Gamma vs Contrast

[CSULB71]

With most photo and video enhancement programs, is a 10% reduction in "gamma" equal to a 10% increase in "contrast"?

[jagabo]

No.

[edDV]

No. Gamma is a non-linear function weighing light to dark.

[edDV]

See http://en.wikipedia.org/wiki/Gamma_correction

Then ask miore.

[jagabo]

evenly spaced shades of gray and a graph:


contrast increased (darks darker, brights brighter, blown out at both ends):


gamma increased (dark lighter but black still back, lights lighter but not blown out):


Some programs use the term contrast to mean gain (black still back, everything else lighter, brights blown out):


Gain and contrast are basically the same thing except gain stretches intensities from the bottom, contrast stretches intensities from the middle.

[CSULB71]

Ouch -- that Wikipedia info melted my brain!
I was curious to ask because years ago somebody told me, as an "easy" explanation, that gamma affects the mid-tones much more than the brights and darks, so if you reduce gamma by x%, it has the visual effect of increasing the brights and darkening the darks relative to the midtones much like increasing the contrast by approx. the same percent. I see now that it is much more complicated than that. Thanks.

[jagabo]

Increase gamma to bring out shadow detail. Decrease gamma to bring out bright detail.

[edDV]

A gamma only adjustment shouldn't affect peak white or black.

Brightness will move the whole waveform up and down. It is used to set black level.

Contrast (aka luma gain) will widen the difference between black to peak white. Too much contrast will crush blacks and saturate whites.

On properly designed equipment these adjustments are independent but in the real world of low end consumer equipment, there is interaction.

[lordsmurf]

"gamma affects the mid-tones much more than the brights and darks" = true.
But I think you've interpreted it incorrectly.

[CSULB71]

I didn't think it was that simple, as "explained" to me years ago, so I thought I'd ask and see what some experts would say.