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02-16-2011, 12:13 PM   #1
Junior Member

Join Date: Feb 2011
Posts: 34
Verifying that old Pentax-A prime lens (setting aperture) works ok

I know this is a stupid question, but I have to ask it anyway ;-) I have old Pentax-A prime lens. When I attach it to my camera (a setting selected), aperture seems to be wide open all the time. I mean that I cannot see aperture blades when looking at the lens. Is this normal behaviour? When lens are deattached I can see the actual aperture size.

If I change aperture value in camera (Av mode), nothing seems to change. Does camera physically move those blades and set the aperture only after the shutter button has been pressed or how should this work? I'm worried, if my lens work ok.

02-16-2011, 12:19 PM   #2

Join Date: Feb 2009
Location: Alameda, CA
Posts: 3,198
A lens with bayonet mount is always fully opened when attached to a camera body. The aperture blades in the lens close down during the exposure, then fully open again.

To verify the aperture works:

- Set the lens's aperture ring at A.
- Set the camera to Av mode.
- Keep the camera still (so that the scene doesn't change), use the camera to adjust the aperture. The shutter speed should change accordingly.


- Set the lens's aperture ring at A.
- Set the camera to M mode.
- On the camera, set the aperture to, say F/16.
- Look straight at the lens and hit the camera's shutter release button. You should see the aperture blades closing down.
02-16-2011, 12:23 PM   #3
Junior Member

Join Date: Feb 2011
Posts: 34
Original Poster
Thank's a lot for the quick response! I'll check those things you mentioned.
02-16-2011, 02:45 PM   #4
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Join Date: May 2009
Location: Twin Cities, MN
Posts: 2,869
you could also do the above (set to say f11 and 2 seconds in M mode) and set a longer exposure speed of a couple of seconds - press the shutter as you look into the lens and you will see it stop down if it is working properly.


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