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What’s the small white plastic dome on old Pentax lenses?

What is the purpose of the small yellow/white "hemisphere" near the aperture scale on some Pentax lenses?

See for example the Pentax M 50 1.4 where the hemisphere is above the 2.8 number of the aperture ring.

Pentax M 50 1.4

  • \$\begingroup\$ @mattdm thank you very much! My English is not so good and I did not find it. Shall we close this question? \$\endgroup\$ Jan 1, 2013 at 18:47
  • \$\begingroup\$ Don't delete it — it will be closed as a duplicate and maybe help others who search for the words you used rather than the ones I did in the other question. \$\endgroup\$
    – mattdm
    Jan 1, 2013 at 18:50
  • \$\begingroup\$ don't delete! My one is distinctly yellow, I could not find the original 'white' answer \$\endgroup\$
    – Ken
    Feb 26, 2016 at 22:18


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