On Brilliance, Contrast, And Signal Processing In Images

In article <01bbd1e5$0e7846a0$06d76cce@Axion.axionet.com>, goldoil@axionet.com says...

"As a newbie to photography, I've often wondered why both film and video images are different in character than the human eye. Film seems to be darker, less sparkling in quality than the human eye (except Safeway flyers and promotional calendars). Video images seem to be brighter with less contrast than the human eye. And when I see movies on TV, sometimes I see a film-like broadcast image, very dark and lacking in detail, and other times, the broadcast image is very sparkling in detail and vibrant (this tends to occur when the major networks run feature films that were recently run on theatres). Was video processing done to enhance the film print so that it looks more detailed when shown on TV? Could anyone please explain these differences? And can't they make film and video respond like the human eye? Kind thanks to anyone who can shed some light on this."

Hmmm, an excellent question, put in an unusual (but
helpful) way.... I think it has to do with what I call
"brilliance", defined as the brightness difference (or
wideness of the range) between maximum dark and light
tones in the image. Starting with the eye, due to some
signal processing in the brain, we are able to see all
at once a brightness range that can exceed a 20 stop
range while at the same time keeping good local
contrasts (for a snappy-looking, but tonally very
wide-range image). Trying to do this in photography
in a medium that places the image on a piece of paper
(which has a very limited range between maximum black
and white) results in a great narrowing of the
differences between adjacent tones (contrast) in
order to fit the wide range of tones into the limited
available span of brightness. Switching to transparency
materials extends the available brilliance somewhat,
but it allows only a slightly extended tonal range if
the contrast is kept constant (and the available
brilliance range is determined by the brightness of
the light behind the film, and by the maximum black
of the film). TV images are capable of great brilliance
and good contrast, if the TV is set up to provide it.
Slides viewed at short distances projected with high
light-level output slide projectors on a good surface
(or on a bright light table) can also provide high
brilliance images. In commercial products, like
advertising, video tapes, display photos, etc.,
manipulations are often done to enhance "snap" and
appearance of sharpness (ever notice how sharp car
commercials can look on standard-definition TV, when
the available resolution is only about 300 lines total
in the horizontal direction?!). So, the image appearance
results from the available brilliance of the medium
used, the contrast selected, the color quality
available (and used), and the "signal processing"
(non-linear manipulations) employed in making the
image.

"Hope This Helps"
David Ruether (d_ruether@hotmail.com)

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