Vented Lens Hood

abruzzi said:

I think the reference was to the photo in the original post, which according to the printing is for a Canon 50mm f1.2. I don’t know how common a lens it is, or how common the hood is, but I can imagine that kind of original hardware might be hard to find and pricy.

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Canon 50 1.2 isn't very rare lens. It is availible on eBay often, if not most of the time. Some of them are with hoods. 55mm thread filters are not as rare as 40mm filters I guess, which is another common size of Canon LTM lenses filter thread.
Original hoods are not easy to find. Some of them where lost after sixty years of use, some of them were worn out or damaged.
Replacement hoods are easelly availible as long as it is common filter size. For some popular Leica lenses exact copy of original hoods is made new and availible on eBay as well.

jim10219 said:

If you're using that style lens hood to close out the surrounding light while pushing the lens up against a pane of glass, like when you're trying to photograph fish at an aquarium without getting reflections in the glass, it can cause a problem. If the sun is directly behind you and shining light through the vent that's reflecting off the front lip of the hood, that can also cause a problem. But those are two rare circumstances.

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I do not know if you simply surmised those scenarios, or you have actually experienced them.
I suppose i am equally impressed either way.

Using a lens shade is the standard procedure when photographing through a window and avoiding reflections in the photograph. Using a rubber shade even enables to tilt the lens.

CMoore said:

What is the purpose of this type of hood.?
Sorry if "vented" is not the right term, but you guys know what i mean.
They look like this..............

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I have that Canon shade for my 1.2. even tho it's vented it still gets in the way of my Canon VI-t viewfinder. 1.2 has a gigantic front element...I'm processing it's first rollmthis very minute, shot without that shade...I'm expecting a lot of glare. That shade would be too easy to lose and is apparently a valuable antique..the answer is tiger a smaller, slower lens.

I did this video on ventilated shades, their history and why to use them.