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# Thread: When using an incident meter, with the camera back a ways.

1. ## When using an incident meter, with the camera back a ways.

Here is something this long time 35mm user doesn't understand. I'm only using a MF but this would apply to any one that uses an incident meter. I figured you guys would be able to explain this as you no dubt have more experiance.

Lets say you are shooting a hall with a large painting at the end. There are other paintings on the sides of the walls so you want to show that they are there, but you really would like to highlight the big painting at the end of the hall. Whether it is lit by strobes (usually a no no in a art gallery) or the natural light coming from windows, you have your camera 40 feet from the big painting. Now the question is, if the camera is that far away, how do you figure in extra exposure? Certainly there is some light fall off over that large distance. With a reflective meter, you are reading the light that comes down the hall. However with the incident you are measuring the light falling on the painting. Do I make myself clear? Thanks. Ric.

2. Originally Posted by Ric Trexell
Certainly there is some light fall off over that large distance.
Thanks. Ric.
Short answer: There isn't any 'fall off', inverse square law at work, or atmospheric attenuation in the circumstance that you describe. No adjustment is needed, just the straight reading.

Some might suggest that you use a flat incident surface (as opposed to a more typical hemisphere for flat art. I suspect that you wouldn't see much offset there under typical art gallery conditions.

Lee

3. Walk down to the end of the hall where the photo is, stick the meter up and point it towards camera position. Take a reading; walk back to where you will shoot from. All that matters is the light that's falling on the subject.

Kent in SD

4. To explain it geometrically, fall off after the inverse square law applies to point light sources, e.g. the head of an enlarger. Doesn't apply to planes diffusing the light in all directions.

5. A different and slightly more correct way of looking at it is as follows:

1) The inverse square law does apply to light reflected from a surface. Therefore, the total amount of light from an object that is collected by the camera lens and focused on the film plane does fall off with an inverse square law.

2) The amount of area an imaged object takes up on the film plane is inversely proportional to the square of the distance.

3) Item 2 compensates for item 1, so the amount of light per unit area on the film plane is independent of object difference.

Therefore, for determining exposure you can just ignore the inverse square law.

6. Sir Alan, you're definitely the greater mathematician!

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