SMC-A on K10D exposure

Discussion in 'Pentax' started by neil_swanson, Apr 18, 2007.

  1. I posted a while ago about A vs M lenses for K10D use. I found an SMC-A 28 2.8. Nice little lens, I like the
    field of view. I does however overexpose by @1-1.5 stops when used on A or M w/green button. If I set a
    little -ex comp. it works fine. Also weird is it doesn't seem to expose this way indoors, just outdoors. I
    thought that by going with the a version I wouldn't have the exposure errors I've read about with some
    older lenses. I have tried changing the custome setting for "use apeture ring" and the same thing occurs. I
    did notice one thing. Operating the diaphram with my finger the diaphram reacts very very slightly slower
    than a new 43mm. The difference is so slight I may be imagining it. Any ideas on the exposre or is it just
    the way I will have to use it. Thanks
  2. It's very likely that your aperture is a little stiff, as you have seen when operating with your finger. The diaphragm can't stop down quickly enough to give the correct exposure in bright light. When indoors, it doesn't have to stop down as much if at all.

    You can test for this: Take a bunch of pictures indoors and out with the lens wide open (f/2.8). Are they all correctly exposed? Diaphragm!

    I had this problem with a 35 2.8 and had to clean the aperture blades -- they were a little oily.
  3. I read your comments as saying you are using the aperture ring on the lens to control the
    f/stop setting. This is the wrong way to use these lenses. The K10D is designed to control
    lenses with an "A" setting via the body and is perfectly accurate when doing so, unless the
    iris mechanism is sticky.

    You should use Pentax-A lenses set to the A position on the aperture ring and forget
    about the green button stop-down metering. For Av mode, set the aperture with the
    thumbwheel. The green button does nothing in Av mode. For Manual mode, set the
    aperture and shutter speed with the thumb and forefinger wheels. The green button in M
    mode will set the exposure in accordance with the program line selected.

    I have two A-series lenses left (sold all my others) and both work perfectly with the K10D's
    metering when used as designed.

  4. Godfrey, thanks for the answers. All my first tests/trys were with the camera on A, lens on A
    and "use apeture ring" set to OFF. In manaul I got the same too bright exposures. Green
    button worked as it should but the final exposre was the same as teh A shots.

    So I was using it as it should be used. The diaphram is fine. Same shot/exposure taken with
    a 43 vs the 28A lens is very different. Also weird, if I take shot (or was it a preview shot) and
    then go to info instead of it saying 125 @f8 as an example it will say very very low shuter
    speeds, like maybe 1/6th? So that is weird but also no way is the image 6 stops off. More
    tests today.
  5. I suspect then that your A28 needs service. MIght have a sticky aperture, might have some
    problems in the electrical connections from aperture ring to camera body. It should work
    identically to the FA43 in terms of exposure. All of my A-series lenses did, and the two I have
    left do as well.

  6. Either yor lens or body is faulty. I have a A 28 F2.8 that works perfectly with my ist D and ist Ds.

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