Focus discrepancies in Rebel XT and XTi, manual and auto focus

Discussion in 'Digital SLR' started by Bob G, Jan 17, 2007.

  1. Bob G

    Bob G Guest

    I have noticed the most puzzling and disturbing focus discrepancies in
    both my Rebel XT and my XTi.

    Please do the following tests with yours (and please report back!):

    Set your camera on a tripod
    Set up a scene with some sharp black-on-white writing in the center
    Set your lens to auto-focus and press the shutter button half way down
    until the lens focuses
    Adjust your eyepiece dioptric to your eyesight
    Shoot a picture at wide aperture (f2.8 will do)

    Now, without disturbing anything, set your lens to manual focus
    Move the focusing ring to dislodge the previous focus setting
    Re-focus carefully on your viewing screen (do not re-adjust your
    diopters!)
    Shoot a picture at the same f-stop

    Now upload your pictures to your computer and compare the two
    side-by-side by enlarging the center to the maximum (try "actual
    pixels" with Photoshop)

    I've repeated these tests with every conceivable variation of lenses,
    cameras, with glasses and without, with additional supplementary
    diopter correction lenses, etcetera.

    INVARIABLY, the picture done in auto-focus is right on, but the one
    done manually shows a pronounced focus shift! You can readily see the
    difference in sharpness between the two.

    I sent the XTi to Canon and it was returned with the same problem,
    after thorough "inspection and adjusting".

    What is going on?

    Do you see the same discrepancies in your own tests?
     
    Bob G, Jan 17, 2007
    #1
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  2. They didn't find a problem since this is normal.
    The focusing screen in your camera is not optimized for manual focus. So,
    every time you see an image that looks perfectly focused you have too much
    tolerance for it not to be perfectly focused.
    No need to. If you want to use this body with MF lenses you should invest
    in a KatzEye screen if they make one for that model.







    Rita
     
    Rita Ä Berkowitz, Jan 17, 2007
    #2
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  3. Bob G

    Bob G Guest

    If that's the case it's a shame. I can't really believe it. Do other
    brands show the same problem?
    And why is not this a problem with film auto-focus Canon SLRs? Or is
    it?
    And why isn't there a warning anywhere in the instruction manual?
    I personally like to set my lenses to manual focus, I hate to press a
    "hold" button and refocus if I don't agree with the camera's chosen
    point.
    That renders the Canon Rebel XT and XTi near useless to me.
     
    Bob G, Jan 18, 2007
    #3
  4. Bob G

    Bob G Guest

    BTW, I spent two hours on the phone with various Canon technicians this
    afternoon.

    They seem to think the problem is with my eyes, that I need to adjust
    the eyepiece diopter.

    Never a word that this baffling camera behavior is "normal".

    That's why I'd like to know if other users are getting the same results
    from the tests.
     
    Bob G, Jan 18, 2007
    #4
  5. I think the current trend is most people use AF and don't play around with
    MF much. Yes, other brands have the same problem to more or a lesser
    degree. Your camera is working within 100% of factory-designed specs. I
    found the website for KatzEye.

    http://www.katzeyeoptics.com/
    It depends on the factory installed focusing screen. Plus, higher end
    cameras give you a choice of screens.
    It would confuse people since most people use AF. It's kinda like the
    debates we have here about IS/VR were some people feel this feature cures
    everything from the common cold to gout.
    I know you don't want to hear this, but if you plan on using MF a lot you
    will need to shell out $100+ and get the KatzEye.
    You'll get used to AF since everybody does sooner or later.







