Question about Digital Cameras as light meters

I am trying to use my digital camera to figure out how bright my flashlights are relative to each other or to a flashlight's different modes. The camera is setup in Shutter Priority - so with aperture locked (say at F4.0) but shutter will vary according to how bright the lights up. I shine a flashlight against the wall about 2 feet away and pick up the beam with the camera. I then take note of the shutter speed (e.g 1/200, or 1/125, etc). I do this repeatedly for the different flashlights or modes I want to compare.

Here's my question: If I have a reading of say 1/200 and another of 1/400, does that mean that the second beam was twice as bright as the first?

You're actually using Aperture Priority if the aperture is locked and the shutter speed varies according to the light level.
As to your question, yes as far as the camera meter is concerned, there's twice as much light, but most camera meters are weighted in different areas of the scene so beam distribution could still sway the results.
You could shoot in manual mode and then compare the histogram for relative difference in the overall exposure.

Hello LiteShow,

You may find this article helpful.

Tom

You are correct RebelXTNC, I meant to say aperture priority. I don't have a full manual camera, so can't do the histogram thing. However, I think I can select between 2 or 3 different types of light metering modes (spot, average, etc), so might adjust that depending on the coverage of the beam.


Interesting article Silverfox - lot's of reading too!

Thanks for your quick answers...

I don't think the histogram will tell you anything about brightness, it will tell you the relative color brightness distribution within THAT exposure, and will be different for each exposure - nothing is held constant from frame to frame.

Your Apeture Priority method should work to an extent with some inaccurracy because cameras usually stick to some steps in shutter speed (1/100, 1/150, 1/200, etc) and the meters arent always that consistant in cameras. You will get variation from one minute to the next (depending on your camera, but I have seen this in practice in many cameras) under the exact same conditions. You should just take several readings over a few minutes until you get a few that agree somewhat closely. Throw out the one or two measurements that don't agree with the rest.

Nice article Silverfox.

Excellent article SilverFox, thanks for the link. I have folks asking me about exposure all the time, and it's nice to have a document that I can refer them to that explains it so well.