Re: [Leica] Mirror

["Dan Post" <dwpost@email.msn.com>]

I am not quite flat enough to pass as a mirror ( The kids say that when I am
in bed, it looks like someone parke a VW in the bed!) but I'll try!

In my reading I find that the contrast of the lens is very much tied to
flare even if that 'flare' is not the kind caused by specular light or other
point sources. ANY internal reflection of a lens can reduce the apparent
contrast, and it doesn't take much to make a significant difference.
for example, if a scene has a brightness range from( ASA 400) EV 3 to say 8,
you have a range of .5 fc to 16fc. This is a highlight that is 32 times as
bright as the shadow area.
Now, if the lens has internal reflections that add a certain amount of
scattered light to all areas, evenly distributed, say .5fc, then the areas
represented by the shadow in the image are seen by the film as 1fc
brightness, and the highlights are seen as 16.5fc of brightness.
So, instead of a lighting ratio of 32:1, the flare reduces it to about
16.25:1, as seen by the film!
So, you can see that internal reflections in the lens is very important. It
even works in the camera- later Rollei TLRs had additional light baffles
within the body to reduce internal reflections, and these cameras perform
better than the older ones with out the baffling.
This is the reason that testing the MTF of a lens is only an approximation
of the performance of the lens alone. If you bolted a fabulous lens with
fabulous MTF on the front of a camera in which you had lined the body with
aluminum foil, the performance would be terrible!
The lens, camera, film, and processing all are part of a system, and the
part of the system with the lowest contrast, and MTF determines the highest
possible contrast or MTF of the total system.
So the contrast of the lens, depending on coatings, internal contruction and
light baffling, is only as good as the camera and film ( and if negative
process) and paper used to produce the image.
Dan
- -----Original Message-----
From: Peter Goldberg <peterg@martinhamblin.com>
To: leica-users@mejac.palo-alto.ca.us <leica-users@mejac.palo-alto.ca.us>
Date: Wednesday, February 17, 1999 10:55 AM
Subject: [Leica] Mirror


>mirror on the wall, please explain to me what is contrast, in the
>context of lens performance.  As in 35mm 1.4 non-ASPH "a low contrast
>lens", etc.  I need to know how this concept differs from contrast in
>the usually accepted sense as in emulsions, processing & printing.
>
>NB: The Focal Press Guide to Lens & Optics declines to list this
>parameter of performance.
>
>