    Rita
     
    Rita Ä Berkowitz, Jan 18, 2007
    #5
  6. Bob G

    Douglas Guest

    :I have noticed the most puzzling and disturbing focus discrepancies in
    : both my Rebel XT and my XTi.
    :
    : Please do the following tests with yours (and please report back!):
    :
    : Set your camera on a tripod
    : Set up a scene with some sharp black-on-white writing in the center
    : Set your lens to auto-focus and press the shutter button half way down
    : until the lens focuses
    : Adjust your eyepiece dioptric to your eyesight
    : Shoot a picture at wide aperture (f2.8 will do)
    :
    : Now, without disturbing anything, set your lens to manual focus
    : Move the focusing ring to dislodge the previous focus setting
    : Re-focus carefully on your viewing screen (do not re-adjust your
    : diopters!)
    : Shoot a picture at the same f-stop
    :
    : Now upload your pictures to your computer and compare the two
    : side-by-side by enlarging the center to the maximum (try "actual
    : pixels" with Photoshop)
    :
    : I've repeated these tests with every conceivable variation of lenses,
    : cameras, with glasses and without, with additional supplementary
    : diopter correction lenses, etcetera.
    :
    : INVARIABLY, the picture done in auto-focus is right on, but the one
    : done manually shows a pronounced focus shift! You can readily see the
    : difference in sharpness between the two.
    :
    : I sent the XTi to Canon and it was returned with the same problem,
    : after thorough "inspection and adjusting".
    :
    : What is going on?
    :
    : Do you see the same discrepancies in your own tests?
    :
    Listen to what Rita says here Bob. Your camera is an Auto Focus camera. If
    you can't get a sharp pic on AF, it has a back focus error. This is not a
    Professional level camera and you can't expect it to perform like one. If
    you truly do want to manually focus this camera, get a manual focus screen
    for it. You may have a problem then with detecting correct AF but it's all a
    compromise on these cheap SLRs.

    --

    Australian Wedding Photography between Kempsy, NSW and Sunshine Coast.
    http://www.photosbydouglas.com
    Digital photos enlarged and printed on Canvas
    http://canvas.photosbydouglas.com
     
    Douglas, Jan 18, 2007
    #6
  7. Bob G

    dwight Guest

    My problem was DEFINITELY fixed with the diopter. Shooting through a
    viewfinder only, I can never trust my own judgement as to whether I've
    manually focused correctly or not.

    I would not be a good subject for your proposed test...

    dwight
     
    dwight, Jan 18, 2007
    #7
  8. Bob G

    G.T. Guest

    Which brings up a question I've been meaning to ask for some time. How
    accurate is the built-in diopter adjustment on a low end Canon? I have a
    Rebel XT and I'm -.5 in one eye and -.75 in the other.

    And if I start doing more manual focus I will definitely get a Katz Eye,
    I've been thinking about it for quite some time.

    Greg
     
    G.T., Jan 18, 2007
    #8
  9. Bob G

    Bob G Guest

    My test removes the possibility of a diopter problem.
    Notice that in my test the diopter is calibrated after the camera has
    autofocussed
    Using manual focus then should give the exact same point of focus,
    since the eyepiece has been corrected for your eyesight already.
     
    Bob G, Jan 18, 2007
    #9
  10. Bob G

    Bob G Guest


    A Katz Eye will not correct an inherent focus discrepancy, should you
    have one.

    These are screens whose only virtue is that they're brighter and more
    contrasty and thereby facilitate focusing, PROVIDED you have a
    perfectly aligned system to begin with.
     
    Bob G, Jan 18, 2007
    #10
  11. Bob G

    Charles Guest


    with the camera turned off, can you see the focusing squared clearly?
    Watch them while you adjust the diopter correction.
     
    Charles, Jan 18, 2007
    #11
  12. Bob G

    G.T. Guest

    Yes, but as others have pointed out the lack of a decent focusing screen on
    current autofocus DSLRs is probably your problem with manual focus, not an
    inherent problem with your Rebels.

