Sharpness issues with Canon 50mm f/1.8 and Canon Rebel XTi

Discussion in 'Digital Photography' started by shaji, Aug 5, 2007.

  1. shaji

    shaji Guest

    Hi all,
    I'm a newbie to SLRs. I bought my Canon Rebel XTi and Canon 50mm f/1.8
    II just a few days back. I feel the pictures I get are not as sharp as
    I expect them to be. An image with 100% crops from pictures taken at
    various apertures can be seen at the following link.[email protected]/1012760583/

    All of these crops except the 3 marked "center" are from the corners
    of the photos.

    1. I get good corner sharpness only if I stop down the lens to f/11.
    Is that usual with this camera/lens combination?
    2. Are these crops of acceptable sharpness?
    3. Sharpness at widest aperture is unbearably soft. Is that usual with
    a Canon 50mm f1.8?

    Thanks in advance,
    shaji, Aug 5, 2007
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  2. shaji

    Steve B Guest

    I'm not familiar with the lens, but a prime should be pretty sharp once it's
    stopped down one stop or so, say f2.8 with a f1.8 lens. Looks out of focus
    to me, with the smaller apertures hiding it with a large depth of field.
    You need to do a back/front focus test to see if it's focusing in front or
    behind the intended focus point. There's plenty of info around, like this
    page for the D70 with a focus chart you can print.
    Steve B, Aug 5, 2007
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  3. shaji

    Pete D Guest

    I just used this test chart to test two new lenses and if you follow the
    instructions it will only take a few minutes to conplete. Do make sure you
    use a tripod though. My FA50mm F1.4 and 90mm F2.8 macro were perfect now all
    I need is some decent weather to get outside and get some decent shots.


    Pete D, Aug 5, 2007
  4. Hello Shaji.

    I assume this is the digital SLR known as the 400D here in the UK. What
    settings for sharpness are you using? You could try taking the test shots in
    RAW mode.
    It is also possible that the lens is faulty so can you try the shots with
    another lens?

    I am trying to look at your photos but I'd recommend that you print the
    photos and look at them on paper rather than on a screen.

    Regards, Ian.
    Fred Anonymous, Aug 5, 2007
  5. shaji

    shaji Guest

    Hi Steve,
    Thanks for this valuable info. I shall try out the test. But, just out
    of curiosity; Could this be a problem with the camera body in any way?
    Need I check that too? If so, how can I check the camera for these
    sort of issues?
    shaji, Aug 5, 2007
  6. shaji

    JohnR66 Guest

    There is always a possiblility that it could be the camera body, but this
    would be noticeable with other lenses and would not be very likely anyway.
    The 50mm f/1.8 is sharp by f/2.8 and critically sharp in the f/4 - f/8
    range. Corners should be excellent at this range as well.

    Your crops look out of focus to me rather than lens aberration caused.
    JohnR66, Aug 5, 2007
  7. With that lens and my 30D it is extremely sharp over the whole field
    at f/4, and even f/2.8. However, the autofocus is not always
    right. Try manual focus and a tripod.

    Doug McDonald
    Doug McDonald, Aug 5, 2007
  8. shaji

    TrevM Guest

    As others have said, it looks as if your shots are basically out of focus.
    They look sharper at higher f-numbers, where accurate focus is less
    critical. How close were you to the targets? This lens will not focus
    sharply closer than 0.45 m (18 inches), so it would be unable to get a sharp
    image of anything closer, and you would get a warning signal in the
    viewfinder. Have you tried other subjects than test targets, such as houses
    across the street?

    Maybe a silly question, but is the switch on the lens (near the red dot) set
    to AF (auto focus), not M (manual)? If so, does the lens try to focus? -
    the front rotates when it does - and you should see the image snap quickly
    into good focus in the viewfinder when you partly press the shutter button.
    If the lens is set to AF and does not try to focus when you press the
    shutter release, or focuses wrongly, there is a fault with either the lens
    or the camera body. Note that you can choose various points in the frame
    for the autofocus to aim at - the centre is best for testing.

