# What does Gamma value mean in Photoshop/GIMP?

Discussion in 'Photoshop Tutorials' started by PGPS, Jun 6, 2007.

1. ### PGPSGuest

I used Photoshop/GIMP to improve the mid-tones of pictures. However, I
don't have access to Photoshop anymore and want to use Irfanview to do
the same thing. The Gamma value (1 to 7) does brighten the image well.

Lately I have been learning that, all along when I used brightness/
contrast slider, it was the wrong way to do good adjustment.

My questions:

1. How does gamma value improve midtones in Photoshop/GIMP?
2. What does gamma value mean exactly?
3. With respect to curves, how does the Gamma value relate to the
curvature of the curves?
4. Is doing gamma value one of the good ways of improving an image?

Thanks.

PGPS, Jun 6, 2007

2. ### Fred C.Guest

Fred C., Jun 6, 2007

3. ### aclGuest

It corresponds to a curve y=x^(1/gamma)
(x is the input value on the curves dialog, y the output), or at
least, it should! So it's equivalent to a convex (curved upwards)
curve for gamma>1.

acl, Jun 6, 2007
4. ### aclGuest

Oops, forgot to say "normalised to 1", ie x and y range from 0 to 1
(thus x=number on x-axis of curves dialog/255 etc). Sorry!

acl, Jun 6, 2007
5. ### Floyd L. DavidsonGuest

One additional comment that might help understand what
is seen in various places, is that often "gamma" is
expressed as the value for 'gamma' in the above
equation, but also it is often refered to by the
'(1/gamma)' part of that equation.

For example, the "gamma" of a standard monitor screen
for Windows is supposed to be "2.2", but that is the
'(1/gamma)' term. In GIMP the way to set that value for
gamma is to use gamma = 0.45.

(1 / .454545) = 2.2

Of course there are other even odder ways to express it!
Some programs use gamma=0.4545 as the default, and all
adjustments relate to that, with 1 being the standard
value set, less than one resulting in a lower gamma and
greater than 1 being higher gamma.

Floyd L. Davidson, Jun 6, 2007
6. ### MarvinGuest

In dealing with digital photographs, I use the definition
that is used for photographic emulsions. By convention, the
relation between the exposure, E, of a point in the image
9i.e., intensity X time) to the blackening, a, is expressed
as log(E/a). When a graph is made with E on the X-axis and
a on the Y-axis, a large portion of the graph will be
linear. Gamma is the slope of the line in that range.
Photoshop, Paint Shop Pro, and some other image editors let
you reshape the curve by changing the gamma for different
parts of the curve.

Marvin, Jun 13, 2007