The ability to apply manual compensation to automatic flash exposures is generally useful, and particularly so for portrait work when using daylight fill-flash. Compensation values between - 1/3 stop and - 2 stops are often used. Automatic cameras communicate with dedicated flashguns to set effective flash levels, but under some circumstances a photographer may wish to intervene to change this predetermined flash setting. Flash levels are probably most commonly reduced rather than increased.
It is worth noting that flash output levels are not generally changed by placing diffusers or filters on a flash head, because the electronic control circuitry merely increases the flash output until the required amount of light is returned from the subject. However, It can also be beneficial to soften and warm the light emitted by a flashgun. A diffuser placed over a flash head softens the light to some extent. Simple translucent plastic covers and more sophisticated miniature softboxes are available. A flash filter such as an 81A or 81B can also be placed over a head to warm up skin tones. This is particularly advantageous when photographing swimsuit models or nudes.