Nikkor 85mm f/1.8 autofocus anomaly

Discussion in 'Photography' started by Pete A, Sep 12, 2011.

  1. Pete A

    Pete A Guest

    This is definitely not a troll post. I am fully aware that similar
    questions have resulted in useful answers being buried under spam noise
    and flame wars. I have spent dozens of hours during the last year
    reading the results of endless web searches to resolve this problem, to
    no avail.

    My Nikkor 85 mm f/1.8 AF (non D version) refuses to focus correctly on
    my D700 set to single servo AF. I'm not talking about a minor focus
    error, the error makes any aperture above f/5.6 totally unusable in
    many situations. I never noticed this problem on my F-601 AF film
    camera.

    So far, I have managed to adjust the camera AF fine-tune, in continuous
    servo AF mode, to give extraordinary focus accuracy and consistency;
    beyond my needs and far beyond my expectations of a screw-drive focus
    servo mechanism with a relatively cheap lens.

    Well, yes, the solution may seem obvious: use it only in AF-C mode.

    This is not the only screw-drive AF lens I have this problem with. It
    is by far the worst and it does cause a problem because AF-C mode often
    takes an inordinate amount of time before it has settled on the correct
    focus.

    Note: I've set the camera away from its default setting such that it
    delays shutter release until it has confirmed AF - I want AF accuracy,
    not a fast sequence of blurred shots.

    I've never had this problem using the following wide-open in
    single-servo AF mode: modern 50 mm f/1.4 AF-S; old screw-drive AFs: 180
    mm f/2.8, 35-135 mm zoom (even in it's macro mode).

    Shared experiences are most welcome,

    Pete
     
    Pete A, Sep 12, 2011
    #1
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  2. Is it useable at an aperture not "above f/5.6" because it
    works somewhat, or just because the Depth Of Field is
    great enough???
    So the lens' AF works... Because the lens does not
    change anything with the switch from Single to
    Continuous mode.
    So you have other lens that do the same thing...

    First, take a cotton swab and use isopropyl alcohol to
    clean the electrical contacts on the camera and on the lens.
    You might try a scrubbing with a mild solution of dish soap
    and then do a very thorough cleaning with alcohol afterwards.

    If that doesn't change anything, I think I'd give the
    camera and the lens together to Nikon's repair service.
     
    Floyd L. Davidson, Sep 13, 2011
    #2
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  3. I have exactly that lens (AF, non D, 1.8) and use it extensively on my
    D700 at full aperture. I do however use C-AF much more (generally I
    leave the camera set that way and use my thumb on the AF-ON button to
    control when focusing takes place; I find that the best of both
    worlds). But I have sometimes used S-AF, either accidentally, or when I
    need AF-assist illumination (like taking flash pictures outdoors at
    night) and have not noticed that it fails horrible. Flash outdoors at
    night might be stopped down far enough to hide a problem, though.
    User-preference item; I use most AF-C, but I won't begrudge your trying
    to us AF-S :) .
    That's strange; I find mine snaps in very fast indeed, and can for
    example track a musician's moving head fairly tightly framed.
    I use shutter priority in AF-C, focus priority in AF-S.
    I don't have other Nikon screw-driver lenses; I don't recall any
    problems with various Tokina or Tamron screw-drive zooms though.

    I shoot at max aperture a LOT; stopping down to f/5.6 even is a rare
    event for me.
    Now you've got me curious about S-AF, I'll try to do some checks next
    time I'm playing with that camera.
     
    David Dyer-Bennet, Sep 13, 2011
    #3
  4. Pete A

    Noons Guest

    Pete A wrote,on my timestamp of 13/09/2011 7:49 AM:


    Hmmm, weird. I'd take the lens to service: it's bound to have slack gearing.

    Given that's the non-D version, any sugestion that it's the electrical part that
    is stuffed, would be silly. The only thing I can think of is the slackness in
    the gears causing the AF-S mode somewhat sudden stop causing an overshot of the
    perfect focus point. Hence why AF-C works: it keeps trying until it gets it right.

    Here is a test to try: fine-tune focus in the camera with AF-S if at all
    possible, then switch to AF-C. Does it then still take heaps of time to focus?
    If so, it's slack gearing. If not, it's something else. Likely a bad contact of
    the focus screw with the drive shaft?

    All of the above need a visit to service, hence my initial sugestion: it doesn't
    sound like you're doing anything wrong or the camera is incorreectly setup.

    HTH
     
    Noons, Sep 13, 2011
    #4
  5. Pete A

    Pete A Guest

    At f/5.6 and f/8 the DOF masks the focus error whereas from f/4 and
    wider the error becomes increasingly unacceptable (at a focus distance
    of about 2 m, IIRC).
    Two other lenses. Initially, I put it down to the AF locking onto the
    wrong part of the scene - operator error. I discovered the problem by
    using a target of black text on white paper, tripod and remote release.
    However, in most practical situations the focus error of these two
    lenses is too small to bother me.

