Quote Originally Posted by Bill Burk View Post
BTZS teaches how to shoot a "black hole". It's a large-ish cardboard box spraypainted black inside with a shade like a lens shade over an opening cut in the front. You put this in your pictures, like a prop, and shoot away. The theory is it is pure black. No light that goes in comes out. So any density over base+fog on the
I did a test like that some years ago while working on an article about flare and the film speed. The set-up consisted of the black box and four cards, white, grey, black, and mixed, with holes cut out of the middle to go over the black box opening.

I based the exposure on a grey card. I then made exposures of the four different set-ups. Being me, I then exposed a sensitometric strip to run with the black box test. I read the density from the area of the box opening and placed the density on the film curve.