How does the autofocus function?

Discussion in 'Nikon' started by Psytrox, Oct 25, 2012.

  1. Psytrox
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    Psytrox Member

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    Hello,

    I was curious as to how the Auto-focus actually functions. I'm mostly interested in how the AF-C functions. For the sake of simplicity, lets say that I only have the center square focus point active, if it matters, its on a D7000.

    How does the auto-focus continuously keep that in focus as it moves? Does it "memorize" the colours in that set of pixels? What happens if the target moves out of the "active focus square"? What happens if something moves between the target and the camera?
  2. Patriot
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    Patriot Active Member

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    I think some sensors measures the contrast of what the focus point is on and tracks the change between them it and the background/foreground.
  3. KmH
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    KmH Helping photographers learn to fish

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    The square isn't the focus point, but the focus sensor is somewhere in the square.
    Also the center focus sensor (and 8 others) are cross-type focus sensors. regular focus sensors are either horizontal or vertical. Horizontal sensors detect vertical lines, and vertical sensors detect horizontal lines.

    Review pages 91-100 of the D7000 user's manual. AF-C is described on page 91.

    Autofocus - Wikipedia, the free encyclopedia


    Understanding Camera Autofocus

    51 Point AF System | Benefits of the Nikon 51 Point AF System Technology from Nikon
    AF Area Modes from Nikon
    Motion Detection from Nikon
    Subject Tracking from Nikon
    3D Focus Tracking from Nikon
    Last edited: Oct 25, 2012
  4. Big Mike
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    Big Mike I am Big, I am Mike Staff Member

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    The AF system looks for contrast...in either a vertical or horizontal direction...or both, if it's a cross type sensor.
    So what it does, is find the lens focus position that gives the greatest contrast (as you go out of focus, the lines blur and you have lower contrast, so the highest contrast is the most in-focus).

    As for AF-C...it just keeps the AF constantly active. If you move the camera, or the subject moves or if something gets in between you and the subject...the camera will try to focus on whatever is under the active focus point.

    This is different from AF-S in that AF-S mode will work until it achieves focus, but then it will stop and lock the focus. This allows you to focus on a subject, lock the focus and then recompose, so that the subject doesn't have to be right on the focus point, to be in focus in the photo.
  5. johncam
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    johncam New Member

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  6. Psytrox
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    Psytrox Member

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    So if I understand correctly, the little red square, is the active focus point(?), if the object, or person moves out side the active focus point, the camera will refocus - the same for that matter if I move the camera. This meaning that you can not use AF-C to get a focus point and recompose the photograph?

    The 3D tracking almost sounds too good to be true... does anyone have any experience using this..?
  7. Big Mike
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    Big Mike I am Big, I am Mike Staff Member

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    That's correct. If the subject isn't in the red box, the camera will try to focus on whatever is now in the box.

    So if you are going to use the focus-recompose technique, you will likely have to be in AF-S.

    Although, as mentioned above, a common technique is to use 'back button focus' where you set the camera so that pressing the shutter button does not activate the AF, and moves it to a button on the back (pressed by the right thumb). This way, you can still use AF-C to continuously focus on your subject, but if you want to lock the focus, you just release your thumb from the new AF button. I've been shooting this way for many years and I now find it awkward on a camera with the focus on the shutter button.

    There are a few versions of AF tracking or hybrid mode. I'm not familiar with the different modes of it or even how it really works. But the idea is great, the camera tries to lock onto your subject (similar to AF-S mode) but then it tries to change the focus and follow the subject if it detects movement (similar to following your subject while in AF-C).
    The issue that may be a problem, is that it's relying on the camera to decide what your subject is. And anytime you allow the camera to make a decision...you are giving up some amount of control and you may thus get unpredictable results.

    What I tell my students is to try the hybrid/tracking mode, but know when to switch out of it when it's not working for you.
  8. Psytrox
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    Psytrox Member

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    I've never thought, or heard of the hybrid mode/setting. I'll try it out as soon as I get a chance. Atleast test it out and see if I like it. At the moment it sounds akward. But I guess its just one of those things that sounds and feels awkward getting used to, once you're used to it, anything else feels wrong.

    Its only now recently that I've started to become a lot more aware of what and where I'm actually focusing the camera. I've always known it in theory. But during the actual execution, that part tends to slip up. It started when I got home from a wedding (I was just a guest with a camera - from what i remember the only one with a camera, but just so that its said no obligations as a photographer;) ). I noticed during post processing, the pictures taken later that evening with the auto-focus (the AF where the camera uses its artificial "intelligence" to decide what to focus on) was absolutely worthless.
    I suppose its just part of the learning curve. I wont learn until i get burned :D
    I also find it easier to learn and understand something, when I actually know how it works...

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