Sunday 18 May 2008

Metering and exposure

For further photography-related information check out my compendium of tips.

The amount of light falling on a subject, or incident light, is what determines exposure. Please read that sentence again. This is what you measure with an incident light meter (by pointing the light dome at the source of light) and is the most reliable method for determining exposure. It's much more convenient though (imagine if the tabby cat above was a tiger) to have a light meter built into the camera. This meter, however, can only measure reflected light from the subject - from this the camera must then infer the amount of incident light illuminating the subject and this is where the problems start. Replace the tabby cat above with a white cat and then a black cat. We'll keep the incident light constant and therefore the exposure will also remain constant - remember, exposure is determined by incident light. The white cat will reflect more light to the camera than the tabby cat, which will reflect more than the black cat. Cameras are programmed to assume that the average reflectance for a scene is 18% - equivalent to a mid-grey tonality - which indeed does work very well for most scenes. The camera does not understand that the three scenes with the cats have different brightnesses due to differing amounts of reflected light, it assumes that the incident light is different in each case and thus determines different exposures for each of the scenes. The solutions are:
i) use an incident light meter (not so practical when the cats are tigers).
ii) meter off a mid-grey subject in the same light - either use a grey card or learn to spot mid-grey tonality by eye. You can zoom in on a particular area to meter or spot-meter, if you have the option.
iii) exposure compensate. In the case of the white cat the increased reflectance fools the camera into thinking that the incident light is stronger than it really is, so the camera ends up underexposing - therefore you need to 'open up' the camera and increase the exposure. The trick is knowing by how much. In the case of the black cat the camera overexposes and so you 'stop down' the camera.
iv) if you shoot digitally learn to read the histogram for a shot - it's instant feedback on exposure and is what the majority of pros do (it's also what I do for my wedding photography).

Please feel free to post a comment if I've confused you!

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