Levels

Overview

Restricts the maximum and minimum brightness levels in the image. This is useful to avoid images being saturated (under or over-exposed) when displayed on some output devices.

Properties

Controls

Lower

Sets the value of the darkest brightness in the image. The lower the value, the darker the lowest level. 

Upper

Sets the value of the highest brightness in the image. The higher the value, the brighter the highest level. 

Clip Levels

Checking this ensures that the brightness values between the upper and lower levels remain unchanged - values are only changed if their brightness lies outside the allowable levels. If this option is unchecked than the image brightness is rescaled to fit between the upper and lower limits.

Notes

This effect is typically used to avoid images with areas of extreme light or dark that can cause display problems on some output devices. VHS tape for example does not like areas of extreme brightness or darkness and this effect can be used to ensure that these levels are not used.

Normally, you should try to use the full range of brightnesses, as this ensures that the contrast remains good in the image.