Re: brightness vs gamma

mark.thomas.7@xxxxxxxxx wrote:
I second Bucky's comments.

*Step away from the brightness control!!*

Use Gamma, and once it looks about right, maybe a tiny tweak of
contrast. Brightness should be avoided except on very dark images
where Gamma just doesn't quite get there... Brightness adjustments can
too easily cause clipping of highlights (ie 'blown' whites).

Play with the two controls, and run them up to extremes to get a good
idea of what they can do.. You'll see a dramatic difference, I think..

They do *different* things, and each should be used appropriately.
I cannot imagine using only gamma adjustment, and not "brightness".

You trimmed the significant (and very well stated) part of Bucky's
article:

Basically, gamma changes the "bend" of the curve. Contrast
changes the slope. Brightness shifts the curve up or down.

If either the slope or the vertical position of the curve is not
correct, adjusting gamma is simply *never* going to be the right
adjustment to make.

Generally it requires, or at least is much easier with, a
histogram for reference when making the adjustments. It is also
very nice to have a display indicating which image pixels are at
maximum or minimum values (for example, flashing to black at
maximum and flashing to white a minimum). That allows you to
know exactly which areas of the image are being clipped.

Basically, the *first* adjustment should be the "brightness",
which commonly would be used to set highlights to be just barely
below the maximum brightness value. Then one would probably
want to set the slope (contrast) to have the lowest values at
"black level" (what that is depends very much on the contrast of
the display device). Then the "bend" of the curve can be
adjusted, though it might be simply "gamma" which compresses
tonal range by applying an exponantial curve, or it might also
be almost any function(x,y), or it might be a manually set curve
that does not follow any specific function.

Ideally, when done in that order there is less influence on the
first (brightness) adjustment as the last ("bend") adjustment is
made. In fact though, there is often interaction, and it may be
necessary to repeat the process multiple times.

--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@xxxxxxxxxx
.

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