In article <4qfuek$611@news.istar.ca>, crs1583@inforamp.net says...

>Back in the days of manual exposure, filter factors had to be
>"factored" into the exposure. You'd estimate your basic exposure, but
>because of that Wratten #25 filter subtracting three f-stops of light,
>you had to add three to the exposure by changing the shutter speed
>from 1/125s to 1/15s, for example. Now, with TTL metering looking
>through the actual filter, the camera does this automatically, so you
>don't have to worry about it.
>However, very strong filters might still confuse the TTL meter. You'll
>have to experiment before you'll know which filters fool YOUR camera,
>but it's not too likely you'll run into trouble.

Um, at least with Nikons, ALL colored filters intended for B & W
photography are read incorrectly by the TTL meters. You need to compare
a no-filter reading of an evenly-illuminated grey subject with the
readings given by the camera with the filters on, and compare those
readings with the filter factor. With Nikon, a dark yellow filter
requires adding 1/2 stop to the camera reading, an orange one stop,
and a red one and a half stops, for correct exposure - these are not
subtle corrections! (Too bad we are still stuck with filter factors,
and they are more complicated now with TTL meters - seems like every
bit of "automation" brings with it a bit of negative baggage that once
in a while makes things more complicated than they were before.......)
Hope This Helps