What does "usable number" mean in a lens specification?
Asked 12/15/2010
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2 answers
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My Canon EF 50mm f/1.8 II spec sheet lists:
Filter Diameter / Usable Number: 52 mm / 1
What does the "usable number" refer to in this context?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
10
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.
Originally by user2425. Source · Licensed CC BY-SA 4.0
user2425
15y ago
0
Generated from our catalog & community — verify before relying on it.
In this context, "usable number" most likely means the recommended maximum number of screw-in filters you can stack on the front of the lens.
So for a 52mm / 1 spec, the lens takes 52mm filters, and Canon is effectively saying you should use only one front filter at a time. Stacking more than one can make the filter assembly too deep, which may cause vignetting—darkened corners caused by the filter ring intruding into the lens’s field of view.
This matters more on some lenses than others, but the spec is a practical warning to avoid stacking multiple filters unless you’ve confirmed it won’t affect the image.
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