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Thread: AF points

  1. #1
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    Default AF points

    Is more AF points mainly for faster autofocus?

  2. #2
    Moderator Octarine's Avatar
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    Default Re: AF points

    No. Fast AF depends on the type of the sensor (cross-type vs. simple) and the processor behind to analyse and send the right signal to the lens. Lower end cams only have one cross-type sensor in the center, all the rest are just simple AF sensors). Higher end bodies have more than one cross-type sensor.
    Multiple AF points help to track moving objects which might not be always in the center of the frame. Also, it helps to avoid the issue with "focus & recomposing" where the focal plain is very thin and slightest movements will result in OOF. Use the AF point that is across the part of the image that must be in focus (e.g. eyes). No need to shift the camera after focusing.
    EOS

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    Default Re: AF points

    Thanks.

    I can understand the part on moving object.

    For off center subject, since there are many AF points cross the image, how do the camera decide which part of the body to focus on, if the camera take the average, then eye could be missed? Or I have to manually set the AF points?

  4. #4

    Default Re: AF points

    Quote Originally Posted by LBL2009 View Post
    Thanks.

    I can understand the part on moving object.

    For off center subject, since there are many AF points cross the image, how do the camera decide which part of the body to focus on, if the camera take the average, then eye could be missed? Or I have to manually set the AF points?
    I suggest you try to read and research on Autofocus if that's really your interest. But the camera will normally focus on high-contrast areas. It can't automatically focus on the eyes for you, that's why you are supposed to manually select the AF point and then compose.
    Alpha

  5. #5
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    Default Re: AF points

    Thanks both of you.

    It is much clearer now.

  6. #6

    Default Re: AF points

    "19 cross-type for F2.8 or faster lens" - Can someone enlighten me what this mean... does it mean that the focus point is supposed to be somewhat faster and more accurate when 2.8 of faster lens is mounted on the body?

    If so, strange that for the newer 1Ds3, it's f/2.8 or faster while 1Ds2 it's f/4 or faster - doesn't this mean the Mark 3 limits you more since you need faster glass...
    Last edited by rendition; 15th September 2009 at 12:20 PM.

  7. #7

    Default Re: AF points

    Quote Originally Posted by rendition View Post
    "19 cross-type for F2.8 or faster lens" - Can someone enlighten me what this mean... does it mean that the focus point is supposed to be somewhat faster and more accurate when 2.8 of faster lens is mounted on the body?
    Exactly.

    Quote Originally Posted by rendition View Post
    If so, strange that for the newer 1Ds3, it's f/2.8 or faster while 1Ds2 it's f/4 or faster - doesn't this mean the Mark 3 limits you more since you need faster glass...
    Well, if you're shooting with a 1DsMkIII, you really should pair it with L glass to maximize the resolution.

    But aside from that, it's not really limiting; in fact, it also indicated that the sensors are much more sensitive and accurate now.
    Alpha

  8. #8

    Default Re: AF points

    Thanks...

    Well, if you're shooting with a 1DsMkIII, you really should pair it with L glass to maximize the resolution.
    L is definitely a must... but just that I have f/4 L glasses now.

  9. #9

    Default Re: AF points

    Quote Originally Posted by rendition View Post
    "19 cross-type for F2.8 or faster lens" - Can someone enlighten me what this mean... does it mean that the focus point will be faster and more accurate when 2.8 of faster lens is mounted on the body?

    If so, strange that for the newer 1Ds3, it's f/2.8 or faster while 1Ds2 it's f/4 or faster - doesn't this mean the 1Ds limit you more since you need fast glass...
    the centre point for the 1D3 is cross sensitive when used with a f/4 or faster lens. the remaining 18 points becomes cross sensitive when used with a f/2.8 or faster lens.

    meaning if you use a f/2.8 lens on the camera, all 19 points becomes cross sensitive. when used with a f/4 lens, the centre is cross while the rest are all horizontal types.

    it is the same as the Mark II except the Mark IIs has the 7 cross type af points all clustered in the centre like the Nikons.

    meaning on Mark II and III, if u use a f/4 lens, only the centre point is cross type.

