Contrast (vision)

Contrast is the difference in luminance or color that makes an object distinguishable from its background. In vision, this difference allows us to perceive shapes, edges, and textures. Human vision is actually more sensitive to relative differences than to absolute luminance. This means our appearance of the world stays fairly stable even when lighting changes significantly. The maximum contrast possible in an image is called the contrast ratio, or the dynamic range.

When an image uses nearly all its available contrast, it reaches a state of contrast conservation. In this state, increasing contrast in one area must result in a decrease elsewhere.

Adjusting the brightness of an image acts like a balance. Brightening an image increases contrast in darker areas, but it decreases contrast in brighter areas. Darkening an image has the exact opposite effect. There is also a specific process called bleach bypass. This technique reduces contrast in the darkest and brightest parts of an image. Instead, it enhances the luminance contrast in areas of intermediate brightness.

This shows how light and dark levels are mathematically linked within a single image.

Scientists use several different formulas to quantify contrast. One method is Weber contrast, which is often called the Weber fraction. This is used when small features sit on a large, uniform background. Another method is Michelson contrast, also known as visibility. This is used for patterns where bright and dark features are roughly equal in size. It is very effective for periodic functions, such as sine-wave gratings.

There is also RMS contrast, which measures the standard deviation of pixel intensities. Because there are so many ways to define contrast, some scientists find the variety inconvenient for comparing research results.

Biological contrast sensitivity describes how well the human visual system discerns different luminances. In 1968, researchers Campbell and Robson discovered that human sensitivity follows a specific shape. They found it acts like a band-pass filter, which means it peaks at a certain point. This peak occurs around 4 cycles per degree (cpd). Sensitivity drops off both below and above this specific frequency.

This means we are best at seeing patterns of a medium size. We struggle to see patterns that are extremely thick or extremely thin.

The limits of our vision are tied to the structure of our eyes. The high-frequency cut-off, or the limit for fine detail, is typically about 60 cpd. This limit is related to the packing density of retinal photoreceptor cells. A finer matrix of these cells allows the eye to resolve finer gratings. At the low end, sensitivity drops because of lateral inhibition in retinal ganglion cells. These cells have a center and a surround region. The center might excite the cell, while the surround region has the opposite effect. This process can even cause color illusions, such as seeing a yellow surround around a blue object.

Contrast sensitivity is a measurable trait that changes over time. It typically reaches a maximum around age 20. After this peak, aging progressively reduces sensitivity. Certain medical conditions, such as cataracts or diabetic retinopathy, also lower this ability. It is important to distinguish this from visual acuity. Visual acuity measures the ability to resolve two points as separate, usually at 100% contrast. A person might have 20/20 visual acuity but still have poor contrast sensitivity.

This can make difficult tasks, like driving at night or climbing stairs, very hard.

Doctors use different tools to test these two different aspects of vision. Standard acuity tests, like the Snellen chart, use high-contrast black letters on white backgrounds. To test contrast sensitivity, doctors use different methods. They might use a Pelli–Robson chart, which features increasingly pale grey letters. They may also use sine-wave gratings, which are parallel bars of varying width and contrast. These tests help create a contrast sensitivity function (CSF) curve. This curve shows the full range of a person's ability to see different patterns and helps identify visual deficits.