Backfocus - different lenses give different focal planes?

Discussion in 'Digital SLR' started by Alan Browne, Mar 14, 2005.

  1. Alan Browne

    Alan Browne Guest

    I tested the backfocus of the Maxxum 7D. Links below.

    For the 100mm lens, all seems well.

    For the 50mm lens, it seems to focus camera side of the target.

    Any ideas?

    Cheers,
    Alan



    Tested:

    100 f/2.8 macro against two targets on an oblique

    50 f/1.7 against one target on an oblique.

    --Max7D + lenses above
    --Tripiod
    --cable release
    --mirror lockup
    --angle finder at 2X for manual focus
    --A/S off
    --ambient light

    100mm results: perfect

    50mm results: appears to focus slightly close to the camera v. target
    (2 samples of each from bunch follow).

    100mm f/2.8:
    http://www.aliasimages.com/images/KM7D/BF/PICT0710a.JPG
    http://www.aliasimages.com/images/KM7D/BF/PICT0721a.jpg

    50mm f/1.7:
    http://www.aliasimages.com/images/KM7D/BF/PICT0717b.JPG
    http://www.aliasimages.com/images/KM7D/BF/PICT0718b.jpg

    note: on the paper target, the focus point is marked with bars with
    ambient light from window.

    ruler target, the focus point is the 15 cm line with
    ambient light from softbox on farside of ruler.

    Comments on why the 100mm shows in-focus at the selected focus point and
    the 50mm does not?

    Cheers,
    Alan
     
    Alan Browne, Mar 14, 2005
    #1
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  2. Alan Browne

    Owamanga Guest

    100mm f/2.8: Although the target is in focus, it's not centrally focused. There is
    more behind it in focus than there is in front of it. At first glance,
    this appears to be very slight back focus.
    Same with this one, although less obvious. The third mm marking in
    front of the focus point is more blurry than the third mm marking
    behind the focus point.

    Having said all that, it's what you'd expect. As you get closer to the
    hyper focal distance, it becomes apparent that the share of DOF that
    appears in front of the focal point vs behind gets smaller. What I'm
    not certain of, (and I'd need a DOF calculator that works in mm)
    should this be visible at such a short distance from the lens?
    You cocked it up somehow?

    Dunno, they look bad.
     
    Owamanga, Mar 14, 2005
    #2
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  3. Alan Browne

    Alan Browne Guest

    eh? In this one, there is more in focus further away. This follows the
    usualt 2/3 in focus beyond the plane, 1/3 focus camera side of the plane.
    Same as above (3 of the marks beyond, and almost the 4th; 2 of the marks
    this side) are in focus, again the 1/3 // 2/3 property.
    Sure... as I said, approx. 2/3 beyond, and 1/3 on the camera side is the
    usual DOF property.

    No. At least I don't think so. I repeated the tests on the 50mm over a
    dozen times. On the 100 it only took about 6 frames to get 6 consistent
    results (refocusing each time).

    On the 50, the focus was always on the nearside (by about 2cm per the
    ruler (ignoring the oblique)). I was extremently careful with the focus
    of the VF diopter (0), then the angle finder focus (on the VF markings),
    then on the the target ... plus cable release, plus mirror lockup. Now
    I have to do these on my 20, 300 and 28-70 and 80-200 ...

    Cheers,
    Alan.
     
    Alan Browne, Mar 14, 2005
    #3
  4. Alan Browne

    Douglas Guest

    "
    ------------------------
    Discovering that KM's idea of "specifications" and autofocus accuracy is a
    little different to what the rest of the world expect, are we? This comment
    from KM Australia (the horses mouth) might shed some light on the matter:
    "These are not really professional cameras so you can't expect them to have
    the same focus tolerance as a pro camera". The fact Konica-Minolta don't
    make a Professional DSLR probably allows them to use such comments to slide
    out from under a real concern a potential customer had.

    The discussion was about the God awful autofocus of their flagship DSLR no
    less. It seems that a "non professional" autofocus camera (according to KM)
    can have a considerable tolerance in how and on what it will focus. Another
    Gem from the horse's mouth: "Anywhere inside the depth of field should be
    considered accurate focus".

