Auto focus accuracy

Discussion in '35mm Cameras' started by fw25hp, Jul 15, 2003.

  1. fw25hp

    fw25hp Guest

    I've got a few questions hopefully someone can help answer for me.

    The other day, I was at a camera store looking at a 24-85mm zoom
    lense. With the lense mounted on a Nikon F5 and center focus area
    selected, I set the lense to 24mm and focused on a large sign at the
    opposite end of the store (the sign was at the center of the frame,
    directly where the selected focus area was). The sign came into
    focus, but seemed a little bit fuzzy. I then zoomed in to the 85mm
    setting. The sign looked blurry/out of focus. So I pressed the
    shutter button lightly to re-focus and the sign looked sharp. I then
    zoomed back to the 24mm setting. The sign looked sharper than it had
    when I originally focused on it. When I lightly pressed the shutter
    button, the camera made a slight adjustment which made the sign look
    slightly fuzzy again.

    I also tried a 24mm f/2.8. Again, when I tried to focus on the sign,
    it ended up looking a bit fuzzy. I also tried focusing on various
    objects at different distances. When I was close to an object (object
    filled up most of the frame), the focus seemed fine. But if it was
    some distance away (about 12 ft or more) the image looked slightly
    blurry through the viewfinder. I had to manually focus the lense to
    get everything to look sharp.

    So here are my questions:
    1. Is the behavior I observed for the zoom normal?

    2. When using a zoom, is it best to zoom in, focus, then zoom back out
    to the desired focal length?

    3. Do you always need to manually focus a lense to get the sharpest
    image (how accurate is the auto focus system supposed to be)?

    Thanks for any information.

    fw25hp, Jul 15, 2003
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  2. fw25hp

    auspics Guest

    This is a very interesting subject.
    Many years ago when Zooms first arrived on the scene (before auto focusing
    cameras) it was good practice to focus at max magnification and then pull
    back the zoom to compose the shot. Yes... I know cars go a lot faster now!
    Recently I purchased a Tokina 19~35 lens for my EOS3. I only wanted the 19mm
    part but didn't want to pay for a prime lens. I shot a few frames of the
    scene I wanted and found it was out of focus. I condemmed this lens straight

    I pulled it out again last week and put it on my new 10D EOS. I discovered
    that it was a good enough lens anyway... Provided I used manual focus at
    35mm and then pulled back on the length to 19mm, Just like the old days!
    auspics, Jul 15, 2003
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  3. fw25hp

    gsks Guest

    Look at this

    gsks, Jul 15, 2003
  4. fw25hp

    John Miller Guest

    Key question: what was the brand of the lens? I've only seen this with a
    John Miller, Jul 15, 2003
  5. fw25hp

    Mxsmanic Guest

    Yes. I've seen the same thing. AF systems appear to focus well enough
    to be within the usual definitions of acceptable sharpness, but they are
    rarely designed to hunt for the absolute best focus possible, since that
    would slow the AF process.
    No. There's no guarantee that the lens is designed to hold focus as you
    zoom. Cinema and TV lenses are designed to do this, but still-camera
    lenses may not be. In moving pictures, you have to be able to hold
    focus as you zoom, but in still pictures, you can afford to refocus if
    you change focal length, so there is no guarantee that a given zoom lens
    will focus at the same point for the same focus setting at all focal
    lengths. The differences are usually pretty tiny, though, so your
    method may not do any harm, even if it doesn't necessarily do any good.
    Yes, although you need good eyesight and possibly a loupe, depending on
    the AF system to which you are comparing it.

    AF can be far more accurate than manual focus, but most camera
    manufacturers build in a certain slush factor in order to keep the AF
    system from continually "hunting" for perfect focus. The AF technology
    itself is capable of surpassing human focusing accuracy quite easily,
    but most real-world cameras only focus to within a certain margin of
    error in order to speed the focusing process and prevent it from
    hesitating excessively when trying to focus (a very accurate AF system
    would be so "nervous" that it would continually adjust simply to track
    the changing distance of a person's chest as his heart beats).
    Mxsmanic, Jul 15, 2003
  6. fw25hp

    T P Guest

    In dim light, yes. The camera will
    find it difficult or impossible to
    obtain correct focus.

    In dim light, manual focus is almost
    always far more accurate than AF.
    T P, Jul 15, 2003
  7. fw25hp

    auspics Guest

    The answer is yes and no at the same time.
    I moved to auto focus cameras due to my eysight degrading with age. I am
    continually amazed to discover many AF shots are out of focus because at
    first, I didn't understand the process. I'm only speaking of Canon gear

    The autofocus on a low cost SLR like EOS 3000 uses a focus system similar to
    the exposure metering. It doesn't necessarly focus on the centre of the
    frame (or for that matter, what you are looking at!). An unforgivable
    incident where I took a once in a lifetime shot was out of focus because the
    camera pulled focus on a leaf in the forground. Since then have learnt to
    switch off the AF for all critical shots and use an eyepiece corrector. The
    10D of course has built in eye adjustment.

    Trust the force Luke!
    auspics, Jul 15, 2003
  8. fw25hp

    fw25hp Guest

    Both were Nikkors (24-85 f/2.8-4D and 24mm f/2.8D). Does this mean
    you think the problem was with the lenses and not the camera body

    fw25hp, Jul 16, 2003

  9. Try

    The location varies, but typically, there is a partially-nonsilvered
    'semi-pass' portion of the SLR mirror, with another mirror behind it
    oriented differently. This allows part of the image to pass through the
    main mirror and be bounced downward to the AF sensor, which is often
    coupled with the light sensor for metering. So the AF sensor is often
    sitting flat on the bottom of the camera interior.

