I'm here and I know how to do this :) Results will be approximate of course, but close enough for our needs as plant growers.

You need a piece of white paper or card. You need to set your camera to point-metering mode so that it is only measuring the light at the centre of the view. You also need to use a shooting mode that shows you the ISO speed, F-stop and exposure, or allows you to fix some settings and shows you the rest.

So, put the paper/card at the spot to be measured and point your camera at it with the metering spot on the card. You will probably need to press the button down halfway to get the camera to measure. Read off the ISO speed (I), the F-stop (F), and the exposure in seconds (E). Focus doesn't matter. Telephoto, zoom, or macro modes don't make any difference, but may reduce the availabe settings the camera can pick. Just make sure the metering spot is completely filled with the white card. The light intensity in foot-candles is given by:

6*F^2/(I*E)

As an example, if the speed is ISO 50, the F-stop F/2.8, and the exposure 1/500 sec, then the intensity in foot-candles is:

6 * 2.8 * 2.8 / (50 * 1/500) = 470 foot-candles

Try it and feel free to come back with any questions or comments.

great, thanks for the explanation. i am a planted aquaria guy, and i want some idea of how bright my lights are with different lamps and reflectors, and esp some idea of how much they will have dimmed over time, by recording this data and checking later.

time to get the manual out for my camera. it hasn't come out of automatic mode yet.

rick

Hello!! I´m sorry I´m like intervining here, but I´d also like to use my camera as a light meter. I have an Olympus C-770, and I still need to read a lot to find all the parameters you mention. But in the meantime, I have a question. What does the value you get after you solve the equation means? What I´m trying to ask is what is (in your example) 470 foot-candles? How is this measure related to the light needed for a shade plant, for example? Can you give me this information or at least tell me where can I find it? thank you very much!

A few web sources will indicate foot-candle values for different plants, but here is a rough guide.

Direct sunlight: 10,000fc, maybe a little more
High light plants: 1,500fc-5,000fc
Open shade: 1,000fc
Medium light plants: 500fc-1,000fc
South-facing window (not direct sunlight): 500fc
Low light plants: 100fc-500fc
North-facing window: 100fc
Domestic indoor lighting: 10fc

These numbers are all approximate, but should give you an idea of roughly where you are. So 470fc would be a very dull day outdoors or a bright indoor position near a south window, and suitable for many plants but not high-light plants.