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Gamma Explained

Sensor "Vision"

Versus Human Vision

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

Highlights

Many

Midtones

Some

Shadows

Few

Sensor Vision

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.

Highlights

Equal Amount

Midtones

Equal Amount

Shadows

Equal Amount

Human Vision

Here's another way of looking at sensor vision and human vision.

Linear Versus Logarithmic

Sensor Vision

Sensors record data as a linear curve.

The curve is a straight line.

With a sensor, if the light doubles in brightness, it's recorded as double in brightness.

Human Vision

Our vision is logarithmic, a curved curve.

Let's say the sun emerges through a cloud.

The sunlight doubles in brightness.

We see an increase in brightness.

But, we see the increase as being less than double.

When we press the shutter release, we record our logarithmic vision as linear vision.

When we display a linear photograph file, we must change the sensors' linear vision back to human logarithmic vision.

Logarithmic Vision (Looking at the scene with our eyes.) →

Linear Recording by the Camera →

Logarithmic Vision (Looking at the monitor.)

Gamma Contrast Curve

Monitors apply a gamma contrast curve to the photograph file data.

The straight line of the data is curved to match human vision.

This gamma processing shifts the data down from the highlights to make more midtones and shadows.

The next section goes into more detail.

Brightness Levels

Photographic files have brightness levels.

The brightness levels are distributed over about a six stop range.

1 2 3 4 5 6
Shadows   Midtones   Highlights

Distribution of the Brightness Levels

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.

Below, the red numbers are the number of stops in a photograph file.

The blue numbers are the quantity of brightness levels.

1 2 3 4 5 6
Shadows   Midtones   Highlights
4 8 16 32 62 128

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 many un-seeable differences in the highlights.

• Huge differences in the shadows.

Redistribution of the Brightness Levels

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.

1 2 3 4 5 6
Shadows   Midtones   Highlights
14 20 27 37 50 69

Why?

Why are most of the brightness levels placed in the highlight areas?

Go to Why?.

By the Way

• 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.

• Film records the world more like the way we see, than do sensors.

Below, whereas a sensor records data as a straight-line curve, film does so with a curved curve.

The x axis, Log Exposure, goes from shadows to highlights.

The y axis, Density, is the darkness of the image on the film.

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