    I seem to recall some models having the AF sensor located just behind
    some surface of the pentaprism in the top of the camera, but not entirely
    sure of this. I do know some models put their exposure sensors there.

    - Al.
    Al Denelsbeck, Jul 16, 2003
  10. fw25hp

    Mxsmanic Guest

    That page is in error. It implies that good SLRs use contrast-based
    passive autofocus, but that it not true.

    Good SLRs use rangefinder autofocus. The AF examines image detail using
    rays from opposite sides of the lens. If the image is in focus, these
    rays will converge on the AF target. If the image is not in focus, they
    will be separate by a distance that is related to the focus adjustment
    required to bring them into focus, and the direction in which they are
    separated will indicate the direction that the focus must be adjusted.

    You know you have a rangefinder autofocus system if your camera's AF
    won't work with lenses that have a maximum aperture smaller than a
    certain size (usually f/5.6). This is because the AF must see rays
    coming through opposite sides of the glass, and this isn't possible if
    the aperture is too small. Contrast-based systems don't care about
    aperture and will work with any lens.
    Mxsmanic, Jul 16, 2003
  11. fw25hp

    Leon Mlakar Guest

    The autofocus on a low cost SLR like EOS 3000 uses a focus system similar
    Some cameras with more than one sensor will choose to focus on whathever is
    closest to the camera. With Pentax MZ-6 I made couple of profile shots with
    the ear in perfect focus until I realized I'm using all three sensors
    instead of center one.
    Leon Mlakar, Jul 16, 2003
  12. fw25hp

    John Miller Guest

    I wouldn't want to guess, but it might be interesting to try those lenses on
    another F5.

    The risk with non-OEM AF lenses is that unless there some sort of
    arrangement between the OEM and the lensmaker, the electronic interface
    between the lens and the camera must be reverse-engineered, and sometimes
    they don't get it exactly right.

    Last year, I thought I'd gotten a deal on a couple of non-Nikkor AF zooms
    (the brand is unimportant -- they've already been thoroughly stigmatized
    here). They had worked fine with F4s, but with an N80 -- newer technology
    -- both were noticeably out of focus at one end of the zoom range.

    The lens manufacturer was willing to update (re-chip) them at no cost, which
    suggests that the problem indeed lay with the lenses, but KEH came through
    with flying colors and gave me the full purchase price in exchange for
    Nikkors, even though it was a day or two past the return period.

    The Nikkors work flawlessly with the N80. I'm through with non-OEM AF
    John Miller, Jul 16, 2003
  13. fw25hp

    Lew Guest

    Thanks Al. I looked behind the mirror on one of my cameras. With the mirror
    up, there is something attached to the back that I am guessing is a second
    mirror that folds out when the mirror is down. There are also visible spots
    on the camera inside opposite the mirror with the same pattern as the focus
    spots on the viewfinder glass. Those spots must be the focus sensors.

    I am surprised that somewhere on the internet there is not a good
    description with pictures of this, but I can't find one. I did find a site,, that tells almost everything wrong with
    autofocus, but does not mention the problem of poor wide-angle focus that is
    the subject of this discussion.
    Lew, Jul 16, 2003
  14. fw25hp

    fw25hp Guest

    I was hoping I would be able to save a few dollars by getting third
    party lenses. But based on your experience (and what I've heard from
    some other people), I'll try to stick with Nikkors (although that can
    get pricey fast!)
    fw25hp, Jul 17, 2003
  15. fw25hp

    fw25hp Guest

    That's an interesting page. There's a lot of information there
    besides a discussion on focus. Thanks.

    fw25hp, Jul 17, 2003
  16. fw25hp

    Mxsmanic Guest

    You bought an F5, and then you try to save money on lenses?

    If you want good pictures, you have to do the opposite: Spend your
    money on the lenses, and then get the least expensive body that will do
    the job.
    You get what you pay for, especially with lenses.
    Mxsmanic, Jul 17, 2003

  17. SAVE MONEY? You falling for that old line?
    Michael Scarpitti, Jul 17, 2003
  18. fw25hp

    T P Guest

    Interesting observations ... sounds like you are one of the select few
    who actually realise just how bad most AF systems are! I have had
    very similar experience of AF including some of the "best" that Nikon
    and Canon can offer ... now I use manual focus almost all the time.
    T P, Jul 17, 2003
  19. fw25hp

    fw25hp Guest

    Actually, I did not buy an F5. I was only looking at it in the store
    since I hadn't really checked one out before. I've heard the "spend
    your money on good glass" advice before and will be following that.
    fw25hp, Jul 18, 2003
  20. fw25hp

    Mxsmanic Guest

    For what it's worth, I always though that advice was hokum promulgated
    by crusty old snobs who just wanted to brag about glass. After I
    splurged on an F5, though (a replacement for a 20-year-old SLR that was
    finally showing a few symptoms of age), it dawned on me that putting a
    cheapo lens on it seemed like defeating its purpose, and so for the
    first time in my life I decided to risk spending big money on expensive
    glass. I wasn't necessarily expecting a huge difference in optical
    quality, but one look through the viewfinder made me understand why the
    expensive lens cost so much, and when I got the slides back, I knew that
    I could never be happy with cheaper glass again. It's an addiction, I'm
    afraid, but it is a nice one (at least when you manage to save up enough
    to satisfy it). You really do get what you pay for.
    Mxsmanic, Jul 18, 2003
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