The Inverse Square Law
Posted: Mon Sep 07, 2009 2:10 pm
Briefly - since my battery is dying - I was on the Basic Studio Photography course at the Met. The instructor was discussing which lens to use for portraiture and recommended about 70mm because shorter would give you distortion and longer would require you to be standing on the other side of the room, killing any creative atmosphere.
This got me thinking - wouldn't you need to change your exposure settings or increase your flash power to compensate if you moved away from the light source? I tried voicing my concerns but no-one in the class seemed to have been taught to think of light as discrete and quantifiable rather than "it's just there and you turn up the bulb to make it brighter". They all looked at me like I was crazy and the instructor said "trust me, you'll go away and think about it and one day you'll think 'ah, yeah, he was totally right'. Since we'd been shown that the studio lights had power dials marked in stops and begun to think about how if you turn up the light you'll have to close down your aperture to maintain an exposure, I thought why doesn't this apply to distance from the subject?
So here's my problem - if I move twice the distance away, the inverse square law says that I'm going to have a quarter the number of photons passing the end of my lens. Yet the before and after photo look the same. I've got a feeling that it's either something to do with the fact that as I move away I'm also zooming back in to keep the framing the same, or (suggested by a physics graduate, though admittedly he works on astronomy and gravitational waves) I'm so close relative to the light levels that the distances between my start and end positions are negligible.
Help?
This got me thinking - wouldn't you need to change your exposure settings or increase your flash power to compensate if you moved away from the light source? I tried voicing my concerns but no-one in the class seemed to have been taught to think of light as discrete and quantifiable rather than "it's just there and you turn up the bulb to make it brighter". They all looked at me like I was crazy and the instructor said "trust me, you'll go away and think about it and one day you'll think 'ah, yeah, he was totally right'. Since we'd been shown that the studio lights had power dials marked in stops and begun to think about how if you turn up the light you'll have to close down your aperture to maintain an exposure, I thought why doesn't this apply to distance from the subject?
So here's my problem - if I move twice the distance away, the inverse square law says that I'm going to have a quarter the number of photons passing the end of my lens. Yet the before and after photo look the same. I've got a feeling that it's either something to do with the fact that as I move away I'm also zooming back in to keep the framing the same, or (suggested by a physics graduate, though admittedly he works on astronomy and gravitational waves) I'm so close relative to the light levels that the distances between my start and end positions are negligible.
Help?