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I just got a new Canon EF50/1.8 II lens. Looking at the specs included with the camera there is a line that says

Filter Diameter / Usable Number      52 mm / 1

What does "Usable Number" mean?

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I believe this is referring to the maximum number of filters you can screw onto the front of the lens before suffering from significant vignetting. Adding more filters, you get too "tall" of a stack, and the edges block corners of the frame.

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