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20mm panasonic - EP2 stopping down

Discussion in 'Native Lenses' started by AceStar, Jan 9, 2011.

  1. AceStar

    AceStar Mu-43 Regular

    26
    Jan 9, 2011
    Melbourne, Australia
    Hey, I've noticed that my E-P2 is stopping down the 20mm 1.7 to less than its full aperture whenever it's in live view mode. I'd estimate it's probably f/4 or a bit smaller. When autofocusing, it briefly opens up to full f/1.7 but then stops back down once it's found focus. This doesn't affect picture taking at all, but I'm just curious as to why the camera does this.

    One positive side-effect is that you can "hear" when the AF has locked on because of the click of the aperture as it opens up and then closes back down.

    However, surely it isn't good for battery life or the aperture mechanism for it to be constantly opening and shutting the aperture whenever it performs an autofocus. You'd also think that it'd be better for the live view preview to have the least noise possible.

    Anyone else noticed this and have a smart explanation?
     
  2. dhazeghi

    dhazeghi Mu-43 Hall of Famer

    Aug 6, 2010
    San Jose, CA
    Dara
    Basically, it's trying to keep the amount of light hitting the sensor from exceeding a certain threshold (and resulting in a blown out image in the viewfinder/LCD, or incorrect metering). So when you move from a dark to a bright area, it will stop down, and in the reverse case, it will open up.

    They could of course keep it always stopped down, but this would result in a noisier image in the viewfinder/LCD.

    As you've noted, it doesn't affect final exposure. It is a bit annoying though.

    Cheers,

    DH

     
  3. AceStar

    AceStar Mu-43 Regular

    26
    Jan 9, 2011
    Melbourne, Australia
    I don't think that's the case, at least not the one I've observed.

    It always does this stopping down even in a relatively dark room at night. When it's autofocusing it opens up briefly and you catch a glimpse of a less-noisy live view for a moment but then it stops down again once focused. I haven't noticed it varying the amount by which it stops down, it seems to always be the same. Also, if it were anything to do with there being too much light for the sensor to work, then why would it open up the aperture while autofocusing?

    I haven't done more scientific analysis; I believe it has nothing to do with the light in the room or the aperture the camera chooses when taking the shot - which, in that dark room, is f/1.7 (even at ISO800).

    I don't know if it's a bug (or mild glitch resulting on using a Panny lens on an Oly) or there is some technical reason for it - I'm assuming there is a reason since it is so consistent. Maybe the sensor uses less power when it's receiving less light (is that physically possible? You'd think it'd be the other way around).
     
  4. dhazeghi

    dhazeghi Mu-43 Hall of Famer

    Aug 6, 2010
    San Jose, CA
    Dara
    You're right, there is more going on. Basically, most lenses are designed to be viewed and focused wide open, and only stop down just before an exposure is taken. For a few fast lenses however, Panasonic seems to have decided that viewing should be at an intermediate aperture (iirc f/2.8). Focusing and metering however are still done wide open. And confusingly, the aperture is also varied somewhat depending on ambient light levels.

    You can of course use the DoF preview button to tell where the viewing aperture is in relation to the shooting aperture.

    The short of it is though that this behavior is not a malfunction - it happens with the 20/1.7 on both Panasonic and Olympus bodies. It also occurs with the Panasonic 25/1.4 (the 4/3 lens).

    Cheers,

    DH