    If KM Canada's attitude to fixing your camera's focus problem (presuming
    they can - which I doubt) is anything like KM Australia's attitude to having
    had 7 attempts in 5 months to fix some of their gear in my shop and, when it
    was finally decided they couldn't fix it, refund my $7500 purchase price...

    You could save yourself a lot future disappointment by taking it back to the
    store you bought it from and swapping it for a Canon DSLR. Trade your
    Minolta lenses in on a 24 ~ 70 f2.8 and get on with your photography. If you
    can't afford the glass, put up with not too shabby kit lenses until you can.
    You might complain about the cost but you'll never complain about the
    quality of your pictures.

    At the end of the line is the overwhelming fact that fewer and fewer
    Professional Photographers are choosing anything else but Canon. Sure the
    'D' series has had some adverse publicity about lock ups and focus issues
    but considering the number sold, they are few and far between. I've looked
    at every option I can to avoid Canon cameras in the past 6 months. There are
    none, Canon rules.

    Konica Minolta cameras are in the same class as Sigma cameras. I have a
    Sigma and it gets me out of a bind now and then but I'd no sooner use it as
    my working camera than I would fly to the moon.

    I started my Photographic career with Minolta cameras. I shot thousands of
    frames with SRT 101s. They got dropped, abused, wet and still took good
    pictures. After the fiasco I had with Minolta over their gear and the
    bullshit they fed me about their camera, I will never own another Minolta
    product as long as my bum points to the ground. Am I biased? Yes and with
    good reason. You never took my advise last time, do yourself a favour and
    take it now.

    Douglas
     
    Douglas, Mar 14, 2005
    #4
  5. Now I'm picturing a dial on the back with which you can tell the
    camera how large you intend to print the image you're about to
    capture -- and from how far a distance you intend it to be viewed.
     
    Ben Rosengart, Mar 14, 2005
    #5
  6. Alan Browne

    Colin D Guest

    An interesting test, Alan. Looking at the 100mm lens results, I think
    the focus is actually slightly behind the 15cm mark - about 15.1, and
    the paper target shows a similar result - but pretty close.

    The 50mm is definitely short-focused, but there may be reasons for the
    discrepancy between the lenses. First, you say you focused manually
    with a 2x angle finder. How did you focus the angle finder? Depending
    somewhat on the type of viewfinder screen design, it's possible to have
    the angle finder slightly misfocused, and to compensate by mis-focusing
    the camera lens. Question: did you refocus the angle finder between the
    two lens tests? If so, I think that would explain the discrepancy
    between the lenses.

    I have 'calibrated' my angle finder for my eye by removing the camera
    lens from the camera and pointing the camera at a bright light source -
    an incandescent bulb, or a well-lit white target - to get a really
    bright diffuse light on the VF screen, and then critically focusing the
    angle finder on the screen pattern. Then I marked the angle finder
    focus ring so I can set it up any time I need it without further ado.

    The second point is to be constantly aware of the VF screen pattern
    while focusing the camera. Some VF screens are partially transparent to
    improve image brightness, and it's possible to focus on an aerial image
    that is not at the plane of the VF screen, since even after critically
    setting up the angle finder, your eye can shift its focus. Personally,
    I think this semi-transparent screen business is what's behind the
    complaints about auto-focus cameras being difficult to focus manually.
    Manual-focus cameras had/have VF screens that are less transparent, and
    make it easier for the eye to see the on-screen image rather than an
    aerial image.

    Compare focusing a camera with a VF screen to focusing a telescope with
    no screen, just an aerial image. There is no fixed point of focus, it
    varies with one's eyesight. In a camera we need a fixed screen to focus
    on, so the focus plane coincides with the film/sensor plane.

    Colin
     
    Colin D, Mar 14, 2005
    #6
  7. Alan Browne

    Colin D Guest

    I understand that the 1/3 - 2/3 rule doesn't apply at very close
    distances.

    Colin
     
    Colin D, Mar 14, 2005
    #7
  8. Alan Browne

    Alan Browne Guest

    1.. I thought I was plonked Dougie?

    2.. Just about every camera out there has had some mention of back focus
    issues, including the Canon 10D and 20D.

    "With the 10D and even with the 20D there are reports of people having
    problems with "back focus" (i.e. the camera focusing behind the
    subject)." http://www.photo.net/equipment/canon/20D/

    3.. the 100 mm f/2.8 focuses just fine... as this image (and many others
    show).
    http://www.aliasimages.com/images/KM7D/ColBill_FD.U.jpg
    (3000 x 2000 pixels, 2.5 MB).

