Missing focus with Canon 50mm 1.8 lens - is it the lens or me?

  • I purchased a Canon EOS 650D and EF 50mm f/1.8 II lens.

    Unfortunately, the auto-focus seems to miss focus at f/1.8. I've tried setting it to single-point auto-focus: it is better, but still not good enough.

    This is my first DSLR, so is this likely a result my lack of skill, or perhaps is the lens at fault?

    I made some tests on a target. Here is the result: test target

    The focus is at 1cm in front of target.

    Shooting parameters:

    Aperture f/1.8, ISO 250m, Exposure time 1/250

    I seem to recall that Canon's DPP software can show you the focus point that was triggered and where on the subject. Not sure if that's for all Canon cameras, being a Nikon user myself.

    It only shows which focus point was selected. If you focus and recompose it doesn't reflect that. Also keep in mind that for many DSLRs including most current Canons, the areas in the viewfinder covered by each focus point are much larger than the squares that mark their center in the viewfinder.

    Did you initially notice this in taking test photos, or did you notice in real photographs first and did the test to confirm?

    @mattdm I noticed this on usual photos, that I had taken. Today I provided test on the target and saw that it is not my imagination.

    Another question: did you try multiple times, defocusing the lens and then starting again? How consistent were the results? I notice that (unless it's cropped) the target is quite close; did you try at further distances, and how do the results compare?

    I doesn't appear the test target is bright enough to return optimal test results.

  • Nir

    Nir Correct answer

    8 years ago
    1. At f/1.8 the depth of field is very small - any movement (of subject or camera) between focusing and taking the shot can push the subject out of focus - so you want to take the photo as quickly as possible after focusing and don't use the "focus and recompose" technique - not at f/1.8

    2. The auto-focus on the 50mm f/1.8 is very slow, if the camera is set up to take the photo even if focus isn't locked it's possible you take pictures before the lens finished focusing - when the camera focuses it will beep and a small circle will light up in the view finder.

    3. And last - and definitely least probable - your camera or lens can be miss-calibrated (at least for each other), there are questions on this site about focus testing (like this). I believe the 650D does not have any way for you to calibrate focus and you need to take the camera and lens to a service center (or at least a good camera shop).

    I agree with `1` and `3`, but regarding to the `2` I always wait for the indication (sound and visual). Anyway, thanks.

    Regarding #1 use a tripod for AF testing --- #2, I believe most cameras (650D included) in single-shot mode will not take the photo until focus is achieved.

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Content dated before 7/24/2021 11:53 AM