Reason for so many focus errors we see today?

Discussion in 'Digital Photography' started by RichA, Jun 23, 2009.

  1. RichA

    RichA Guest

    Plastic? Thermal expansion of plastic is much greater than metal and
    it could very well be why we are seeing focus issues that need "lens
    re-calibration" at service depots or that we see the need for in-
    camera focus fine-tuning. Even cameras and lenses that appear to be
    metal today may have plastic cells holding lenses, components in
    cameras. The cameras are produced in a control temp environment but
    that isn't real life use where temps can vary by 10's of degrees. I
    don't remember all metal AF SLRs needing focus fine-tuning (or having
    that facility) in the film days.
    RichA, Jun 23, 2009
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  2. RichA

    Pete D Guest

    You mean like with Canon D1's and L glass, cos they all have this problem?

    Sometimes Rich even I think you are an idiot.
    Pete D, Jun 23, 2009
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  3. RichA

    Doug Jewell Guest

    Bzzt, wrong.
    Coefficients of Linear thermal expansion (10^-6 m/m/C):
    Aluminium 23.1
    Magnesium 26
    Brass 19
    Stainless Steel 17
    Steel 11-13
    Glass reinforced Polycarbonate 22
    So there's not a lot in it.
    To put expansion in perspective, a 30 degree C change in
    temp on a 300mm lens made of Magnesium would be a mere
    0.2mm. Every lens I've ever seen has that much play in it's
    movement if not more.
    Because of the lower resolution of the sensor it wasn't as
    critical. Consider that a typical 35mm frame of film can
    resolve the equivalent of maybe 12-15MP. Modern DSLR's will
    cram 12-15MP on a sensor that has half the surface area, or
    are putting 24MP onto the 35mm frame. Because of this higher
    resolution they require more critical focus from the lens,
    and so errors that have always existed are now noticed.
    Doug Jewell, Jun 23, 2009
  4. RichA

    Don Stauffer Guest

    I can see that in open loop focusing, where you estimate the distance
    and dial that distance on lens. However, in any closed loop operation
    that source of error would not lead to a focus error. Also, there are a
    some plastics that have a thermal expansion less than many metals. So
    one cannot use generalities on this.
    Don Stauffer, Jun 23, 2009
  5. RichA

    Nobody Guest

    How thick is a film emulsion, versus the sensor plane of a chip. I would
    hazard to suggest a CCD/CMOS is more critical than film.
    Nobody, Jun 23, 2009
  6. RichA

    Charles Guest

    Agreed about the separate sensor array. However, the phase system (at least
    on some cameras) seems to be a hybrid servo (open loop for fast response,
    then switches to closed loop for the final tweak ... that's where the
    hunting comes in). There was a protracted debate about this on one Canon
    forum and it never was resolved, since some of the information is
    Charles, Jun 23, 2009
  7. True, but thermal expansion in the lens *is* within the AF closed
    loop, so that particular problem won't give rise to focus errors in an
    AF system.
    Chris Malcolm, Jun 23, 2009
  8. John Navas wrote:
    But, contrast detection relies on detecting a maximum, with no information
    about what direction the focus system should be driven. With phase
    detection you are seeking a zero, and the sensor output tell you which way
    to drive the focus, making a very fast, one-shot, open-loop movement
    possible. Iterate further /i/f there is any need. On the other hand,
    detecting a maximum can be quite a lot slower as you need to seek on
    either side of the maximum and make a best guess as to the peak. Swings
    and roundabouts for both systems.

