Question about auto-focus and AF points

Discussion in 'Photography Beginners' Forum' started by MDesigner, Jun 12, 2008.

  1. MDesigner

    MDesigner TPF Noob!

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    Warning, this is quite the newb question.

    My Canon EOS 40D has 9 AF points. In the default mode, it basically somehow decides where to focus.. so it chooses one or more AF points and then tries to achieve focus using those points. My question is, how could I ever shoot a spur-of-the-moment photo.. take off the lens cover, turn on the camera, and shoot? I never know which of the 9 points the camera is going to use. I feel like any time I want to get the right shot, I need to manually set the AF point, and then shoot.

    How do others deal with this? Do you skip AF altogether and use MF instead? I have trouble using MF through the viewfinder, I don't think my eyes are good enough, but the Live View feature rocks (when I'm on a tripod) and if it's a still object I'm shooting, it lets me achieve perfect focus.
     
  2. NateWagner

    NateWagner TPF Noob!

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    well, the way I understand it, doesn't the camera stick with whatever you had it set to last?

    Well, I often do one of two things. I either set it to one of the outside focus points and leave it there (I often use those for portraits because the faces end up there instead of in the center) or I set it to the center focus point. By doing that you automatically know where the camera is going to choose.

    On my camera it stays with the most recent one I've chosen even after I turn it off and back on, but it may be different with yours (I've never played around with a 40d)

    Nate
     
  3. MDesigner

    MDesigner TPF Noob!

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    Yes, if I use the center point, it remembers that. What I mean is, if I set it to automatic where it chooses which focus point(s) to use, it chooses them at will.. very unpredictable. I feel like I always need to override that and manually set my focal point to get the shot I want. I suppose I could just leave it set to center point, and use the traditional compose, focus, then re-compose technique.
     
  4. Sarah23

    Sarah23 TPF Noob!

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    Why don't you just leave it on a focus point that you most use, and then change it if you need to? Why let the camera choose? It doesn't know what you are photographing. :) Once you get comfortable, you can switch focus points pretty darn fast. It becomes second nature. I never ever let the camera decide.
     
  5. BoblyBill

    BoblyBill TPF Noob!

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    On mine I always keep it on the center focus point. That way I know everytime I pick up the camera, I can expect to aim what I want to shoot into the middle, recompose, and take the shot (with a little practice you can do this very fast). I also have mine set to AI servo. That allows me to shoot moving objects just moments after I turn on the camera and get a sharp focus. Then I can direct that focusing point to other places in the view finder while shooting so that I can get a better composision on moving objects. The AF-ON button is also my friend because on AI it will continue to focus when I press the shudder button half way down and I use that to stop focus. I have to practice at that because with my grip and portrait shots it becomes a little hard to reach the button but I manage.
     
  6. MDesigner

    MDesigner TPF Noob!

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    Good points all around! Pun intended. ;) I think I'll just have it set to center point by default.

    Not sure if I follow.. can you elaborate a bit? AI Servo mode is the one, if I recall, that never beeps a confirmation as it focuses.. as long as the shutter is half-pressed (or AF-ON is pressed), it continues to focus no matter where you move. AI Focus is similar but it supposedly uses some sort of "intelligence" to determine if your subject is moving.. if it is, it goes into servo mode. I tried AI Servo and holding the AF-ON button didn't stop it from focusing. I'm probably misunderstanding something..
     
  7. BoblyBill

    BoblyBill TPF Noob!

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    Ah... yes... In the menu I have fnIV (1) I have it set to Metering + AF start/AF stop. Then if you have fnIV (2) set to disable then when you hit the AF-ON button it will lock the focus in AI Servo.
     
  8. MDesigner

    MDesigner TPF Noob!

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    Nice.. thanks!
     
  9. Mav

    Mav TPF Noob!

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    For landscape and even a lot of people photos I just leave my D80's AF in full autopilot mode. Usually it selects what I want. Sometimes it doesn't and I have to re-focus and let it re-guess. For people photos as long as I'm at f/4 or f/5.6 or a smaller aperture, the AF's best guess in full autopilot mode is usually good enough. If I want to shoot at f/4 at a longer focal length or close range (smaller depth of field and less room for error) or larger (f/2.8 and bigger) I select AF points manually and leave the camera on continuous focusing mode (AF-C in Nikon terminology).

    If you just want to be able to grab and fire, that's what the old "f/8 and be there" comes from. Even if you have AF (or even MF) focusing error, you'll have enough depth of field that what you want to be in focus will still usually be in focus. And if you leave the camera on full auto AF it'll definitely lock onto something. If you only have a single AF point selected and it doesn't happen to fall on anything that it can focus on it may lock up and not shoot at all.
     
  10. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    You can set the camera to use a specific AF point or to use all of them. It's designed to pick up on areas of contrast and maybe movement. It might sometimes miss but the 40D has a pretty advanced AF system and is said to be pretty good.

    Personally, I keep my AF set to the centre point and use the focus-recompose method with my 20D. Another advantage of this method is that the centre focus point is more sensitive than the others. With a lens that has a max aperture of F2.8 or larger, it's a cross type sensor rather than just a linear type sensor. I believe the 40D has several cross type sensors or is more sensitive.
     
  11. MDesigner

    MDesigner TPF Noob!

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    All nine of the 40D's AF points are cross type sensors. This was one of the advantages over the XSi/450D.
     
  12. Jon0807

    Jon0807 TPF Noob!

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    I too have mine set to center since I can always lock focus on a subject then recompose. Once in a while I'll let the camera decide for me and have it at all 9 points when I"m shooting landscape. And on occasion I'll specify the other points as well, especially when I'm using a tripod.
     

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