Light meters

James Hope

James Hope

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Hello everyone.

I'm getting myself a little confused and hope somebody can help. I understand that with a light meter you set the Shutter speed and ISO you'll be using, and then measure the light from whatever source you are using. This then gives you an f number to use to gain a good exposure. I've seen lots of video's on line where people then use this f stop as a starting point and work from there. My confusion is that surely the inverse square law means that if you're standing further away from your subject then the exposure could be quite different from that given on the meter. Does the meter assume you'll be a certain distance from your subject?

Thanks in advance,

James
 
No, it doesn't work like that.
There are two basic metering methods, reflective and incident.
Reflective means measuring the light reflected from the subject, and has the same disadvantages as using the meter built into a camera, i.e. that very light or very dark subjects will give a false reading.

But the Inverse Square Law, although applicable, is self-cancelling. The further away the lit subject is away from the camera, the less light will reach the camera BUT as the distance increases and the relative size of the subject becomes smaller, the light is also concentrated into a smaller area, cancelling out the effect completely.

The other (and generally favoured) metering method is measuring incident light, or the light falling upon (incident upon) the subject. Here, the meter is placed at the subject position (if possible) and the incident dome is pointed back to the camera to take the reading. If that isn't possible then the meter in placed in a position that is receiving the same light as the subject. Again, the effect of the Inverse Square Law is cancelled out and is irrelevant.

Good question though:)
 
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