I have an old metal Hoya lens hood which I used to use on a 35mm lens that looks perfect on it but it nearly doubles the length, so although no darkening don't like it. Also can't put lens cap on when in bag, using a filter.
Not that it has direct bearing on this thread, but the EF-M 22mm f/2.0 uses a functionally identical hood (although it's a different size/part number).
What's the real amount of protection you get with this anyway? Think of the amount of ways the lens could actually be impacted and saved by a 8mm lip that protrudes less than the actual distance from the glass to where the hood starts... I mean, you'd have to hit literally hit a surface exactly head on and even then, is it really any more protected than it is without the hood?
And what light source is it going to protect from flare? A laser thin strip of light coming in from almost perpendicular to the lens surface?
Id be interested to see if something so seemingly insignificant really makes a difference. A before and after shot would be awesome...
... The 50 1.8 hood might be an option for more extreme situations (mosh pits and the like). Can the lens cap be used while the hood is attached? I assume the hood can not be reversed like the bayonet types can be?
I have the Vello ES62 hood (pretty much identical to the Canon version) for the 50 f/1.8 II and it does indeed work on the 40mm. Yes, the lens cap can be used while hood is attached. And yes, the hood can be reversed.