    Repeating the test with the 85 mm at f/1.8 was interesting: in AF-S
    mode the text was unreadable in each of 10 shots; in AF-C mode 9/10
    were perfectly acceptable and one was marginal (may have been caused by
    operator error).

    I don't like doing these tests because they induce an unpleasant
    feeling of mistrust for a few days afterwards. The last thing I want is
    to become obsessive about it. I dream of the day when the only thing
    spoiling my photography is an AF error :)
    Many thanks, Floyd. Now I know how to clean the camera and lens
    contacts I'll have a go. I'll also carefully check the lens
    spring-loaded contacts and focus drive. I rarely use the 85 mm and it's
    quite old so it may be suffering from lack of use.
     
    Pete A, Sep 13, 2011
    #5
  6. Pete A

    Pete A Guest

    Thanks, David, that's very useful info. I've setup both Shooting Menu
    Bank C and Custom Setting Bank C for my tripod night photography. I use
    C-AF mode and focus obtained only via the the AF-ON button. It works
    superbly with all of my AF lenses. Unfortunately, the light is too low
    for contrast-detect AF in Live View, but it is just high enough for
    phase-detect in C-AF mode. My mind boggles at how well it works because
    there isn't enough light to manually focus using an old
    split-image/macroprism screen.

    I had never considered using the AF-ON button for hand-held shots so
    I'll setup a spare bank and give it a try.
    Like you, I generally use single-servo AF either by mistake or because
    I need the AF illuminator.
    I shall try Floyd's cleaning advice. My lens snaps focus very quickly
    in S- and C-AF. In C-AF, after a very short delay, it hunts for a
    variable amount of time before settling down to micro-fine adjustments.
    Sometimes this take a second or so and the focus indicator in the
    finder confirms this strange behaviour.
    I changed the setting simply to get my 85 mm to focus correctly. None
    of my other lenses seem to need it, implying that they manage to
    acquire focus quicker than I need.
    I use my zooms wide-open a lot of the time because their max. apertures
    range from f/3.3 to f/5.6. Luckily for me, their focus accuracy and
    speed is astonishing and none of them have required an AF fine-tune.

    Using Auto-area AF mode with non-D lenses I find the camera focuses on
    eyebrows and/or nostrils instead of the eyes, but I haven't yet had the
    chance to see if my more modern lenses behave differently. The camera
    manual clearly states that D or G lenses are required to utilise its
    enhanced functionality.
    I would be very interested to know your findings.
     
    Pete A, Sep 13, 2011
    #6
  7. Pete A

    Pete A Guest

    Thanks, Noons. I shall get some isopropyl alcohol and clean the
    contacts because they probably need it.

    What you've said has been in the back of my mind for ages, I think the
    focus error could be caused by a mechanical issue.

    Earlier today, I carefully checked the manual focus control. It has
    some play, but why shouldn't it? It is an auxiliary to the screw-drive
    internal focus positioning mechanism. I could easily feel each gear
    tooth as it engaged, but it seemed impeccably smooth with very low
    friction. What I did notice was an inertia that seems very high for an
    an internal-focus lens, but the inertia was not like that caused by the
    grease in my manual focus lenses.

    Judging by how hard the D700 kicks as it's trying to focus this lens,
    there is a strong possibility that the momentum of the large manual
    focus ring is knocking off the focus after the camera has stopped
    turning the screw. Free-play would doubtless make this worse.

    My F-601 has a much less powerful AF motor than the D700 so although it
    takes longer to focus this lens, the focus ring momentum is much lower.
    I never got an out of focus shot using this combination in single-servo
    AF unless the subject was moving.

    It was impossible to fine-tune the D700 in single-servo AF because the
    direction of the focus error appeared to be random. In C-AF, I've
    nailed it.

    My guess to date is that my lens was either insufficiently greased at
    the factory or not greased at all.

    I need to be realistic and decide on the right time to send my camera
    and lenses for a service. It's a hard decision because the cost will be
    about that of a nice new lens of some sort.
     
    Pete A, Sep 13, 2011
    #7
  8. Pete A

    Noons Guest

    Bingo. That must be a mechanical problem on the gearing, then.

    or a nice 2ndhand one in epay? ;-)
     
    Noons, Sep 13, 2011
    #8
  9. Pete A

    Pete A Guest

    Update. Trying to obtain some isopropyl alcohol from local chemists was
    an amusing experience. Luckily, one of the many pharmacist actually
    understood that 99% IPA is good for cleaning electrical contacts - some
    of the others got the idea that I wanted to sterilise the camera and
    lens so they insisted that 70% IPA was the best option for killing
    bacteria. Oh well, I've learnt a few things in the process.

    Cleaning the contacts hasn't resolved the problem. However, it appears
    from what I've since read that this lens was designed to give
    reasonably fast rather than totally accurate focus with older bodies.
    Its tendency to hunt on the F-601 used to annoy me, but AF was accurate
    enough once it got there. On the D700/D3 series it's very fast and
    doesn't hunt, but it seems to be a lot less accurate. By f/5.6 AF is
    more than good enough, but so are my much more versatile zooms.

