Subject to camera distance

markrichardson

markrichardson

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I entirely understand the inverse square law in relation to flash to subject distance.

What I don't understand is the science of why subject to camera distance doesn't have an effect on exposure. Any physics geeks who care to explain?
 
Now that you've called me a geek, no:)
 
I've got lots of boring work to do, but this question is more intereresting, so I'll forget that you called me a geek...

Basically you're not photographing a subject, you're photographing the light reflected from it. Like any immutable law of physics, the ISL does apply - but it doesn't seem to.

What actually happens is that the same amount of light reaches (and so reflects from) the subject regardless of the size of the subject on the film/sensor. As you move further away, the subject appears to be smaller and the light has to travel further, but the same amount of light is concentrated into a smaller area, therefore the same amount of light reaches the camera, so the exposure is the same.
 
Ah I see...that sort of makes sense. If you double the distance from subject to camera, you would also be quartering the size of the image projected onto the sensor.

Using a lens with double the focal length of the original (I.e. retreating the field of view of the original situation) would presumably then in effect gather four times as much light?
 
Oh, and I'm not sure there's any getting away from being a geek!
 
It's the same inverse square law applying just the same.

Try it this way. If you frame up a subject with a 50mm lens, then move back to double the distance with a 100mm lens, the subject remains the same size.

But for the same f/number, the 100mm lens will have an aperture four times the area.
 
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