EF 50/1.8 AF Experiment?

Discussion in 'Digital SLR' started by Wilba, Dec 20, 2009.

  1. Wilba

    Wilba Guest

    It doesn't matter whether you stop and recheck or hold a half-press all the
    way through, you see the same thing.
    I find that a very artificial concept, like saying that on average a man who
    is one half of a heterosexual couple is half female.
    New and interesting, but a long shot I think. :- )
    I haven't gone very far with stopping down (maybe up to f/5.6 or so), but I
    don't see a drift. We know the lens applies a looked-up correction for that
    in AF.
    So we come back to where we were three weeks ago. If the remote Live View
    image (at 100% on the PC monitor) is in optimal focus, either by AF or beep,
    where's the shift? There is no shift that I can see. The misfocus happens
    under identical conditions, except we started with the POF on the other side
    of the subject. I still cain't see no shift.
    Well, we can eliminate the influence of the correction tables by doing the
    beep test, and we get the same results (same two focus points) as we do with
    AF, so where's the shift?
     
    Wilba, Jan 20, 2010
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  2. A hysteresis band is a kind of dead band, but a dead band isn't
    necessarily a hysteresis band, because a hysteresis band is
    directional.
     
    Chris Malcolm, Jan 20, 2010
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  3. Different kind of directional. When a system is to seek a target and
    stop when it gets there there can be a problem of dithering or hunting
    close to the target due to tiny movements over the threshold, possibly
    due to noise. So some kind of delay between on-target and off-target
    conditions is imposed, often by giving some kind of snap action over
    the target, so that it has to run past the target to be triggered as
    target reached, and then under-run it by some amount to be triggered
    as off-target.

    It's a kind of snap action. In the past often implemented with
    snap-over springs on a mechanical switch. Now often implemented by
    control software.

    Wilba was originally describing something very like that, but since
    other cameras behave the opposite way in that situation I wasn't
    convinced he hadn't got it back to front, and didn't want to get
    involved in that argument until it got a lot clearer what had in fact
    happened.
     
    Chris Malcolm, Jan 20, 2010
  4. Wilba

    Wilba Guest

    Yeah, sure. :- )

    I took many 'nother looks, but once you get an idea stuck in your head...
    :- )

    That wasn't the only bit, and far from the most important bit, that Paul
    wasn't getting.
     
    Wilba, Jan 21, 2010
  5. Wilba

    Wilba Guest

    I don't, but David seems to, unless I'm reading him wrong.
    Whatever. Not my term.
     
    Wilba, Jan 21, 2010
  6. Wilba

    Wilba Guest

    The AF behaviour and the beep test finding the same focus points are still
    as originally described, only the directionality of the beep test that has
    been corrected.
     
    Wilba, Jan 21, 2010
  7. Wilba

    Wilba Guest

    But you did say, "you will likely stop at a different position, always
    within that dead-band", which doesn't happen with this system (it always
    stops at the ends of the beep band).
    Correct. Even CDAF has shot-to-shot variation. PDAF is a touch more
    variable. But we're not talking about micrometric precision, just not
    focussing between the end points of the beep band. The beep test shots
    match, so it seems that PDAF triggers focus confirmation on the transition,
    on entering or leaving the beep band.
     
    Wilba, Jan 21, 2010
  8. Wilba

    Wilba Guest

    That sounds like exactly what Chris has been talking about - a static focus
    error due to the mismatch of the lens exit pupil and the AF sensor's
    required exit pupil ("effective aperture"). I wonder what would happen if
    you put on an external aperture which reduced the exit pupil to that
    f/2.8-f/4 range.
     
    Wilba, Jan 21, 2010
  9. Wilba

    Wilba Guest

    Except it does the exact opposite - it makes front-focus out of gross
    back-focus.
     
    Wilba, Jan 22, 2010
  10. Wilba

    Wilba Guest

    Well slap my kneebones to the ground! That's exactly where we started a
    month ago. :- )
     
    Wilba, Jan 23, 2010
  11. Then my guess is that this is the programmed behaviour of that
    camera's AF control algorithm. As someone suggested earlier, while not
    the best behaviour for accuracy, it's probably the best behaviour for
    speed, and most people don't seem to want more AF accuracy if it slows
    things down.

    Have there been any updates to the camera's OS? Does the camera maker
    boast of superior AF in more expensive or later models in terms which
    give any clues? Have any details (or educated speculations) been
    published in technical forums about how that camera's AF is
    programmed?
     
    Chris Malcolm, Jan 24, 2010
  12. That sounds like more movement, which sounds like it wouldn't be the
    fastest AF, but if it's the result of an estimated dead reckoning
    movement from a distance which involves that kind of error, then
    improving it would involve another iteration through the focus
    measurement process, which would slow the AF down. So that behaviour
    could still be the result of going for AF speed.

    AF speed seems to be what most people want, and the camera makers seem
    to prefer to offer manual focus, or manual focus trimming, for best
    focus precision, rather than offering two modes of AF, one approximate
    and fast, one precise and slow.
     
    Chris Malcolm, Jan 24, 2010
  13. Wilba

    Wilba Guest

    Not AFAIK.
     
    Wilba, Jan 24, 2010
  14. Wilba

    Wilba Guest

    I don't think so, because when _I_ do the focussing movement at very low
    speed (the beep test), I get the same two focus points as AF does. So it's
    however focus is confirmed that determines where the focus ends up. One part
    of that is how the light from the lens is turned into two signals by the AF
    sensor, and another is how the phase difference between the two signals is
    deemed to be close enough. Somehow that comes out as two distinct points,
    and the one you get can be predicted from the initial conditions.
     
    Wilba, Jan 24, 2010
  15. I'm not sure why you feel it necessary to make that comment, because
    the rest of my post went on to deal with the various circumstances in
    which that can be the case, and why. It's a complex topic to which
    there is not a simple single answer.
     
    Chris Malcolm, Mar 23, 2010
  16. Not me, some odd system delay.
     
    Chris Malcolm, Mar 23, 2010
  17. Probably your system then, because the newsserver doesn't change the Date-
    header.
     
    Robert Spanjaard, Mar 23, 2010
  18. Wilba

    Wilba Guest

    Interesting. I don't have access to a 50/1.4 or 85/1.8, but my 50/1.8 II
    focusses differently depending on which side of the subject it's coming
    from, leading to the conclusion that the central "enhanced precision" PD AF
    sensor is sufficiently confused by the fuzziness for it to confirm focus
    over a range of the order of the DOF.
     
    Wilba, Mar 25, 2010
  19. Setting Macro Mode used to do that on the Nikon 990.
    I haven't checked it on the DSLRs.

    David
     
    David J Taylor, Mar 26, 2010
  20. Wilba

    Mikewhy Guest

    That doesn't sound at all convincing. PDAF works the same way split-prism
    focus screens work. The aberrations would have to be pretty bad to prevent
    the AF from focusing. The 50/1.8 is about as cheap mechanically as you'll
    find, but the optics are quite good. I don't doubt for even a moment that AF
    inaccuracies are due solely to the coarse stepper motor and whatever other
    mechanical compromises they made. A certain amount of backlash sounds
    consistent with the directionality you saw.
     
    Mikewhy, Mar 26, 2010
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