    David J Taylor, Jun 24, 2009
  9. RichA

    Rich Guest

    That is for glass-filled. Non-glass filled is three times that. Not
    all polycarbs are glass-filled. It is
    also the differential of expansion between two different materials
    that is the problem. All aluminum and maybe all plasic on their own
    might be ok, but combined it causes problems, like the binding of lens
    elements when temperatures drop.
    Rich, Jun 24, 2009
  10. Most is open loop with respect to focus on the image sensor, and
    closed loop for at least the final approach on the AF sensor. There's
    a lot of misunderstanding and misinformation on this topic. Try the
    experiment of locking focus with a half press on something near when a
    big slow focusing lens is at infinity. Then complete the press, and
    after the lens has started to move, but before it has stopped, remove
    the thing it's focussed on from the view of the AF sensor. If the lens
    runs past that point without stopping then it's a closed loop system
    whose terminating condition has not been found. If it's a predictive
    system it will stop focused on the distance the object was earlier
    found to be at, even though it's no longer there.
    Chris Malcolm, Jun 24, 2009
  11. Please explain how lens errors are affecting focus detection in
    DSLRs, but not when using contrast detection.
    Sure, and P&S cameras focus near instantly, too. Pull the
    other one.

    Wolfgang Weisselberg, Jun 24, 2009
  12. RichA

    Don Stauffer Guest

    I was under the impression that the OP was talking about lens cell
    materials. The structure holding the image chip and the focusing chip
    is in the body of the camera, so a movement of something within the lens
    itself should affect both chips the same, shouldn't it?
    Don Stauffer, Jun 24, 2009
  13. I think rather than open loop "in general" you mean "at first". It's
    difficult getting at the technical details of phase detection
    autofocus, since it's both highly technical, and at least in some
    cases the details are regarded as a trade secret by the
    manufacturer. However, having read the workshop repair manual for one
    set of Canon lenses, which in passing mentioned various aspects of
    autofocus as relevent to a service engineer, the impression I was left
    with was that in those particular lenses, which were made for specially
    fast focusing, what happens is as follows.

    The AF phase detection makes a first measurement of the direction
    and approximate distance to focus. The focus motor control then sets
    up the parameters for the focus movement. If the distance is long
    enough to warrant it this is a four phase movement.

    The first phase is an acceleration of the lens up to its maximum speed
    of focus travel. This is terminated automatically when the motor is
    spinning at maximum speed, and the second phase begun. The second
    phase travels at maximum speed until it gets to the deceleration
    position, at which point it automatically starts the third phase, the
    deceleration ramp down to the final focus approach speed. That final
    fourth phase approach speed is slow enough to allow a closed loop
    approach to focus detection by the AF sensor which given the inertia
    of the lens etc.. can be stopped close enough to exact focus that
    there will be no overshoot.

    If the lens overshot the focus point it would have to
    reverse. Reversal of direction is expensive in time, so like a car
    approaching a stop light it decelerates at a speed at which it can
    reliably stop close enough to the line. In the case of these lenses
    "close enough" is a permissible error parameter previously read by the
    camera from a table in ROM in the lens, and which is alterable by
    service engineers in the lens focus calibration procedure. That
    parameter may also be adjusted by the camera depending on the chosen
    aperture. In that way smaller apertures can focus the lens a bit
    faster. That parameter may also incorporate an early stop factor to
    take account for the delays, inertia, etc. in the system, which mean
    that it will always stop a little bit further on than where it was
    when the software loop decided to stop the motor.

    When the deceleration phase finishes the lens is now not quite at the
    required focus point, and moving slowly enough to operate in closed
    loop manner, closed on good enough focus detection by the AF
    sensor. That'a the fourth final closed loop phase. It drives on at
    that speed reading the sensor, and stops when it detects near enough
    focus. Of course sometimes it won't detect focus, because the squirrel
    may have moved away or the light changed enough to lose the necessary
    contrast in the sensor while the lens was moving. So there is also a
    timing watchdog to terminate this loop if it goes on for too
    long. Other lenses/AF systems don't use a watchdog and simply keep
    running the lens until it hits the end stop. Simpler, but slower.
    Termination on the final focus approach loop on a focus find failure
    condition will automatically initiate another complete focus from
    scratch loop. This is how "hunting" happens, and whether it runs from
    end to end of complete focus travel, or dithers about the focus point,
    depends on whether the system uses timer watchdog or end stop failure,
    and what kind of outer loop control system is operating.

