I was contemplating how much filter factor (light cutoff) I get with a cheapo Quantaray Circular Polarizing filter that I inherited when I bought my Canon 50mm f1.8 second-hand. It finally occurred to me that I could switch over to M (Manual) mode on my camera, aim it at something, dial in an exposure (I happened to set it to '0'), then drop the filter over the lens and watch the exposure level indicator ( -2..1..0..1..+2 ) adjust (usually downward) and note how many stops the exposure dropped. The camera focuses and meters through-the-lens (TTL) (and thus through-the-filter), so it takes this into consideration anyway when metering a scene.

For my 52mm Quantaray Circular Polarizing filter, the exposure drops 1 1/3 stops. I'd been working with the filter on most of the time, not realizing that I was losing 1 1/3 stops unintentionally, turning the "nifty fifty" f/1.8 effectively into a f/2.8. Oops, lesson learned, check your filters out.

Tags: photography test filter dslr canon camera slr nikon technology howto