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Focusing techniques

CedCed Member
edited March 2011 in general photography
Hello,
I have sometimes hard time understanding why some of my pictures are out of focus.
Is it me, the lense, my camera?
- Me: I select the autofocus point closest to the eyes of the subject,focus and then frame as I wished with as little move as possible.
- The lense: I tried to do the focus on my eyes subject and shot without reframing, so no move at all, some shot are out of focus...
- The camera: I see a clear difference when I use the center cross point or one of the side points(that are not cross type on my camera). Also, there is a big difference in low light condition.

It looks like my camera says it has focused although it has not(especially in low light).And that the non cross type focus point are inefficient.

Is that so or it can only be me ? Would I benefit from the USM lenses, I don't have any.

Ps:my camera is a canon 1000D, the lense a canon 50mm f1:8, I did the experiments at f1:8 to f2:8,and with a tripod.
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Comments

  • StephenStephen Member
    edited March 2011
    1) You are changing the plane of focus when you focus on the eyes and then recompose for the frame. So, those eyes that you originally focused on are not quite in focus. The shallower your depth of field (i.e. wide apertures), the more pronounced the out of focus effect is.

    Here are some articles explaining this effect, and the mathematics behind it.
    http://visual-vacations.com/Photography/focus-recompose_sucks.htm
    http://www.bobatkins.com/photography/technical/focus_recompose.html
    http://www.mhohner.de/recompose.php

    You can still use focus and recompose, but you just have to be aware of this situation.

    2) The AF sensors in the camera are actually a little larger than the AF points shown in the viewfinder. Just because you placed your AF point on your target, you may still be off.

    http://www.cambridgeincolour.com/tutorials/camera-autofocus.htm

    The most accurate AF point is the center point. The least accurate AF point are on the edges.

    In addition, I sometimes accidentally move my zoom ring or manual focus ring by accident while taking the shot, and that will cause out of focus. Did you touch these controls when you shot?

    3) The sharpest images are usually obtained one or two stops down from the widest aperture.
  • CedCed Member
    I did not touch the focus ring while shooting, my guess is that the non cross type focus point are inefficient under low light conditions, and as mentioned in your links, being close to the subject does not help(it was the case). I don't see a solution though, for this type of photo: low light, close to the subject,shallow depth of field, and only one cross type focus point on my camera,so I have to recompose. Maybe with some more practise!
  • The only other solution I can think of is to throw some light on the subject, but if you cannot do that, then it is a problem. Perhaps the professional photographers here can give you advice.

    For me, I would still rather take my chances with the non-cross AF point in low light than try to focus and recompose, which I know will make the focus even worse.
  • The best solution I have found is to use a speedlight mounted on the camera. You can disable the flash if you like but still use the focus assist pattern that the speedlight puts out.

    Of course using fast lenses helps because the camera uses the widest aperture that the lens is capable of for autofocus regardless of what aperture you have set on your camera. I was shooting an event in very low light last night and even my f2.8 lenses struggled in comparison to my f1.8 ones, I have given up on shooting anything slower than 2.8 in low light. It's just a fact of life that the gear does make a difference in tough situations. But you can pick up a fast 50mm or 85mm at a reasonable price and it's worth it if you shoot low light.


    BTW the situation you describe is the only time I actually move my focus off the centre, I don't recompose if I'm wide open and close to the subject.
  • Neil vNNeil vN Administrator
    Ced ... your problem may be as simple as it being equipment.

    I had focusing problems with the Canon 50mm f1.4 USM. It just wasn't predictable.
    (Not that the Nikon 50mm f1.4D is all that much better in AF consistency.)

    Then also, your equipment might be out of calibration.
  • What is the best method of AF - single point, dynamic or auto? I have been using Single point and focusing on the eyes, then recomposing the frame to suit. this means the eyes are sharp..
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