  10. #10

    Default Re: AF points

    Quote Originally Posted by rendition View Post
    L is definitely a must... but just that I have f/4 L glasses now.
    Should be fine. You'd still benefit from the additional accuracy and sensitivity.
    Alpha

  11. #11

    Default Re: AF points

    if u use a f/4 lens, only the centre point is cross type.
    Cheh..... hmmm OK then. Now I know... Thanks.

  12. #12

    Default Re: AF points

    in fact im quite blur bout AF even after reading so much... for instant.. if i mf... and i select the point already... wad do u mean by re compose? very blur on focus

  13. #13

    Default Re: AF points

    Quote Originally Posted by rendition View Post
    Cheh..... hmmm OK then. Now I know... Thanks.
    the pro series are more geared towards users who are using the f/2.8 lenses than the f/4 lenses. which makes sense because of the difference in performance.

  14. #14

    Default Re: AF points

    Quote Originally Posted by hyori View Post
    in fact im quite blur bout AF even after reading so much... for instant.. if i mf... and i select the point already... wad do u mean by re compose? very blur on focus
    This is in your manual under the "Focus" section. Or google "focus and recompose"
    Alpha

  15. #15

    Default Re: AF points

    Quote Originally Posted by Rashkae View Post
    I suggest you try to read and research on Autofocus if that's really your interest. But the camera will normally focus on high-contrast areas. It can't automatically focus on the eyes for you, that's why you are supposed to manually select the AF point and then compose.
    Hi sorry can i query on this issue... when u say compose does it mean .. i select one AF point on the eye... den aeb lock it.. den focus again?
    Canon 450D/ F1.8 50MM/ EF-S 55- 250MM/ IS 18 - 15 MM

  16. #16

    Default Re: AF points

    Quote Originally Posted by hyori View Post
    Hi sorry can i query on this issue... when u say compose does it mean .. i select one AF point on the eye... den aeb lock it.. den focus again?
    No. Please read your manual or google what I suggested in my previous post. COMPOSITION is different from FOCUS.

    IF you have composed the picture and the selected AF point is already on the model's eye, great, just snap.

    IF it is not, then prefocus on the eye using the closest AF point; you do this by half-pressing the shutter. Then, while it's half-pressed, you can recompose the image then fully depress once you're done.

    This is in your manual, and in every basic newbie guide to photography. I suggest reading up a bit more.
    Alpha

  17. #17
    Senior Member limwhow's Avatar
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    Default Re: AF points

    Hi, gentlemen. I was following your discussions with keen interest. I use the 5dMkII.
    I like to use the AF point on one side of the VF to focus on my subject, so that I do not need to recompose.
    If I may OT slightly. My question is on metering: Personally I find that when use the side AF point and while on spot metering, I review the pictures and it seems that the camera meters on the centre of the VF, instead of on the AF point on the side.
    Some CS members have said that the camera will only meter in centre of VF, but my camera shop people say that the spot metering will follow my AF points.
    I am a little confused. Can you all advise me?

  18. #18

    Default Re: AF points

    Quote Originally Posted by limwhow View Post
    Hi, gentlemen. I was following your discussions with keen interest. I use the 5dMkII.
    I like to use the AF point on one side of the VF to focus on my subject, so that I do not need to recompose.
    If I may OT slightly. My question is on metering: Personally I find that when use the side AF point and while on spot metering, I review the pictures and it seems that the camera meters on the centre of the VF, instead of on the AF point on the side.
    Some CS members have said that the camera will only meter in centre of VF, but my camera shop people say that the spot metering will follow my AF points.
    I am a little confused. Can you all advise me?
    it meters on the small circle in the centre. for canon, only the pro series has spot af on selected af point.

  19. #19
    Senior Member limwhow's Avatar
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    Default Re: AF points

    Quote Originally Posted by flipfreak View Post
    it meters on the small circle in the centre. for canon, only the pro series has spot af on selected af point.
    5DMkII = pro series?

  20. #20

    Default Re: AF points

    Quote Originally Posted by limwhow View Post
    5DMkII = pro series?
    Nope. It's an enthusiast series. Pro is the 1D series.
    Alpha

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