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flash metering/distance question
a nighttime shoot, off camera flash, no ambiant light to factor in. the subject is flash metered at f5.6 and it's 10' from the cam. now, keeping the flash and subject exactly where they are, i move the camera back 10' more. it's now 2x as far away, and the flash to subject is same as before. using the same exposure, will the subject appear just as bright or darker? does subject to camera work similarly to flash to subject distance, i.e. increased distance = decreased brightness? Sep 01 05 12:21 pm Link The subject will be correctly exposed. It's flash to subject distance that matters, not camera to subject. Paul Sep 01 05 02:50 pm Link Pauls right Sep 01 05 02:54 pm Link Yep, though I've never been able to figure out why. I mean, if the light intensity from the flash falls off with distance, why doesn't the light intensity reflected from the subject to the camera fall off the same way? Sep 01 05 05:24 pm Link Camera to subject is based on exposure not distance. Flash exposure is based on the correct amount of light reaching your subject before the shutter closes. If you know the exposure it takes to expose your subject and the light distance never changes then the required exposure setting never changes no matter where the camera is. It's sort of like shooting the moon at night which is lit by the sun. It doesn't matter that you're thousands of miles away, it's still lit therefore all you have to do is set the correct exposure. Sep 01 05 07:21 pm Link Ditto x3 Sep 01 05 07:39 pm Link Hehe the following is how you know I have no life at all. . . light falls off at the square of the distance. paul is mostly correct, though there's likely to be a slight drop. but back to this square thing (because I have no life). . . Move a light 4 feet, and your light will decrease 16 times (relax, that's only actually 4 stops because everything in photography (at least originally) was based on a factor of 2- either half or double (100 ASA 200 ASA 400 ASA- same as 2.8, 4. 5.6, 8 (f stop is a funky german formula indicating the ratio of the length of a lens to its narrowest opening (i.e. the apeture), but every full F stop is exactly twice as wide as the one before it and half as wide as the one after it, but once you figure in that wacky Pi, it ain't as simple as 1, 2, 4, 8. . . Lets not even start with T stops. . .) This is just all random trivia (though the half and double stuff in the field and in the darkroom can simplify things immensly, or at least make them easy to understand for the mathematically challanged like myself) For all of you who think I'm crazy, just be glad you're shooting stills and you don't have to factor in frame rates and shutter angles too. Sep 01 05 08:36 pm Link As to question about light falling off with increasing distance: You are right, the light intensity does fall off with distance. But exposure is NOT about registering light, it's about registering brightness. And brightness (by definition) is light per surface that receives it. When you increase the distance, the light falls off at the square of the distance. But so does the visible size (in Russian we call it 3D angle) of the object that produces / reflects light. And so the quantity of light per visible object remains the same. So the brightness stays constant regardless of the distance between you and the object provided the lighting of the object is constant. Hope this helps Sep 03 05 01:24 am Link BrooklynPhoto wrote: If you move a light four times further from the subject then your light will decrease 16 times. Four feet is true only if your original light-to-subject distance was 1 foot. But I'm sure that's what you meant. Sep 03 05 11:00 pm Link Tim Hammond wrote: Yes, correct. 4 X (X=initial distance) would have been clearer. Sep 04 05 08:02 pm Link Think about it like this: what if the subject was lit by daylight? Increased subject-to-camera distance would make no difference then, right? So, if the amount of light on the subject remains the same..... And by the way, it's great that it works like that. I mean, if I had to worry about light to subject AND subject to camera, well..... Sep 04 05 08:13 pm Link I am getting dizzy. So the short answer is if you don't move the light but you step back, the exposure remains the same. The rest is gibberish that clouds the mind. But it is a lot of fun to listen to. I love to hear them argue about such minutia just to correct each other slightly. Alan Sep 04 05 08:31 pm Link Alan from Aavian Prod wrote: Hey, I did start by saying I had no life, Didn't I? Sep 04 05 08:48 pm Link Gary Davis wrote: because when the subject to camera distance is doubled (and thus the subject illumination reaching the camera is reduced to ¼ of the prior amount) the sensor (film) area onto which that "focused" light falls is reduced to ¼, thus balancing out. Sep 04 05 08:57 pm Link steve_bevacqua wrote: thx that daylight point totally helped me understand. Sep 05 05 01:36 am Link |
