assume there are two lenses
under consideration, A and B.  For discussion, let's say they are both 50mm
lenses and I'm testing both with Kodachrome 64 under similar circumstances
(i.e. same subject, same roll of film, on a tripod, nice even overcast sky
lighting, etc.).  When I get the slides back I notice that the OVERALL slide
contrast appears to be greater with lens A.  In other words, the dark areas
of the slide appear darker with A and the light areas appear lighter.  But
let's also say that subtle tonal gradations are more easily seen in slides
from lens B. In other words, in various areas of the slide (but especially
in the mid-tones), B seems to have more micro-contrast.  While slides from A
tend to have more "punch" and overall contrast, slides from B look possibly
more "natural".

I have several questions about these two lenses, and lens in general.  Can a
lens (e.g. A) be "too contrasty" or is the appearance of "excessive"
contrast really a film contrast issue?  Second, can a lens (through coatings
or otherwise) ADD contrast to a scene or can it only let pass what light is
available to be passed to the film?  Third, can a lens have greater overall
contrast, and at the same time, lower micro-contrast, or is a high contrast
lens simply a high contrast lens, in all respects?

Thanks.

John McLeod


- ----------
From: Erwin Puts <imxputs@knoware.nl> 
To: leica-users@mejac.palo-alto.ca.us 
Subject: Re: [Leica]Lens contrast 
Date: Thu, 9 Apr 1998 17:19:54 +0200 

>That is, what are the sources of the high and low
>contrast in a lens, in terms of optical aberations?  Is there someting more
>going on then simple flare in the lens?

Yes, there is more to it (as usual). One obvious source of low contrast is
the unwanted reflection of light rays from every glass surface of a
lenselement. The more lens elements, the more reflections (non image
forming illumination). Another source is the mechanical reflection, due to
bad internal construction. A  most imporant source, hardly ever mentioned
is more difficult to explain. In general we have two different planes of
sharpness from any lensssystem: the plane where we see the highest contrast
and the plane where we see the highest resolution. In both cases only a
fraction of the infinite number of light rays emanating from the object
(point) is needed for a sharp reproduction of the object point.Then we can
use this small selection of light rays to produce a sharp image with very
small circles of of least confusion. The rest of the rays are not
eliminated, they are present in the optical system,and have to be dealt
with. One option is to produce very wide circles that do not produce any
clear images, but will be present as general illumination, contributing to
a general low contrast. Older lenses are invariably tuned to the plane of
highest resolution. Therefore the designer needs to spread the rest of the
light rays as uniformly as possible in order to enhance the resolution.
That is why older lenses with high resolution are almost always lenses with
low contrast. If you go for the high contrast plane, the desigher can
handle the rest of the rays differently. The relationship between optical
aberrations (implying low circles of least confusion), resolution and
contrast is complex. But the general low contrast of a lens depends partly
on the way the designer handles the aberrations in relation to his choices
between contrast and resolution.
Erwin