    4. Owamanga's comment above, is in fact wrong. The image is correctly
    focussed with the right about of focuse on each side of the focus plane
    (1/3 on near side, 2/3 on far side).
    I can assure you that not only is Maxxum 7D AF accurate, but it is fast
    as hell (with the 100 f/2.8 and 28-70 f/2.8). OTOH, I don't use MF very
    much in any case.
    The only Minolta item I've ever had break, was repaired for no charge
    .... despite the warranty having expired. (Replaced tube and other
    components in a high end flash (5400HS)).
    I have a 300 f/2.8, 100 f/2.8, 50 f/1.7, 20 f/2.8, 28-70 f/2.8 and
    80-200 f/2.8. All Minolta. Three of those lenses are sharper or as
    sharp as their Canon counterpart.
    If I were starting from scratch, it would most likely be Canon. But
    that's not the case...
    http://www.aliasimages.com/images/KM7D/ColBill_FD.U.jpg
    Maxxum 7D with 100 f/2.8 macro. Be sure to zoom in, this is 3000 x 2000
    pixels.

    Sigma? Forget it. 10D or 20D with a Canon 100 f/2.8, almost.
    10D or 20D with a Tamron 90 f/2.8, certainly.
    What advice? I've had Minolta's since the ealry 90's. I've had
    consumer lenses, of which 1 out of 3 was actually very, very good, and
    one that is quite good. Now, all of my lenses except one are pro, and
    the only one that is not *great* is the 300 f/2.8. It is weak in flare,
    but reasonably sharp.

    I'm not sure who pisses you off more, Dougie, me or K-M, but you should
    just keep posting your rants, they show you to be exactly what you are.

    Cheers,
    Alan
     
    Alan Browne, Mar 14, 2005
    #8
  9. Alan Browne

    Alan Browne Guest

    I dropped the paper target as, as you can see, it is not sufficiently
    flat, nor the contrast sufficiently high for this test, esp. with the
    100mm lens.

    It was so easy to focus the 100 that I may have been careless in
    accuracy. The 50 was difficult to focus, but over a dozen+ tests, the
    result was consistently on the camera side of the reference.
    Very carefully using the viewfinder marks as the focus points. I
    refocussed it a few times as I sometimes removed the angle finder for
    other reasons. So it was always first to be checked before focussing on
    the target.
    Similar technique, but you don't need to remove the lens, just defocus
    it against a white backround and the VF marks are very clear.
    AF cameras ARE difficult to focus manually for the reason you state
    above plus the lack of drag in the focus mechanism when it is unclutched.
    I have to use the diopter adjustment. My left eye has no astigmatism,
    and is about -1.75 (and is my dominant in any case). I can get that
    sharp with the diopter or with the angle finder (with the angle finder I
    need to put the VF diopter to 0, of course).

    Thanks for your feedback, Colin.

    I'll be testing other lenses tonight or tomorrow and we'll see how it goes.

    Cheers,
    Alan.
     
    Alan Browne, Mar 14, 2005
    #9
  10. Alan Browne

    Alan Browne Guest

    Never heard that... but I hear new things all the time! :)

    Got any references?

    Cheers,
    Alan
     
    Alan Browne, Mar 14, 2005
    #10
  11. Alan Browne

    Alan Browne Guest

    Should have read: "OTOH, I don't use _AF_ very much in any case."

    Cheers,
    Alan
     
    Alan Browne, Mar 15, 2005
    #11
  12. Alan Browne

    Scott W Guest

    The 1/3 2/3 rule is in fact pretty much useless. When focusing far
    enough away infinity come into focus and so clearly the ratio of front
    to back is not 1/3 2/3. On the other hand when the DOF is very small
    you will get close to the same amount in focus in front and behind,
    this happens when the DOF is small compared to the distance to the
    object, which does tend to be when you are focusing in close with the
    lens wide open. It should be noted that the ratio changes just with a
    change in the f-number.

