Inverse Square Law and window-light.

[ki6mf]

All the more reason to have a hand held meter to measure the quality of light from all directions so as to create dramatic lightning by moving camera subject lights etc! There are times when I use my Sekonic for incidence over my Pentax V spot meter

[Robert Hughes]

I believe the inverse square law refers to how light intensity drops off with the square of distance from the light source to the illuminated object. As the sun is 93 million miles away, a couple extra feet into the room is not going to make any difference whatsoever.

[ki6mf]

Inverse Square Law would apply from the point where light enters. In this case a window is the source where light enters and the square of the distance determines how the light falls off in intensity. With the subject 10 feet from the window the light is 100 times weaker even though the sun is 93 million miles away! This is why you can shoot a digital picture of a subject with on camera flash in your point and shoot camera and if the wall behind your subject is 10 feet away the background goes dark!

[Bruce Watson]

The window is an aperture and as such behaves in the same way as your camera aperture or any point source of light does - the position of the aperture is your light source and all calculations start from that point not not the position of the sun.

+1. Exactly right.

[Bruce Watson]

We are talking about a window, a large rectangular light source facing the north sky. The conclusion, with regard to the Inverse Square Law, seems to be that the origin of the source is at the window frame, regardless of whether the window is covered with a diffuser or left wide open. Is that it?

That's it.