Re: Contrast and Realists

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Wed, 25 Sep 1996 19:52:46 -0500
>From: "P3D  Dr. George A. Themelis"  <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
>Subject: Re: Contrast and Realists

>I had similar experiences with *clear* sunny days in Cleveland.  BTW, have
>you noticed that some "sunny days" are different that others?  I fell in
>love with Velvia when I used in Greece one summer.  It was sunny but also
>hazy.  There was plenty of light around but no distinct shadows.  Velvia
>performed miracles under this light.

One thing that surprised me when I started photography was the discovery that
the darkest skies (relative to the brightness of objects on the ground) are
found on the clearest days. On a very clear day it's usually easy to get a good
picture including both ground and sky - on a hazy or cloudy day the sky tends
to be too bright relative to the ground.

>>I think any transluscent material such as oil or vaseline might affect
>>sharpness by altering the wavelength. 

Actually, it alters the bidirectional transmission distribution function
(BTDF).    :-)

>You are right.  What I said was obviously wrong.  The dirty filter will
>reduce resolution.  I think that a picture fogged or diffused this way
>still looks natural and very acceptable, unlike a picture that is clearly
>out of focus.

It would be interesting if someone could come up with a "working definition"
of 'sharpness' and 'resolution' for the purpose of this discussion - is there
a rec.photo FAQ on line anywhere?

Intuitively, a "hazy" filter should affect the modulation transfer function
(MTF) of the optical system across the picture, but the effect should be
most noticeable near sharp boundaries with high contrast. It would add light
to a shadowed area mostly around the outer edges, with the center part of the
shadow relatively unaffected. Brighter areas would be somewhat darker around
the edges than in the centers of the bright areas, but this might not be
very noticeable to humans because of the relatively low sensitivity to
transitions of low spatial frequency.

Some of the posts seem to indicate that just adding uniform exposure to
film (i.e. shining a dim flashlight on the film) will add detail to the
shadowed areas of the image. I can see how it would reduce overall contrast,
but I don't see how it would actually add detail to the darker parts of the
picture. Could anyone take a shot at describing the mechanism? Perhaps it
boosts the total exposure of the film in these areas up to a level where the
response curve has a better shape? Could it be just a matter of human
perception when viewing the result?

I would think that deliberately adding haze/scattering would be much trickier
in 3D than in 2D because of the the challenge of getting a close match between
the two optical paths. (Stretching one nylon stocking across both lenses, with
uniform tension on the stocking might work fairly well.) Other special effects
would be even harder - I've had accidental star effects (caused by the shape
of the lens apertures in extremely bright light) that matched very well, but
getting proper alignment with add-on filters could be a problem. Has anyone
had any luck with filter-related special effects in 3D?

John R


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