Lens auto focus issues

Discussion in 'Reality Check' started by ShutterMOM, Feb 6, 2009.

  1. ShutterMOM

    ShutterMOM
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    Hey guys. I am having focusing issues with the Tamron AF 70-300 lens. When I have the AF on, the lens can't seem to find the focus point I'm going for even though I can see that it is within range. Is this happening because this is a crappy lens or user error?
     
  2. HatMan

    Pentaxian Extraordinaire

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    Is it possible that the target is too dark to focus on? It may be apparent to your eye what you're trying to focus on, but the camera's sensor requires contrast on one of the sensors. If there's not enough, it can't focus.

    There are several good articles on the web about how AF systems work and it's a good idea to understand how the sensors work.

    It's also important to know whether your camera has cross type or vertical bar type sensors and in which location they are. For example, if you're using a focus point near the left of the frame to focus on a tree, chances are you're trying to focus a vertical line with a vertical sensor and the sensor does not "see" the contrast required to focus even though it seems plainly clear to you.

    Here's a link to an article specific to the Nikon D70 and an issue many dSLRs experience, although I don't necessarily agree with all the information given, it gives a very good description of how most modern AF systems work and makes for a short and informative read: http://www.focustestchart.com/chart.html

    What focal length are you using and what's your aperture? If you're at 300, you're at the larger number (likely 6.3 or higher) which means you may also need to up the sensitivity to let in more light. You might also try shining a flashlight on what you're trying to focus on and then turning it off to take the shot.

    Hope this helps!
     
  3. novascotiaskier

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    I had problems with my D70s in low light and ended up using a small LED flashlight to temporarily light up the subject to get a lock. If you are shooting in low light, this is an easy thing to try to see if it is your problem or not.
     
  4. Squashtowner

    herp derp

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    I have that same lens for my Nikon. IT's slow to Autofocus, and if it's darker outside, it will hunt for a focal point all night :(
     
  5. ShutterMOM

    ShutterMOM
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    Focal length was closer to 300. I was at the widest aperture(smaller number) It would let me go. It probably didn't have enough light to find a focus point. I'll try the flashlight thing and see if that works.
     

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