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My old Pentax lenses have a small white dome between the aperture ring and the focus ring. See the photo below:

smc Pentax-M 1:1.7 50mm

The clear white dome calls to mind the larger white plastic domes used in light meters, but there's no such thing in this lens (and that'd be a very strange place for one). You can't press it or anything; it's not a button or control. It's just a white plastic bead half-embedded in the metal. It's common to many or most Pentax manual focus lenses (take a look at any of the K or M primes shown here); I haven't seen it elsewhere.

This dome is on a part of the lens body which does not move. It lines up with f/1.7 when the aperture is set to f/16, or f/2 (unmarked) when the aperture is set to f/22. It's not in a place where it seems likely to provide any meaningful tactile feedback — in fact, when holding the camera in use, I'd characterize it as "out of the way". And it's offset from the orange dot used for mount alignment, so it's not that either.

So, what is it there for? It can't be just to look pretty, can it? It's not very pretty.

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My SMC Pentax-A 1:2 50mm has yellow plastic dome. –  asalamon74 Nov 16 '11 at 19:47
    
I saw that in some pictures, but couldn't tell if it was yellowed white plastic or originally yellow. My f/2 50mm got thrown in a box once I got this one, which is no-contest better. I'll have to dig it out and inspect. –  mattdm Nov 16 '11 at 19:52
    
I'm going to take a random guess that it lines up the mount for putting the lens on –  Dreamager Nov 16 '11 at 19:56
    
There is an orange dot used for that, and it doesn't line up with that. But now that you mention it, it does line up with the release lever when mounting or unmounting. –  mattdm Nov 16 '11 at 20:02
2  
Pentax calls it: white plastic bump. –  asalamon74 Nov 16 '11 at 20:09

1 Answer 1

up vote 9 down vote accepted

As stated here ( and guessed by Dreamager) it is aligned with the lens release button of older Pentax cameras. It's much easier to change the lens in the dark. ( I don't really understand the advantage of the newer orange dots. )

From K-1000 user's manual:

In the dark, when red dots are difficult to see, align the white plastic bump on the lens barrel with the lens release lever by touch.

share|improve this answer
    
Well, okay then. Thanks to both you and Dreamager! :) –  mattdm Nov 16 '11 at 20:03
    
yay for guessing :D –  Dreamager Nov 16 '11 at 20:09
    
Manual also available directly from Pentax. Cool. –  mattdm Nov 16 '11 at 20:35
    
@asalamon74 Is the bump fluorescent? –  uvts_cvs Jan 2 '13 at 8:34
    
No, my bump is not fluorescent ( maybe it was 30 years ago ). –  asalamon74 Jan 3 '13 at 8:04

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