This Leica Rectangular Metal Lens Hood with Cap is specifically designed for the 35mm and 50mm f/2.5 M lenses. It comes supplied with a custom cap that is especially designed for this hood.
Lens hoods are primarily designed to prevent unwanted stray light from entering the lens by extending and shading the end of the lens. In addition, since the end of the lens is extended, you also get the added benefit of some extra protection from accidental impact.
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Comments about Leica Rectangular Metal Lens Hood With Cap:
This is a screw-on lens hood and it doesn't screw on tight, so the lightest touch against it can and will cause it to rotate, which, with a rectangular hood, is bad.
There's a mechanical stop built into the design of the lens and hood that prevents it from being turned beyond a certain point on the thread (and insures proper alignment). The problem is that there is absolutely no tension at the point where it hits the stop.
I'm sort-of surprised that Leica would allow this given their attention to detail.
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