Originally posted by WPRESTO I have both single & double versions of the OptiCaps. The rubber seal means they seat snuggly, no tendency to become too loose (so far). They have the same disadvantage as other non-Pentax rear caps: they only fit one way and you must align a fairly inconspicuous ridge with the red lens dot to get the cap to mate correctly. Once installed, all the rear lens contacts are very well protected. The double cap doesn't really save space but it keeps two lenses in one padded slot with no danger of bumping/scratching against each other. The double cap is best used on smaller lenses. I double up a 70mm with either a 40mm or 35mm macro.
I like it for the small lenses (my 15mm and 40mm are usually mated and fit into a small neoprene lens pouch, and my 77 and 31 are connected and reside in a neoprene pencil case). But it's also great for carrying a teleconverter connected to the lens you use it with most -- I often carry my 1.5x kenko connected to my 60-250 using one, in case I need extra distance.
I don't think I'd trust just glueing two lens caps together with anything heavier than a small limited or plastic fantastic. The opticaps feel like a single solid cap. There's a seam, but I'm not sure if it goes all the way through or not. They are, on the whole, much nicer quality than any pentax, canon, nikon, sigma, or tamron lens cap I've seen. I have 3 of the doubles and... I forget, maybe 5 or so of the singles. I don't use them on everything all the time, but most of the lenses I use regularly have them.
And yeah, it would be nice if they were slightly more flexible with the location of the mount.. but the ridge makes them so much easier than other caps. I wish all the lenses had a little bump where the red dot is, too, so I wouldn't have to fumble when trying to mount a lens with the body facing down in snow or whatever.