    Scott
     
    Scott W, Mar 15, 2005
    #12
  13. Alan Browne

    Bob Hickey Guest

    How's this? Lenses, except macro, have curved fields and therefore curved
    DOFs. You tested one in the center and one on the far left edge. I'd try
    scribing an arc and shooting that to see if it's in, edge to edge. Or maybe
    that's not it at all. Bob Hickey
     
    Bob Hickey, Mar 15, 2005
    #13
  14. Alan Browne

    Colin D Guest

    Yep. Some simple math should do it. All dimensions in millimetres.

    First, we need the hyperfocal distance for a given lens. Let's say we
    will use a 50mm lens set at f/8. The HD for such a lens at that stop is
    given by F^2/(f:number x diameter of chosen circle of confusion) (coc).
    Coc generally accepted for 35mm film is 0.03mm, so:

    50^2/(8 x 0.03) = 10,416.6 mm - say 10,000mm

    OK. The near distance for acceptable dof is given by (Hyperfocal dist. x
    focus distance)/(hyperfocal dist. + focus distance),
    and the far distance for acceptable dof is given by (Hyperfocal dist. x
    focus distance)/(hyperfocal dist. - focus distance).

    Let's set the lens focus distance at the hyperfocal distance. Then we
    have:

    Far distance: (10^4 x 10^4)/(10^4 - 10^4), which is 10^8/0 = infinity
    Near distance: 10^8/(10^4 + 10^4), which is 10^8/(2 x 10^4), which is
    5,000mm. So, our dof in this case is 5 metres to infinity. The near
    depth is 5 metres, and the far depth is infinite, which is what we
    expect when focused on the HD of 10 metres.

    Now, let's do it again for a close distance, say about 10 inches, or
    250mm, and we'll switch to metres to keep the zeroes under control.

    Near distance is (10 x 0.25)/(10 + 0.25), = 2.5/10.25, = 0.244 metres.
    Far distance is (10 x 0.25)/(10 - 0.25), = 2.5/9.75, = 0.256 metres.

    Since we focused on 0.25 metres, the near dof is 0.25 - 0.244, =
    0.06mm, and the far dof is 0.256 - 0.25, = 0.06mm.

    The dof is equal either side of focus.

    In fact, the near/far dof ratio varies with distance from infinite at
    the HD to practically 1:1 at close distances, and is more or less 1:2
    only at some intermediate distance.

    Congrats if you managed to wade through that lot, Alan!

    Colin
     
    Colin D, Mar 15, 2005
    #14
  15. Alan Browne

    Alan Browne Guest

    All focus was at dead center of the lens. The crops I provided were
    hastilly done, not symetrical top-bottom or right left. Cropped in the
    interest of your bandwidth.

    But keep throwing ideas.

    Cheers,
    Alan
     
    Alan Browne, Mar 15, 2005
    #15
  16. Alan Browne

    Alan Browne Guest

    The math doesn't bother me ... but it's 10 past midnight so I'll do a
    spreadsheet tomorrow and look at the front-DOF/BACK-DOF ratios as
    function of distance.

    I assumed that the 1/3 - 2/3 was one of those unchanging properties.

    Thanks!

    Cheers,
    Alan.
     
    Alan Browne, Mar 15, 2005
    #16
  17. Alan Browne

    Owamanga Guest

    <massive math that proves the point snipped>

    Colin, I came to the same conclusions in 20 seconds using a DOF
    calculator, but couldn't get exact numbers for the close-ups, thanks
    for that.

    :)
     
    Owamanga, Mar 15, 2005
    #17
  18. Alan Browne

    Owamanga Guest

    No it isn't, read Colin's math.

    :)

    The words Owamanga and wrong just don't sit well together.
     
    Owamanga, Mar 15, 2005
    #18
  19. Alan Browne

    Alan Browne Guest

    The jury (me) is still out. You might scrape through yet.

    Cheers,
    Alan
     
    Alan Browne, Mar 15, 2005
    #19
  20. Alan Browne

    Owamanga Guest

    Ah! As I suspected. Do you have any diagrams/math behind this
    statement?

    If it's correct (and I can see why it would be), then this website is
    wrong:

    http://visual-vacations.com/Photography/focus-recompose_sucks.htm

    It makes the assumption that the plane of focus is truly a plane, not
    an arc.

    This site was discussed recently in rec.photo.digital, starting with
    messageid: <>
    Subject: Why "Focus and Recompose" causes back-focus issues
     
    Owamanga, Mar 15, 2005
    #20
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