No need, Helen adequately answered the question. Thanks Helen, as usual you're explanations are concise and clear.
Then if you took a telephoto lens, would exposure time be shorter with a wide angle lens under similar magnification (i.e. closer to the subject), provided that the f-ratio stayed constant?
OK, but I had already sent the email -- so what the heck, here's his answer.
My son teaches physics at the Univ. MN and is a Phd candidate working in bio-physics (proud dad here).
Joe
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The inverse square law is true for a light source that is a point - infinitely small. More precisely, the inverse square law is an approximation that is valid if you're far enough away from the light source that the physical extent of the source is negligibly small. So if you have a small flash, say 1 square centimeter, and you're 10ft from it, the inverse square law will give you reasonably accurate results. If you have a larger source, and you're not super far away from it, then you're dealing with something more complicated.
You can think of the large source as being composed of many dim point sources. (You can build something like a light box by gluing 500 small LEDs onto a piece of plywood.) Mathematically, to derive the "modified" inverse square equation, you would add up (integrate) the intensity contribution from each little light bulb. We know that the contribution from each little bulb is = (bulb intensity)/r^2, where r is the distance from a light bulb to the point you place your exposure meter. r is different for EACH little bulb. After doing the geometry and the integral, you will end up with an equation that depends on how far you are from your light box and the shape of the light box (ie, circle, triangle, rectangle, etc). Most light boxes have lengths that are approximately the same as their height (3' x 4'), so we might approximate all light boxes as something simple like a circle or a square. We can then express our approximate formula in terms of the surface area of the light box.
One interesting note: If you have a spherical light source (like one of those oriental paper lanterns), it still follows the inverse square law no matter how close you are to it. You can think of this as a quirk peculiar to spheres. Of course, this isn't going to be too interesting to photographers since most light boxes you'd encounter in a studio have flat surfaces.
Isaac