> Any filter that has some color tint to it does not pass all the light > entering it. The color tint is due to absorption of some "color" light. > Your light meter does not measure the difference because it is not > sensitive enough for these small differences. If you had a color temp. > meter it probably would read different due to a sky light filter. I > don't know what the threshold is for filter maker to say the filter > factor is not 0.
Right-oh. Also, most meters do not have the exact same response to all kinds of radiation as either the film, or our eyes. Fred picker of Zone VI Studios had Paul Horowitz try to modify a Pentax 1-degree spot meter so that it had the same response as Tri-X 320. (It required several additional filters and additional baffling.) Transmission also depends on the light that the filter is passing. A red filter will pass less light if it is pointed at blue sky than it will if pointed at a red blanket, obviously. Most meters are not completely linear, and all through-the-lens meters are affected by flare (including spot meters with lenses). Meter cells are now usually pretty good, though. But few metering systems are good enough to accurately register thirds of stops, much less tenths or less. And a sky filter may only cut the light transmission by 1-3%, which is, as you say, far below the tolerance of the metering system. Incidentally, here's my rule on filters: you should either absolutely not use them, or else you absolutely should, or else something in between. Ever the empiricist, --Mike
