Pentax 50mm f1.7 curiosity

Discussion in 'Photography Beginners' Forum' started by nic.nmd, Jun 26, 2010.

  1. nic.nmd

    nic.nmd TPF Noob!

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    Hey guys

    I have a Pentax k100d super. Recently I got an Asahi Pentax SMC 50mm f1:17 lens, which is quite old with no AF.

    I didn't expect any auto functions so I went straight to manual mode and everything works perfectly, I use the ring to adjust aperture and choose my own shutter-speed. However out of curiosity I tried aperture priority mode and something strange is happening. I set the ring to f1.7 and the camera chooses an appropriate shutter-speed and all shots come out fine. When I turn the ring to f22 the camera continues to fire with automatically controlled shutter-speed, but the shots that come out were obviously still shot at f1.7! :confused:

    So even with the aperture ring set to f22, when the camera is in av mode, somehow the aperture blades are still wide open - which suggests the the camera is somehow locking them open or something? It's just annoying because I would like to be able to use auto-metering in av mode at apertures other than f1.7!

    Anybody have any insight? Thanks in advance.

    Cheers, Nic
     
  2. LCARSx32

    LCARSx32 TPF Noob!

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    I don't know much about Pentax, but with my Canon 300D, I would expect to have to tell the camera that I was using f/22 by changing the setting (I have to hold a button and use the scroll wheel).

    Even though you are manually changing the aperture, the camera has no way to tell (since it's a manual lens). So it thinks you are still using it at f/1.7 and sets the shutter speed wrong. Look on your camera's display and see if the aperture is displayed at f/1.7 or at f/22. I'm willing to bet it's reading f/1.7.

    -Ray
     
  3. Flash Harry

    Flash Harry TPF Noob!

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    As your camera has no control over the lens aperture operation then its not possible for it to stop down the lens as you hit the shutter, all lens operate this way and are wide open for focus etc till either electronics or a manual trigger stop down with shutter operation, even if you manually stop down the aperture won't change when firing as no communication between body & lens. H
     
  4. usayit

    usayit No longer a newbie, moving up!

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    Yes..... olld manual lenses are operational on pentax dslrs depending on the lens. Enev modern pentax dslrs have the proper aperture linkages to control aperture diaphragm of the lens. So its possible to use the camera and old af lens in av mode.

    First, does the lens have an A marking on the aperture ring?
     
  5. usayit

    usayit No longer a newbie, moving up!

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  6. usayit

    usayit No longer a newbie, moving up!

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  7. Dwig

    Dwig TPF Noob!

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    RTM (appendix section titled "Using the Aperture Ring")

    That's the way its supposed to work. One of the economies in that model, and many other Pentax models, is that there is no support for using the f/stop ring on the lens when in an autoexposure mode. The camera has no mechanical mechanism to "feel" the position of the f/stop ring. It only supports electronic control of the aperture in any of the automatic modes.

    If your old lens is in the original K mount (no "A" on the f/stop ring) then you can only use it in Manual exposure. Using Av will cause the camera to only shoot at f/1.7 regardless of f/stop ring position and exposure error may occur, generally when the camera can't set an adequately high shutter speed for the lighting conditions.

    If your old lens is in Ka mount (there is an "A" setting on the f/stop ring) simply set "A" on the f/stop ring and use the body controls instead of the f/stop ring.

    Since you are shooting successfully in M with the lens you've apparently made the setting change in the menus to allow use of the f/stop ring.
     

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