    Of course if the camera is doing a more complicated kind of focus such
    as predictive focus of a moving object, or approaching a compromise
    focus between the readings from a number of previously selected AF
    sensors, a more complicated procedure is used. I'm describing here the
    simplest long distance high speed AF procedure. If the focus point is
    close to where the lens is of course it will only use the final
    approach phase.

    The manual also made it clear that not all Canon lenses were
    sophisticated enough to use this sophisticated long distance high
    speed focus method. Simpler modelds have only a single speed focus

    I also get the impression that there is another two speed AF system
    used by some lens/body combinations, where the high speed checks the
    AF sensor only every several motor steps, and when it gets near slows
    down to checking every step.

    I think it's an oversimplification to talk of THE phase detection AF
    system of DSLRs, since in the search for highest speed and highest
    accuracy the camera makers have kept improving the sophistication of
    both their AF sensors and the focus control loops using them. In some
    cases we've seen a camera maker offer a camera model software upgrade
    which has improved focus performance not by fixing a bug but by
    introducing a more sophisticated control system.

    There are a number of different phase detection AF systems out there,
    of different levels of sophistication and accuracy. Some, I suspect
    these days at least most, always operate a closed loop approach of
    monitoring the AF sensor during the final approach to the focus point.

    Some camera makers have caused confusion on this question of whether
    their DSLR AF systems are open or closed loop by describing them as
    open loop because the closed loop they operate is closed around the AF
    sensor and not the image sensor, whereas their compact cameras operate
    a loop closed around the image sensor. In terms of control theory it's
    technically correct to describe DSLR phase detection autofocus systems
    as open loop, because they don't close the loop by measuring the
    achievement of the purpose of the system, which is an image focused on
    the image sensor. Instead they close the loop around the proxy of the
    AF sensor. Hence all the focus calibration problems of DSLR autofocus.

    Unfortunately that technically correct description of DSLR autofocus
    as "open loop" without qualification has misled a lot of camera users
    unfamiliar with the technicalities of control systems theory into
    supposing that that "open loop" means that no DSLRs use the AF sensor
    in a closed loop fashion.
    Chris Malcolm, Jun 25, 2009
  14. RichA

    Wilba Guest

    Thanks for that Chris. There's a lot of stuff there I didn't know.

    Is that "workshop repair manual" available online? I have a copy of the "EF
    50mm 1.8, 28mm 2.8, and 15mm 2.8 Service Manual", but AFAIK that doesn't go
    into the operation like you described.
    Wilba, Jun 25, 2009
  15. RichA

    Don Stauffer Guest

    There was a big patent fight between Honeywell and someone (I forget who
    right at the moment. I followed the case because I worked for Honeywell
    at the time. The lawyers even gave a noontime seminar one day for folks
    interested in the case). It was over the details of that particular
    autofocus method. The material disclosed in court is voluminous, but is
    at least part of public record.
    Don Stauffer, Jun 25, 2009
  16. RichA

    Guest Guest

    Guest, Jun 25, 2009
  17. No, the OP was continuing a decade long rant about something he has
    little knowledge of but finds it a convenient subject to troll with.
    Kennedy McEwen, Jun 25, 2009
  18. RichA

    Lloyd W. Guest

    [Demented ramblings deleted]
    The list may go on and on but so do you. This smells like that pompous
    blowhard, Semi-Yawning from the "Anything for a Perfect Shot" thread.

    Lloyd W., Jun 25, 2009
  19. Please don't feed the pests.
    John McWilliams, Jun 25, 2009
  20. RichA

    John Navas Guest

    Please trim huge quotes to just a relevant portion, not the whole thing.
    John Navas, Jun 25, 2009
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