Pentax Cameras - Page 6 - Recent Questions, Troubleshooting & Support
Pentax K100 problem
Might be temporary lack of communication between lens and body - try cleaning the contacts on both gently with a lint-free cloth and a very little IPA.
It is an older lens and won't work on my camera
You've not said what model DSLR you have, but if you enable 'Use Aperture Ring' in the menu (check manual for exact setting), then it should work OK. If you use SR, you may have to input the focal length as well.
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