Auto focus / AF points

Discussion in 'Canon EOS' started by gerald_roston, Aug 1, 2009.

  1. All,
    I am using my EOS 40D, with a 300 m lens, to photograph birds at our bird bath. I have the camera on a tripod and am using the EOS utility with a long USB cable as a remote release.
    My question has to do with the auto focus. Since I know exactly where in the frame the bird will be, and since I have this spot centered in the frame, it seems that I should select only the central AF Point. However, when I was playing with this (just pointing the camera around a room), that with only the central AF Point selected, the camera struggled to focus in many cases.
    For example, when pointed at a flat wall, it could not focus. When pointed at a tripod leg, about 5 feet away, it would seek into focus, then seek all the back out of focus. This process of seeking to focus, then seeking back out occurred quite often.
    Given that birds do not pose, I am reluctant to set up and potentially waste a lot of time waiting for the bird, etc., if the focus will misbehave. Can you explain why the camera is behaving this way? More importantly, can you suggest what I might do to minimize out-of-focus risks when trying to take these pictures?
    Many thanks.
     
  2. The 40D has one of the best AF systems on a consumer camera. However it still requires points of contrast to acquire focus. No AF system can lock on a blank white wall or clear blue sky.
    If you are in a low contrast situation, you can achieve AF most of the time by manually selecting the AF sensor and placing it on a point of contrast, e.g., a point of light or a border between a light and dark material. The center AF sensor is the most sensitive although the other 8 are almost as good in all but the lowest contrast. If contrast is so low if will not focus, use AF assist from a Speedlite or St-E2 (there is a CF to enable AF assist/disable flash). With AF assist you can even nail that many fabled blank white wall...
     
  3. Hi,
    I have to ditto Puppy Face and add that the 300mm closest focus distance is 4.9 feet. Your tripod leg shot maybe was too close for the 300mm.
    How far away from the bird bath are you. Does your 300mm lens have a focus limiter? If so, and you are "far" then try using that and maybe pre-focusing on the bird bath. Then when the bird comes the lens will not have as much work to do to focus on the bird. And/Or Put a flash on the camera and experiment with that and the AF assist as Puppy Face suggested.
    Somebody correct me on the focus limiter and such if that is wrong.
    Best Wishes
     
  4. Put a toy in the bird bath where you expect the birds to be, and focus ahead-of-time. Now remove the toy, switch your lens to manual focus, and wait for the birds.
     
  5. I'm with Peter: in a situation like this where you know in advance where the subject is going to be, or very nearly so, manual focus seems safer. There is always the risk that the bird will move slightly off of the AF point and send your camera into a long, slow hunt for focus where you could have just pushed the button and had the shot.
     
  6. I have a 40D and when taking pix from 2-3 meteres away with a flash on, I use center AF, I lock AF on the subject and very often even without recomposing the AF slips away on the very far end so everything I wanted to have in focus is OOF, I can delete evertyhing... happened to me 2 weeks ago, when I doing a job and yesterday as well, bad back focusing (only with 16-35 F2.8) then I usually switch to 70-200 F2.8 and things are fine.
    I might update software for 40D if there is one, will search now, otherwise PITA.
     
  7. Thank you all. I did start using flash and I also tried manual focus. Now, if the birds would just sit still a little longer...
     

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