> These precisely cancel each other, and the "light density" stays the same. This is very useful for a photographer! Want to correctly expose some small part o

> These precisely cancel each other, and the "light density" stays the same. This is very useful for a photographer! Want to correctly expose some small part of a scene? You can walk right up to that thing, use your light meter (either the one in your camera or a handheld) to set the exposure, and then walk away, recompose, change your lens... and still get the correct exposure. Until I understood this, I didn't really "get" how a hand-held light meter could be useful. There are two caveats to this. Make sure you don't cast a shadow over the subject when you're metering it; and if the subject is emitting light or has specular reflections, then it might not emit equally in all directions. On this latter point, an interesting question is why the moon appears as a disk of constant brightness, but the sun is darker around the edges (BUT DON'T LOOK AT THE SUN WITHOUT PROPER PROTECTIVE EQUIPMENT, obviously)