    Greg
     
    G.T., Jan 18, 2007
    #12
  13. Bob G

    Bob G Guest

    And by what mechanism do you suspect the focusing error occurs?
    As I have explained, in the test procedure the diopter is adjusted
    after the camera has autofocussed the lens. The lens is then refocused
    manually until the image, already adjusted for my eyesight, again
    appears tack sharp. The second picture is then taken.
    How do you explain the fact that the image focused on auto is right on
    but the image focused manually shows a very perceptible focus shift?
    It is quite obvious to me, and I fail to see why it isn't to everybody,
    that the focusing screen brightness plays absolutely no role in the
    test procedure.
    Please explain to me then how the focus discrepancy occurs.
    Better yet, do the test on your own Rebel XT, it won't take you 10
    minutes.

    PS - I have also observed that the manual focused image INVARIABLY
    shows 1/3 of a stop more exposure than the autofocus image. Explain
    that one also.
     
    Bob G, Jan 18, 2007
    #13
  14. Bob G

    G.T. Guest

    As "Rita" said:

    "The focusing screen in your camera is not optimized for manual focus. So,
    every time you see an image that looks perfectly focused you have too much
    tolerance for it not to be perfectly focused."

    I'm not going to do the test because I don't trust the combo of my eyes and
    my Rebel XT's screen.
    PEBSAC? Can't think of any other reason. How do you explain it?

    Greg
     
    G.T., Jan 18, 2007
    #14
  15. Bob G

    Bob G Guest

    Yes, after adjusting the diopter on the autofocussed image I can then
    see the viewfinder perfectly with the camera turned off.
    I don't see why the reverse would help any - I probably did adjust it
    that way when the camera came out of the box, which, BTW, was when I
    first noticed that the screen was a tad off when focussed at infinity
    and what started me on this unpleasant voyage.
     
    Bob G, Jan 18, 2007
    #15
  16. Bob G

    Charles Guest


    The screen could be in the wrong place, such that when the image is
    focused properly on the viewfinder screen it is not focused properly
    on the imaging sensor. One of the Photo magazines, either Pop or
    Modern ran an article on this problem many years ago, back in the days
    of film.

    since what is seen in the viewfinder is not an aerial image I can't
    figure how the diopter or bad eye could make a mis-focused image look
    good. there's a lot of things I can't figure out, this is just
    another one of them.
     
    Charles, Jan 18, 2007
    #16
  17. Agreed. I've always had MF problems with my D30/D60/DRebel. Amazingly, I
    can live life normally like you and most of us.
    I saw the light and am giving up on Sigma and Tamron permanently due to the
    superior AF on Canon lenses.
     
    Phil, Non-Squid, Jan 18, 2007
    #17
  18. Bob G

    J. Clarke Guest

    And have a split prism and a microprism ring.

    As for an "inherent focus discrepancy", how many SLRs have you
    encountered that actually have one of these? On every SLR I've taken
    apart you're going to to have to bend the primary structure to get an
    "inherent focus discrepancy" . On a Leica the rangefinder can get
    misaligned without a whole lot of difficulty, on an SLR you have to
    quite a lot of damage.
     
    J. Clarke, Jan 18, 2007
    #18
  19. Bob G

    J. Clarke Guest

    In the procedure you are applying you should be adjusting the diopter
    until the image is tack sharp, not adjusting the diopter then changing
    the focus to accomodate your misadjustment of the diopter.

    Of course it may be that you have the focusing screen in upside down.
    If you can see 1/3 stop more exposure you've a better eye than the lot
    of us I suspect.
     
    J. Clarke, Jan 18, 2007
    #19
  20. Bob G

    Bob G Guest

    I can just barely see it in the image - but I can see the shutter speed
    in the camera below the screen change from 1/20 sec to 1/15 sec (I've
    been shooting on "A") at the same aperture.

    I'm told by a knowledgeable friend that these cameras use two different
    systems to focus (and probably to determine exposure thereby) - one
    system is independent of the viewing screen and uses intersecting beams
    (!?) and I don't know what to focus the image; the other uses the
    traditional groundglass screen and top prism to focus and possibly also
    to determine exposure.
     
    Bob G, Jan 18, 2007
    #20
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