Photoshop Elements /

Monitor Calibration /

Monitors are set to a gamma value.

This gamma value is a contrast curve.

The gamma contrast curve is applied to the photograph file so the monitor displays it the way we see the world.

Sensors produce files with uneven numbers of shadows, midtones, and highlights.

There are very few shadows, some midtones, and many highlights.

That's not the way we see.

Human vision has more even amounts of shadows, midtones, and highlights.

The gamma contrast curve shifts the data down from the highlights to make more shadows and midtones.

Photographic files have brightness levels.

The brightness levels are distributed over a six stop range.

1	2	3	4	5	6
Shadows		Midtones			Highlights

A JPEG file has 256 brightness levels.

Before gamma processing, these brightness levels are not distributed evenly.

There are 128 brightness levels of highlights.

We need at least a 1% difference between two adjacent brightness levels to see them as different.

In a JPEG, the 128 highlight brightness levels are so closely spaced that there's is less than a 1% difference between them.

There are many more highlight brightness levels than we can distinguish.

Before Gamma Processing

There are only four brightness levels of shadows.

There are huge differences between the four brightness levels in the shadows.

So, we have too few differences in the highlights, and too many in the shadows.

The gamma contrast curve redistributes the brightness levels to better correspond to human vision.

Your monitor displays the photograph file the way we see photographs.

Where there were 128 brightness levels of highlights, there are now 69.

There are now 14 brightness levels of shadows, instead of only four.

• The term gamma is from the exponent in the equation that "fixes" the brightness-levels range produced by sensors.

• 2.2 is the Windows default gamma setting.