Why not transparent lens caps?
1. Highly unlikely a general purpose lens can focus on something millimeters from it's front element. Probably not even close. So the text would be completely unreadable and nearly invisible. (Look through a camera viewfinder and put your finger right in front of it; notice that you can barely even make out the finger).
2. For anything with a through the lens viewfinder, is there anything more obvious that you have the lens cap on then an all black picture?
So I think transparent would actually be worse, because if you didn't pay closer attention (or if you were drunk), you'd look through the viewfinder and see a picture (albeit blurry and distorted, but on tiny digicam screens/viewfinders it can be tough to tell), and take the picture, thinking you were all right. But what you'd get would be an awful quality image. Better to not see anything, and know you have to take the lens cap off.
This only applies if you have a through the lens preview of course. If your viewfinder doesn't go through the lens, then the camera manufacturer better have provided some signal in the viewfinder that the lens cap is on.
My Sony mini dv camera has a heavily opaque lens cap, and I think it's succeeds at screaming "take off the lens cap." When I first figured it out, I thought it was genius.
I'm with Rob: isn't this a really great example of fail-fast?
With my DV camera, having an opaque lens cap tells you when the camera is on but you have the lens cap on. If the lens cap was solid black, you wouldn't know if the camera was off or the lens cap was on.
It's a subtle clue that is useful.
With my SLR, it's totally irrelevant since the viewfinder displays what the lens sees wether or not the camera is on.
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