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Auto-Focus Accuracy

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Auto-Focus Accuracy

  • I recently bought a new Sigma 24-70 F2.8 to pair up with my EOS 350D. Had it out there at the weekend and it seems pretty good, definately a good step up from the kit lens I had been using for the last year in terms of image quality and wider aperture available.

    One thing I did notice however was the auto-focus accuracy doesn’t appear to be as good as the kit lens. Is AF controlled by the lens, or by the camera? I was shooting a lot of photos with people in them outdoors and I could often see the AF point in the viewfinder lighting up on insignificant elements in the shot. I found I had to do a fair bit of manually selecting focus points or full time manual focusing to get solid results. Is this just a downside to buying lenses from a company other than the camera manufacturer? Is there anything I can be doing to help improve accuracy such as changing metering? Is the fact I only have 7 AF points on the 350D compared to more on newer models going to make much of a difference?

    Would be interested to hear peoples comments on this, I know people have mixed views on lenses such as Sigma but unfortunately that is the budget I am on. Thanks :)

    Alan Rossiter
    Participant

    I never use the various points of focus on a camera. They have their uses in sport or wildlife photography. There should be a setting on your camera to select the centre point as the focus point. Select this then when you’re taking the shot find your main point of interest, half depress the shutter to lock focus, recompose and shoot.

    I have a 24-70 f2.8 with no problems focusing.

    Alan

    bingbongbiddley
    Participant

    The problem you’re describing sounds like it is the camera’s fault, not the lens.

    irishwonkafan wrote:

    I never use the various points of focus on a camera. They have their uses in sport or wildlife photography. There should be a setting on your camera to select the centre point as the focus point. Select this then when you’re taking the shot find your main point of interest, half depress the shutter to lock focus, recompose and shoot.

    That’s interesting as I rarely use this technique, although I have done it a few times without really giving thought to it. I nearly always rely on the focus points and when they don’t match up with the focal point I want then I either manually select a different focus point or just switch to manual focus. That said this is the first time I’ve really given any thought to how a camera focuses and the options available.

    Looks like it isn’t the lens to blame anyway which is a relief, perhaps I just noticed it more because I was curious to see how the lens performed.

    aoluain
    Participant

    Focus is controlled by the camera !

    The lens does the focusing and the body tells it when to stop.

    aoluain wrote:

    Focus is controlled by the camera ! The lens does the focusing and the body tells it when to stop.

    This is what I had assumed, but I was curious why then people will say things along the lines of “when I used the Sigma lens I found it hunted more than my [insert Nikon/Canon equivalent]….”. Is that technically incorrect? If the camera is controlling the AF then surely the only difference in focusing between lenses would be the actual speed of the focus motor such as USM or HSM?

    Sorry for all the questions, and thanks for the replies so far, just trying to get my head around this :)

    Alan Rossiter
    Participant

    An opinion can’t really be technically incorrect but it can be an observation that many will relate to. The camera looks for a definite line to auto-focus. If you have a photograph of say, a plain curtain it will have difficulty unless it can find a dark crease to find that contrast between light and dark. In low light your lens will hunt – especially if it is f5.6 and beyond as the camera cannot make out a definite contrast. So it may be the case that Sigma lenses may hunt more…but not necessarily because they’re Sigma.
    There is another point to consider. Some lenses can go through their complete range of focus in about 1/5 of the rotation of the barrel where some older lenses which may have been geared for manual focus will go a lot further. So if you can imagine the motor rotating the focus it may take longer for the older lenses than the newer ones with less rotation so may take longer to focus…or perceived hunting.

    Alan.

    aoluain
    Participant

    When I was using L lenses they still hunted sometimes based on Alan’s post above but
    it wasnt as noticable because the focus is so fast and silent with the Canon USM.

    Thanks for the replies, some interesting info there. I did use a 70-200 F4L in the past and I always thought it was superb for focus accuracy but as you say the focus was very fast and quiet thanks to the USM so I probably just didn’t think about it the same.

    Pretty impressed with the 24-70 so far, seems like a decent lens for the money. Just need to further develop the muscles in my hand to support the weight of it when using it for a full day :D

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