Infiniity on 12-40mm

Discussion in 'Native Lenses' started by Imjinman, Jan 25, 2014.

  1. Imjinman

    Imjinman Mu-43 Regular

    54
    Sep 22, 2011
    Sydney
    I have the Oly 12-40mm lens but with manual focusing I notice that you can go past infinity vs the red marker line on the lens. What's the deal on that?? Why?

    Thanks for any thoughts
     
  2. walter_j

    walter_j Mu-43 Veteran

    364
    Sep 10, 2013
    Hagwilget, B.C., Canada
    Walter
    I think in cold weather the barrel contracts, so you'd have to go past mark to focus to infinity.
     
  3. BobbyTan

    BobbyTan Mu-43 Top Veteran

    560
    Dec 26, 2013
    Long Beach, CA
    Most lenses are made this way - so you do have to make sure that you do not go past the infinity mark when you are in MF mode.
     
  4. Imjinman

    Imjinman Mu-43 Regular

    54
    Sep 22, 2011
    Sydney
    But why??
     
  5. quatchi

    quatchi Mu-43 Veteran

    326
    May 17, 2012
    Munich, Germany
    I have read once, that "past infinity" is required for AF. The camera pushes the lens behind the perfect focus in order to recognize that it gets worse (= not so sharp) again. It starts out somewhere and randomly decides on a direction to go (closer or further away). It then periodically checks how sharp the images within the focus point is. In case it gets sharper each iteration it goes on. It repeats the pushing forward until the previous images appeared to be sharper. the camera then tells the lens to go back one step and locks the focus. That's also the reason why AF on the newer m43 sensors is faster compared to the older ones (e.g., Pana G1). The sensors have a higher (double?) refresh rate, meaning that the sensor delivers a new images for processing twice as often.

    You can see the AF process when a lens is hunting in the dark and no real focus can be locked. The camera goes through the complete range in order to find the "sweet spot".

    (Disclaimer: I read about this some where and it seemed sound to me. But I have no real knowledge about that whatsoever).
     
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  6. janneman

    janneman Mu-43 Veteran

    414
    Dec 6, 2012
    Netherlands
    Jan (John) Kusters
    focus 'beyond infinity' became normal when more exotic types of glass were used, back in the late seventies. Some of those glasses are used because they have a very high refractive index. Barrel and glass are prone to temperature induced expansion, and those high refractive types of glass even more so. To make sure they would still focus till infinity with higher temperatures, lenses were allowed to focus past infinity. At first it was just for those lenses using that type of glass, later it became normal for all lenses to focus past infinity. I am not sure whether that was with the introduction of AF motors, but it might very well be for the reason Quatchji wrote (I came rather late to the AF party :smile:). I can also imagine AF-motors not being to happy with forced stops at the ends.
     
  7. Ricoh

    Ricoh Mu-43 Top Veteran

    906
    Nov 2, 2013
    UK
    Steve
    If you want rapid response in a control system (which we do) then a degree of under damping will be built-in. And as someone said the motor drive will overshoot slightly, then back off, repeating a few times about the set point until the error signal is zero (the error will be integrated out). All done in a fraction of a second. It seems logical to assume that the camera has an outer control loop and the lense has a secondary - inner control loop. The camera acts as the auto pilot and the lens as the slave.
    If the motor could not drive beyond infinity, I.e if the physical end stop was at infinity, the servo system (electronics, motor, gear train) would attempt to drive against the buffers. First thing to suffer would probably be the electronics, the drive FETs, then the motor, due to over- heating.
     
  8. EarthQuake

    EarthQuake Mu-43 Top Veteran

    836
    Sep 30, 2013
    I don't think the typical "AF lenses focus past infinity" applies here:

    1. The 12-40 isn't a true mechanical coupled focusing lens, even with the MF ring pulled back, it is not actually mechanically focusing the lens, instead it sets the focus electronically (hence the slight lag).
    2. The lens can't AF when the ring is back anyway, so I can't see any reason to allow focusing past infinity in this mode.

    This is true of all the snap-focus Oly lenses as well. Its really strange that there is no hard stop for infinity, as this is one of the biggest benefits of having a fixed focus scale vs a continually spinning focus ring. In theory, any variance in the actual position of infinity focus could be electronically calibrated (again, as focus is still fly by wire) so I don't think that is a viable reason to explain it either.
     
  9. Imjinman

    Imjinman Mu-43 Regular

    54
    Sep 22, 2011
    Sydney
    Many thanks all- hadn't realised it was quite so complicated and controversial!
     
  10. Trankster

    Trankster Mu-43 Regular

    104
    Jan 19, 2013
    Irvine, CA
    Michael
    I asked the same question a couple of months ago and found this answer in the Panisonic section https://www.mu-43.com/showthread.php?t=50612 My conclusion is the lenses are designed primarily for AF and the range markers including infinity are approximations requiring you to focus by eye even at infinity (manual focus).