View Full Version here: : Help with Clip-in filter and EOS to PK lens

31-10-2017, 10:08 PM
Hi Folks

I am using a PK to EOS adaptor to mount a 300mm Pentax lens to my old Canon 40D. The lens mounts securely to the camera with the adaptor, but the lens has a protruding aperture tab that is preventing me from using a clip-in light pollution filter because the tab hits the face of the filter once mounted in the camera body. Anyone know if there is a workaround? I could use a front filter, but at 77mm, it is an expensive option Ė also I am of the understanding that front filters donít perform as well as clip-ins (I could be wrong).

Any help appreciated!

Thank you

Regards Steve

01-11-2017, 01:27 AM
Can't the tab be kept pressed, by taping it, or something like that? If not, probably your only chance is to remove the tab, as you cannot modify the lens distance from the mount, or it won't focus to infinity.

01-11-2017, 10:08 AM
This is always a problem with older mechanical aperture lenses.
If the lens is never going to be used for a manual camera, I would file/cut the offending 'tab' away sufficient to clear the filter. that should leave enough to manually stop the lens down with whatever method you're using.


01-11-2017, 03:33 PM
Thanks fsr, yes thought of removing it but there goes my resell value to someone using it for non-astronomy. There is not only the aperture tab but a raised (to the same height as the tab) metal shoulder that protects the aperture tab from damage. Its actually part of the inner body of the lens. So looks like my only option other than removing it is a filter at front.

Thanks for your thoughts... cheers Steve

01-11-2017, 03:39 PM
Thanks for your comments Trevor - yes I figured as much. I read that a filter on the front of the lens is not the way to go, but this was in context of a wide-angle lens discussion. Why do they make asrto filters at 77mm (and larger) if they don't perform in front of the lens? Any thoughts?

Perhaps I'll start another thread and as the question.

Thanks again Trevor