    If none of the above applies, your dealer should be able to try the body
    with another lens, and try the lens on any Canon autofocus SLR (film or

    Hope this helps.

    TrevM, Aug 5, 2007
  9. shaji

    Robert Coe Guest

    : Hi all,
    : I'm a newbie to SLRs. I bought my Canon Rebel XTi and Canon 50mm f/1.8
    : II just a few days back. I feel the pictures I get are not as sharp as
    : I expect them to be. An image with 100% crops from pictures taken at
    : various apertures can be seen at the following link.
    :[email protected]/1012760583/
    : All of these crops except the 3 marked "center" are from the corners
    : of the photos.
    : 1. I get good corner sharpness only if I stop down the lens to f/11.
    : Is that usual with this camera/lens combination?
    : 2. Are these crops of acceptable sharpness?
    : 3. Sharpness at widest aperture is unbearably soft. Is that usual with
    : a Canon 50mm f1.8?

    Because of its multiple focus points, the XTi can be tricky to handle at
    first. It's easy for it to lock onto a focus point other than the one you
    intended. Until you get more experience with it, try setting it to use only
    the center point. This won't constrain your placement of the subject in the
    picture, because you can reposition it after it has focussed, as long as you
    keep the shutter button halfway down. (Obviously, you can't change the
    *distance* to the subject without allowing it to refocus.)

    Robert Coe, Aug 5, 2007
  10. shaji

    Pat Guest

    A f1/8 lens at short distance has an incredibly shallow depth of
    field. At 2 feet (approx 60cm), your DOF is maybe an inch (2 to 3
    cm). Therefore, you are well below the tolerance that you can
    "eyeball" whether your camera is square to the image you are copying,
    or not. When you stop down to f11, the DOF increases to about 3

    My first thought would be that your camera is not precisely square to
    the image you are copying. Therefore, while the center of the paper
    is in focus, the corners are out of focus because they are outside
    your DOF.

    Visualize it this way. Say you have your image square to the camera
    and everything is in focus. Now someone walks by and hits your
    copyboard and it pivits along the center of the image. The center
    remains exactly the same distance to your camera and remains in
    focus. But one side is now closer to your camera and the other size
    if farther -- thus they are out of focus.

    At f 1.8, it doesn't take much to be off. That's why they use really
    bright lights for copy-work -- so they can stop down the lens.
    Pat, Aug 5, 2007
  11. shaji

    shaji Guest

    HI TrevM,
    Thanks a lot for your thoughts.
    I was 3 feet away from the target when I took the test shots. The
    lens, of course, was in AF mode and I chose the centre auto focus
    point. I took the back focus test and can't find anything wrong there

    Could it be that the lens has a focusing issue?
    If this is an issue with body, what could be the possible reasons?

    Other than this 50mm lens, I've got only the "kit lens" with me( I'm a
    newbie to SLR :) )
    shaji, Aug 6, 2007
  12. shaji

    shaji Guest

    Hi Steve,
    I took the back focus test you mentioned and can't find anything wrong
    with it other than the softness at the point of focus. I shall take
    out the test with manual focus and let you know the result.
    shaji, Aug 6, 2007
  13. shaji

    shaji Guest

    Hi Pat,
    I tried my best to set the camera square to the target.
    Now, I tried manual focus and at f 2.8 the images are tack sharp. So
    is it safe to assume that the lens has a faulty auto focus? Need I
    check anything else?
    shaji, Aug 6, 2007
  14. shaji

    Pat Guest

    I've never felt that autofocus is as precise as manual focus, so the
    first thing you might want to consider is swapping out your focusing
    screen and getting one that is better suited for manual focus, such as
    a split ring screen.

    Otherwise, you can either do things the easy way or do things the hard

    The easy way is to got to the store and get a couple of florescent
    light fixtures -- say 2 with 2 bulbs each (the 4' bulbs). Put one on
    each side of your work and really light it up. Then stop it down to f8
    or f16 and get to work. Total cost, under $50.

