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I made a "standard" light source out of a LUXO lamp and a piece of opal glass (plus miscellaneous stuff to put it together). Then I made 2 dimming masks out of half-tone screen, with a different pitch so that I can stack them to get an additive effect. This gives me 3 light levels: full, one-screen, and two-screens down. The 3 values I happen to have wound up with are (ISO 100) f/16-250, f/5.6-60 and f/2.0-15... a good general purpose range that goes beyond full sun at the high end. I don't know how easy it is these days to still find halftone screen; something like fine wire mesh would work well too. I established the "correct" light values by checking it with several, relatively new and very good cameras and meters and, if there was any disagreement (there wasn't much), making a value judgment as to which was the better number. This still leaves me with the question of whether the brightness of the bulb stays constant over time. It isn't on much, so there shouldn't be too much drift, but it doesn't hurt to recheck it periodically to make sure that you still agree with what you're getting. |