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The fascination of flash photography.

Highlight/Shadow Readings

Complex contrast situations confuse the flash computer

People can appear to have an excessively dark or pale complexion on a photo, which is often the outcome of an extreme imbalance between highlights and shadows within the measuring field. This is the point where the application range of the automatic flash unit can be extended.

The sensor takes an overall reading in automatic flash mode, i.e. it measures the average brightness over the entire measuring field without considering specific image sections. This ensures that the automatic flash mode will always provide the correct exposure if the highlight/shadow ratio in the picture is approximately 1:1. However, if the subject covers an extreme range of contrasts, and if the metering field is aimed at one of the extreme ends of the highlights and shadows scale, this will result in the automatic flash mode producing a weighted meter reading in favor of the given brightness. A person dressed in dark clothes standing in front of a large white wall will appear much darker because white was the dominating value in the metered reading. The automatic flash mode shifts white, as the dominating brightness, towards grey thereby adding "darkness" to the already darkly dressed person. Inversely, a person dressed in white standing in front of a black wall will invariably be overexposed as the automatic flash mode shifts the dominating black towards grey to enhance brightness, thereby adding extra "white" to the person already dressed in white.

A flash of gracefulness with Metz mecablitz: exposures of blossoms from Katrin Sdun-Heinlein

The photographer has to take corrective measures in such extreme situations. For the person dressed in white standing in front of a black wall this means that the computer's aperture must be reduced by one stop compared with the camera's aperture (e.g. camera f/8, computer f/5.6). Consequently, the flashgun will diminish its light output by one f-stop, thereby preventing overexposure of the person. The setting is reversed for a person dressed in dark clothes standing in front of a white wall, i.e. an aperture of f/8 is set on the camera and automatic aperture of f/11 for the flash unit.

An alternative method of overcoming such extreme situations is to change over to manual mode, and to use for the camera aperture the setting indicated by the flashgun's aperture calculator or to calculate the setting by the following formula:

Aperture = guide number ÷ distance of subject

This method invariably covers all the different brightness levels of the subject for correct rendition in the picture.