    The new AF-S 85 mm f/1.4 is reported to be slower to focus yet nails
    nearly every shot wide-open. This new design makes sense to me because
    my f/1.8 has too much coma at f/4 and wider for it to be generally
    useful. The new AF-S has very low coma wide-open making it a much
    better general-purpose fast optic plus it retains the charming
    characteristics of the older 85 mm Nikkors.

    Looks like I need to trade in my long-serving 85 mm for the latest
    version. I shan't miss it because its ghosting is by far the worst of
    all my lenses. Cleaning up 15 to 20 ghosts in each of its night shots
    isn't my ideal way of having fun with photography.
     
    Pete A, Oct 31, 2011
    #9
  10. I use mine mostly at f/1.8 and fairly close up, and it focuses the
    eyeballs just fine. I don't have more than an inch of depth sometimes.
     
    David Dyer-Bennet, Oct 31, 2011
    #10
  11. Pete A

    Pete A Guest

    Just re-read your earlier post on this. With AF-C my 85 mm has a quirky
    (sometimes jittery, sometimes very fast) yet astonishingly accurate
    focus; with single-servo AF it is the only useless lens I have.

    Having recently got out-of-focus shots of a fireworks display due to my
    ED Nikkors manually focussing beyond infinity (yep, my stupid choice of
    lenses to carry), can anyone confirm that the AF-S 85 mm f/1.4 has a
    hard stop at infinity? It ought to have because there's no mention of
    it using ED glass, but times have changed resulting in wider production
    tolerances becoming a "user beware" caveat.
     
    Pete A, Nov 3, 2011
    #11
  12. I do run in AF-C essentially all the time (using the AF-ON button with
    my right thumb, I can then get AF that's continuous until the instant I
    life my thumb, and resumes again whenever I want).

     
     
    David Dyer-Bennet, Nov 3, 2011
    #12
  13. Pete A

    Robert Coe Guest

    : On 2011-10-31 23:16:52 +0000, David Dyer-Bennet said:
    :
    : > I use mine mostly at f/1.8 and fairly close up, and it focuses the
    : > eyeballs just fine. I don't have more than an inch of depth sometimes.
    :
    : Just re-read your earlier post on this. With AF-C my 85 mm has a quirky
    : (sometimes jittery, sometimes very fast) yet astonishingly accurate
    : focus; with single-servo AF it is the only useless lens I have.
    :
    : Having recently got out-of-focus shots of a fireworks display due to my
    : ED Nikkors manually focussing beyond infinity (yep, my stupid choice of
    : lenses to carry), can anyone confirm that the AF-S 85 mm f/1.4 has a
    : hard stop at infinity? It ought to have because there's no mention of
    : it using ED glass, but times have changed resulting in wider production
    : tolerances becoming a "user beware" caveat.

    If your camera supports AF microadjustment, I wouldn't think you'd want your
    lenses to have a hard stop at infinity, since that limits the range of
    correction for front-focussing. I suppose that in principle you could design a
    lens whose infinity stop was dependent on the preset microadjustment point,
    but it would figure to be expensive.

    Bob
     
    Robert Coe, Nov 4, 2011
    #13
  14. Pete A

    Pete A Guest

    I thought the AF micro adjustment was to compensate the AF sensing
    system rather than an error in the lens mount to sensor distance. If
    any Nikkor with an infinity stop has a front-focussing problem at
    infinity it will be adjusted under warranty. The infinity position
    should be totally reliable on both MF and AF bodies. Likewise, a camera
    body will be adjusted or replaced under warranty if it gives a focus
    error with lenses set at infinity.

    Mirror alignment is critical in MF bodies for the finder to be an
    accurate focus guide. Sub mirror alignment is extremely critical in AF
    bodies that have a choice of focus points. Sometimes a new camera has a
    misaligned mirror and high-usage cameras may suffer from mirror pivot
    wear.

    I've never noticed an infinity focus error on the D700 using a lens
    with an infinity stop and the aperture wide-open, but my lenses
    probably aren't sharp enough to show it. Using the 85 mm AF at f/1.8 on
    a subject a few hundred meters away, I've noticed that AF takes it just
    slightly away from the infinity stop. I couldn't detect it was sharper
    than manually setting it to infinity, which doesn't prove anything
    except that the infinity stop is accurate enough to be useful for
    taking distant subjects at night. It's hyperfocal distance using a 10
    micron CoC is 406 meters so my subject was just about "in-range" with
    the lens at infinity taking the combination of lens sharpness and
    camera resolution into account.

    It seems that AF micro adjustment has two purposes: I guess it reduces
    the number of returns under warranty of both bodies and lenses; it is
    extremely useful for fine-tuning a lens for the working distance and
    aperture most often used by the photographer. As far as I'm aware, only
    Hasselblad lenses and bodies automatically correct for focus shift with
    aperture.
     
    Pete A, Nov 4, 2011
    #14
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