I shoot with an SLR, and all lenses need a rear lens cap when not on the camera. Every SLR shooter knows the three-handed ritual. (Four handed if the Camera's not on a strap.) You take one lens off the camera. You pick another lens and remove the rear cap from it. Holding the old lens, new lens and rear cap and camera, you put the new lens on the camera, then put the rear cap on the old lens. (Or you put the cap on the old lens first, put it down and put the new lens on the camera.)
Anyway, a simple invention I have already built is a doubleheaded rear lens cap, namely two lens caps glued together. Custom-built it would be a lot smaller and solve some of the problems I have experienced.
With the doubleheader, you can take your lens off the camera and put it immediately onto the open end of the doubleheader cap on the new lens. Then with a twist you remove the new lens from the resulting docked lens pair, and put it on the camera. In theory one less hand or less dexterity.
However, the catch is the docked lens configuration tightens both as you twist one way and loosens both as you twist the other way. So you must master the art of making sure the lens you want comes loose.
How this works varies from lens to lens and how well it fits the rear cap. Sometimes pressing them both together causes one to undo reliably. The most reliable trick is to grab the old lens around the rear neck so you can get a finger on the cap, and then pull the new lens off.
It seems one might be able to design ways to make this more reliable, such as a small flange on the cap to hold with your finger to make sure of what twists off, or a ratcheting twist-off that requires a release button.
If both become equally lose when you untwist, then gravity will help you in that the cap will stay on the lower lens. You must later twist it back to stay on. I think the ideal motion would be to twist on so both are tight, then either hold the cap or release a ratchet so only the lens you want comes off without loosening the old lens.Of course, if you want to redesign the lens mount -- not really an option here -- you could design lens caps that quickly snap on and off or which release differently. Owners of digital SLRs would love a system where the rear lenses and most importantly the camera body area are exposed for as little time as possible, to stop dust from getting in to the chamber and then onto the sensor where it plays havoc.
An iris in the body mount would be great for that. Any faster changing mount would be great.
It's pointed out that the other side of the cap, if left exposed on the lens, would get dusty. Since part of our motive in a fast-switch is to avoid exposure, it might make sense to use a body cap there. It adds extra steps -- remove body cap (place somewhere). Swap lenses. Return body cap to other side of doubleheaded rear cap now on your old lens.