Focus accuracy: Tamron 17-50 Vs Me Vs Canon 17-55

Discussion in 'Canon EOS' started by willhl, Nov 7, 2010.

  1. I've been using my Tamron 17-50 for a few years now to shoot weddings, social events & portraits and I love the image quality but I lack confidence in it's focus accuracy under certain circumstances and I'm trying to determine if a lens or technique upgrade would give better results.
    The times I have the most issues are when focusing on more distant objects, from full body length @ 35mm and greater distance (including @ infinity), to the extent that I'm not comfortable shooting at f2.8 because I'm not certain it will focus accurately enough to be sharp. I'm mostly using the lens on a 50D and see the problem with both centre and outer focus points. I have the camera set-up to only focus with the AF button on the back and I typically use servo autofocus.
    I guess my questions are:
    1. Is there an issue with my technique that could be causing the issues?
    2. Have other experience similar issues with the Tamron?
    3. Does the Canon have been focus accuracy in the conditions I outlined above?
    Thanks!
     
  2. Why do you use servo autofocus (I assume you mean AI Servo mode)? This is usually used for moving subjects and it tends to work best when the subject is moving at a constant speed and direction. It's much better with a subject that's some distance away and moving fast like a running dog or a moving car. It doesn't work as well with a subject that moves erratically and is unnecessary for still subjects. I think you would do much better using One Shot mode.
     
  3. +1 for the one shot mode.
    The only problem remaining would be the AF speed, I have compared the VC version of that lens (which is known to be faster Afing than the non VC version) to the canon 17 55 is, The canon is faster.
     
  4. +100 against AI servo. People don't run that fast in the weddings
    You have used the Tamron for a few years. Depending on the build quality and how you use it, the lens may already start to wear out and becomes inaccurate
     
  5. I own that lens and have used it a ton on my crop body. Mine focuses great. And as a portrait lens I
    actually prefer it over my 24-70 f/2.8 on my full frame.

    Something to realize is that AF targets aren't perfect in location on some cams. If you're shooting distant
    subjects with other objects visually nearby in the horizontal plane but spaced at different distances, you
    may be locking onto one of those.
     
  6. AI servo doesn't actually lock focus. It simply stops focusing when you release the * button. Not a bad a technique with ultra fast ring-USM/IF but really iffy with slow front extension focus designs like the tammy 17-50. I agree with the others, most of the time you'd be better off in one shot mode. At least it won't fire until it actually locks and confirms focus.
    And, yes, 17-55 IS AF is uber fast and reliable. Internal focus and ring-USM make a huge different over old front extension focus designs. Also you won't have to worry about the spinning MF ring snagging your fingernail...
     
  7. As already started earlier don't use AI servo. Under some sercomstances it may think your hand motion is subject motion and adjust the the focus due to that. For me One shot is very reliable.
    The focus points you see in the view finder are not the same size as the actual focus points. So the camera might focus on something just outside the point you want to be in focus. For example If you point the camera at a small bird in a tree the camera may not focus in it. Instead it may focus on a nearby branch or leaf. The best way to avoid this is to be aware of it and if necessary switch to manual focus.
    Avoid letting the camera sellect the autofocus point. the camera has no way of knowing what you want in focus It will instead just focus on the easiest item it finds. Most people just set there camera to the center autofocus point and leave it there.
    There are several ways to deal with the shallow depth of feild when using F2.8. First off do you need F2.8. If not just stop down a little. that will increase your depth of field and make it less likely that a focus error will be visible in the final image. Another option is to just back up a little. Depth of field is very narrow when you are close to the subject. Backing up just a few feet will greatly increase it. At 50mm at F2.8 anything you focus on that is about 15 Feet or more from you will be in focus. For shorter focal lengths the the lens infinity focus distance moves closer to you. If I recall correctly at 24mm anything beyond 6 feet and beyond will be in focus.
    Based on what you have said I don't see a reason for you to replace the Tamron.
     
  8. I've been using the Tamron as a bread 'n butter lens for a few years on both Pentax & Canon cropped sensor bodies. Focus
    issues have been known on all platforms. Fortunately the newer bodies have AF adjustment capability. In fact I tweaked the
    Tamron on my 7D last night for the first time. It only needed a +5 adjustment. Of course none of this helps you directly.

    This is a very sharp lens at a fair price. Perhaps you can conduct some simple tests using One Shot AF to validate your
    concerns.

    ME
     
  9. Thanks guys, I've been using the AI servo mode because I had assumed it would 'lock' on fine if I just released the focus button before I took the photo, plus it saved switching modes in a hurry when walking down the aisle etc. I liked the idea of always being able to track the subject and just shoot when I needed to. I also used to shoot quite a bit at dance events so it was essential then. For the more static cases, I'm quite sure focusing time/speed is not the issue.
    Steve, I don't always shoot at f2.8 or always want to but I do want to have the option when required otherwise it is a bit of a waste having a f2.8 lens. Some churches are pretty dark!
    Michael, I did test the camera for AF microadjustments but I don't think it needed any correction, my Sigma 70-200 need about +7 one end of zoom range and -7 the other though ... (works pretty good on my 30D after calibration though)
    Anyway I guess it's time to perform some more tests, thanks for your help.
     
  10. Thanks guys, I've been using the AI servo mode because I had assumed it would 'lock' on fine if I just released the focus button before I took the photo, plus it saved switching modes in a hurry when walking down the aisle etc.​
    Walking down the aisle is not very fast motion compared to the speed of the focus motors on many lenses thhese days (although I have no experience with your lens). Dancing should also not need AI Servo. AI servo works best for subject that ar moving in a straight line tward or away from the photograher. One Shot should be fast enough for most applications.
     

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