    The hard way it to precisely align your camera with the image. Here's
    how I've done it in the past. Find a wall in your house that is
    precisely plumb (exactly vertical), which isn't always easy. Draw a
    line across it that is exactly horizontal. Put a dot in the middle of
    that line. That will be your centering point. Set up your tripod at
    the distance you want. Level the top of your tripod so it's exactly
    level in all directions. Use a level, don't trust the built-in
    levels. Mount your camera on your tripod. Use the crank to get your
    center focusing point exactly on the dot (you can't use any other
    tripod controls, just raising and lowering the neck.

    Here's where it gets a bit tricky. You easiest way is to drop a plumb
    line from the centering dot to the floor. Then draw a line out from
    that line at a right angle to get straight away from the wall. Then
    hold a plumb bob's string on the bottom of your lens and drop a plumb
    bob to the floor (at the line). Position the camera so that the
    center of the lens is over that line at the same time the center
    focusing point is on your focusing point. With your camera level,
    you're then ready to shoot.

    A more difficult way is to measure the distance from the center of the
    lens to the four corners of the object being photographed. When that
    distance is the same, then you're centered and all you need to do is
    then point the center focus point at the center of the object being

    Finally, don't forget to use your auto timer on the camera so that you
    don't move the camera when you push the shutter. Also, use the mirror
    lock-up to avoid movements. All movements will be magnified as you go
    farther from the center.

    After you do all of that, then I'd worry about the lens.
    Pat, Aug 6, 2007
  15. shaji

    Rich Guest

    Canon needs to embrace the idea of the digital lens. Making the
    exiting light rays as perpendicular to the sensor plane as possible.
    They've neglected this for far too long and the results show. Biggest
    buyers of old or new non-Canon fixed focal length wide angle lenses?
    Canon FF users.
    Rich, Aug 6, 2007
  16. shaji

    TrevM Guest

    [...] snipped
    Hi Shaji:
    OK - 3 feet should be fine.

    If you get sharp results with manual / eyeball focussing at f/2.8 (as you
    mentioned in another reply), that means that the plane of correct focus is
    the same both for the viewing screen and for the image sensor - that rules
    out some possible lens and body faults.

    Have you tried the kit lens on AF and manual settings, and at its widest
    apertures (lowest f-numbers)? If that behaves the same way - OK on manual
    and fuzzy on autofocus - that points to a body fault which is giving the
    wrong "correct focus" signal to all lenses.

    However, if the kit lens focuses correctly both on manual and auto, it would
    definitely point to the f/1.8 lens being faulty on autofocus. It could be
    worth checking that the lens's electrical contacts (small gold pads on the
    back of the lens body) are clean - a dry rub with a clean lens cloth or
    tissue should be enough.

    TrevM, Aug 6, 2007
  17. shaji

    UC Guest

    Who told you to get an auto-focus digital piece of crap to start with?

    UC, Aug 6, 2007
  18. shaji

    acl Guest

    If what he wants to do is a way to get the film/plane sensor
    perpendicular to a sheet of paper, one way would be to place a mirror
    somewhere, put the camera on a tripod and look through the viewfinder;
    the central AF point should be in the middle of the reflection of the
    lens (assuming the viewfinder, sensor etc are where they should be).
    acl, Aug 6, 2007
  19. shaji

    Pat Guest

    Sweet. That should work.
    Pat, Aug 6, 2007
  20. shaji

    shaji Guest

    Hi TrevM,
    I tried with my 18-55 mm kit lens and , yes, the problem persists on
    that too. What more, when in auto focus mode, the shutter speed
    increases causing the picture to become darker. This could definitely
    be a body issue, right? How reliable is Canon's service network? Any
    idea? I wonder how such a faulty piece got past the Canon QC
    shaji, Aug 8, 2007
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