Main light thru the Spot is the EL 500 at full power 500Ws. The incident light aperture reading using a Minolta Flashmeter IV is f11. There is no background light. To get a threshold trace of backdrop color, a red-gelled EL 250 at minimum power (64Ws) was moved until incident light on the black seamless paper was 2 stops less (f5.6). For the result, see next frame. If you don't see a change, your monitor is set too dark or contrasty.