how to correct image gamma to a desired value in PS

[steven_woody]

hi,

 

in PS, does the gamma correcting mean play with the middle slider in the level

tool? if i have a linear tiff file without gamma correcting ( maybe i should

say the gamma is 1 ) and want to correct it to a gamma value of 2.5, what should

i do in the level tool?

 

thanks.

 

-

woody

[colinsouthern]

Image -> Adjustments -> Exposure.

[steven_woody]

thanks. but what exact value should i apply? 1/2.5 = 0.4? and, i also noticed the Image->Adjustments-Exposure->Gamma seems identical to the middle slide in the Level diagram, except the value inversed. am i right?

[teeuwen]

The behavior of the middle slider in the levels dialog is inversed.

 

To apply 'gamma encoding/correction' to a linear file you should use the inverse of gamma 2.2 being 1/2.2=0.45. As the scale in levels dialog is inversed you should use 2.2

[digitaldog]

That slider in Levels may be called a gamma slider but that's not what it's doing (it doesn't

follow the gamma formula). I also don't know why you'd want to apply some arbitrary gamma

value on a linear encoded image. Apply a tone curve that makes the image appears as you

want it to (unless you're doing some science).

[teeuwen]

> but that's not what it's doing (it doesn't follow the gamma formula).

 

See "Note on Photoshop Gamma" on pages 18 and 19 of David Dunthorn's "Maintaining Color Integrity in Digital Photography":

 

http://www.c-f-systems.com/Docs/ColorIntegrityCFS-243.pdf

[bernardwest]

My understanding of it is this (hopefully Andrew can correct me if and where I am wrong):

 

The gamma correction is normally applied before white balance adjustment is applied in the raw conversion. If you try and apply the gamma yourself to a linear raw converted file it will be different as when done by the raw converter, as one or more channels may clip in the white balance correction.

 

Also, the destination colour space will determine how the gamma is applied. For RGB it isn't a true gamma curve. There is some extra voodoo going on in parts of the curve (particularly shadows I think).