In article <4ebrm3$1bt@ipof1.fla.net>, levine@ipof.fla.net says...

>I purchased a tiffen 25a red filter. The literature says the filter
>requires an adjustment of three stops. When I meter with a spot meter
>thru the filter I only get a one stop difference. Why? What adjustment >do I use? If I use the exposure given by tiffen does that mean thru
>the meter is inaccurate for filter readings?

Yes, most meters (including the ones in cameras) will not read accurately through colored (B and W) filters. If you use the spot
meter (which, BTW, will not read saturated subject colors accurately, either), use it normally, then apply the three stop correction when
setting the camera controls (or change the ASA setting on the meter
three stops lower and use it normally - BUT, remember to change the
ASA setting back when the filter is removed). If you use the in-camera
meter with the filter, meter a daylight lit grey subject without
the filter, drop the shutter speed setting three stops, and change
the ASA (or compensation) setting without changing the aperture or
shutter speed controls until the meter indicates correct exposure
("+" compensations will probably be about 1/2 stop for dark yellow,
1 stop for orange, and 1 1/2 stops for red filters).
Hope This Helps