focusing speed?

Discussion in 'Photography Beginners' Forum' started by Hardrock, Oct 1, 2009.

  1. Hardrock

    Hardrock TPF Noob!

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    Does aperture affect the focusing speed? I believe the lens is wide open when focusing before the shot and then stops down(unless shooting wide open) correct? So for example: Will the Canon 70-200 f2.8 focus faster then the 70-200 f4 due to more available light?
     
  2. KmH

    KmH Helping photographers learn to fish Supporting Member

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    The short answer is no.

    When the aperture opening gets smaller than f/5.6 most camera makers claim no guarantees that AF will work at all.

    AF requires contrast. If a lens performs poorly at rendering contrast it will slow down the AF process.

    Using a lens hood usually helps any lens render contrast.
     
    Last edited: Oct 1, 2009
  3. Hardrock

    Hardrock TPF Noob!

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    below f5.6 meaning f2.8 or f8? So with those 2 lens mentioned the focusing speed should be equal?
     
  4. Big Mike

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

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    Maybe.

    When the light gets low enough most AF systems will start to struggle, a wider aperture gives you more leeway when it comes to low light AF performance. With adequate light, it probably isn't an issue.

    Some cameras have certain AF sensors that are better at F2.8 or larger. For example, my 20D cameras have a centre AF sensor that is a cross type sensor when used with F2.8 lenses but only a linear sensor with slower lenses. That could certainly make a difference.

    The actual speed of the AF is mostly a result of the camera's AF reading and processing abilities and the motor in the lens.
     
  5. benlonghair

    benlonghair TPF Noob!

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    As I understand it, until you push the button, your lens is wide open, to allow more light in. So the slower the lens, the slower (or at least less accurate) your AF will be in lower light.

    In other words, a 2.8 lens is always focusing with the amount of light a 2.8 lets in even if you're stopped down to 16 or something.
     
  6. fiveoboy01

    fiveoboy01 TPF Noob!

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    Here's another question:

    Does ISO affect AF performance?

    (never mind I'm guessing not, as the mirror is in the way of the sensor - DUH)
     
  7. KmH

    KmH Helping photographers learn to fish Supporting Member

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    opening smaller than, like f/8. I edited my post to make it a bit clearer.
     
  8. Big Mike

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

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    No, they are not connected in any way.

    ISO really only enters into the mix after the light hits the sensor. Turning up the ISO is like turning up the volume on your stereo, it just amplifies the signal that is already there. And just like a stereo, if you turn it up too high, you get distortion which results in noise.
     
  9. fiveoboy01

    fiveoboy01 TPF Noob!

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    Mike did you miss this part of my post:mrgreen:

     
  10. musicaleCA

    musicaleCA TPF Noob!

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    Unless you're using the contrast-detection in Live View, instead of the phase detection of the AF sensor. Though in that case the camera is, I think, choosing an ISO that is sensitive enough for it to focus, and then changing back to your selected ISO when you depress the shutter release.

    To expand on that, (it may look like I'm picking on Big Mike with these quotes; I'm not, just expanding) this is something true of all Canon cameras. Generally, at f/2.8, all of your AF points are working quite swimmingly. On the 7D, for example, this means that all 19 AF points are effectively cross-points. On the 1Ds MkIII, all fifty some odd points are at their highest sensitivity. But, on Canon cameras, once you start using slower and slower glass, those cross-points fall-off FAST, turning into less-accurate horizontal or vertical points. Around f/5.6, even on a 1Ds MkIII, only the centre point is still a cross-point. Same goes for the new 7D. Nikon cameras, on the other hand, have more sensitive AF systems—or so I'm told—and this fall-off of sensitivity at the outer cross points is less pronounced.

    The overall effect of this is, that in any situation where fast, accurate AF is a necessity, on Canon cameras, f/2.8 or faster glass is the most preferable option, by far. Even at f/4 things fall off quickly. So the main difference between two lenses with different max apertures, when put on the same camera body, is not AF speed, but rather AF accuracy.
     
  11. Hardrock

    Hardrock TPF Noob!

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    Thanks guys for all of the great information!:thumbup:
     

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