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B+W Pro Snap-On Lens Cap$11.95
The B+W 311 Snap-Cap Pro is a lens cap with two spring-loaded plastic claws that click into the filter thread. This ensures that the cover sits firmly and securely without any time-consuming screwing and unscrewing. The claws can be operated from the side as well as from above.
The cap can therefore be fitted and removed through a lens hood, as long its barrel is not too long.