Improvement On Canon Auto Focus Points

Discussion in 'Digital Photography' started by Peter, Jul 22, 2006.

  1. Peter

    Peter Guest

    I am interested to know why the canon auto-focus points are all around the
    centre of the image and not spread out more. Also, why have a diamond shape
    and not a square shape?

    I would have thought that the AF points should be square and cover the areas
    directly where a lot of shot are composed (for example the old rule of two
    thirds framing)?

    Example:

    1D Mk II 45 AF points
    http://i5.tinypic.com/20qjp6e.jpg

    20D 9 AF points
    http://i5.tinypic.com/20qjq53.jpg

    My design 13 AF points
    http://i5.tinypic.com/20qkhs6.jpg

    Granted, I am no camera designer, but why not have focusing points where I
    have positioned them?
    Peter, Jul 22, 2006
    #1
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  2. Peter

    Clive Guest

    "Peter" <> wrote in message
    news:...
    >I am interested to know why the canon auto-focus points are all around the
    >centre of the image and not spread out more. Also, why have a diamond
    >shape and not a square shape?
    >
    > I would have thought that the AF points should be square and cover the
    > areas directly where a lot of shot are composed (for example the old rule
    > of two thirds framing)?
    >
    > Example:
    >
    > 1D Mk II 45 AF points
    > http://i5.tinypic.com/20qjp6e.jpg
    >
    > 20D 9 AF points
    > http://i5.tinypic.com/20qjq53.jpg
    >
    > My design 13 AF points
    > http://i5.tinypic.com/20qkhs6.jpg
    >
    > Granted, I am no camera designer, but why not have focusing points where I
    > have positioned them?

    A wild guess, but if using AF, Canon may assume that the person taking the
    picture is going to have the 'subject' in the centre of the frame.

    If NOT, then you can either use focus lock, or switch to manual focus?

    Just my guess.

    Clive
    Clive, Jul 22, 2006
    #2
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  3. Peter

    Bill Hilton Guest

    >Peter wrote:
    > I am interested to know why the canon auto-focus points are all around the
    > centre of the image and not spread out more.


    They need a certain amount of contast to 'define' focus, so I guess
    it's much easier to get this near the center.

    This document from Canon on the 1D pro series has some interesting info
    on autofocus, starting on pg 6 ...
    http://www.photoworkshop.com/canon/EOS_Digital.pdf

    > I would have thought that the AF points should be square and cover the areas
    > directly where a lot of shot are composed (for example the old rule of two
    > thirds framing)?


    On the Pro models with 45 points the points seem pretty useful to me
    ....

    > Example:
    >
    > 1D Mk II 45 AF points
    > http://i5.tinypic.com/20qjp6e.jpg


    The active area is 8 x 15 mm for all the 1D bodies, so with a smaller
    sensor like the 1D Mark II the 45 points fill more of the screen,
    allowing more flexibility ... here's an example to compare to the
    full-frame one you just posted ...
    http://members.aol.com/bhilton665/bunny_af.jpg

    Bill
    Bill Hilton, Jul 22, 2006
    #3
  4. Peter

    Peter Guest

    Fair comment, but focus lock is not so practical when the subject is moving
    and you don't want to put the subject in the centre of the frame.

    Manual focus is difficult when the subject is unpredictable, moving quickly
    and you are concentrating on framing the subject.


    "Clive" <> wrote in message
    news:44c27398$0$941$...

    > A wild guess, but if using AF, Canon may assume that the person taking the
    > picture is going to have the 'subject' in the centre of the frame.
    >
    > If NOT, then you can either use focus lock, or switch to manual focus?
    Peter, Jul 22, 2006
    #4
  5. Peter

    Peter Guest

    I will look at the link you provided.

    The photo of the bunny is perfectly focused on the eyes in this photo, which
    is what I am looking for, however what if you managed to get closer to the
    bunny or you had a greater focal length lens. The focus point would need to
    be higher and more to the left than these focus points.


    "Bill Hilton" <> wrote in message
    news:...
    >
    > The active area is 8 x 15 mm for all the 1D bodies, so with a smaller
    > sensor like the 1D Mark II the 45 points fill more of the screen,
    > allowing more flexibility ... here's an example to compare to the
    > full-frame one you just posted ...
    > http://members.aol.com/bhilton665/bunny_af.jpg
    >
    Peter, Jul 22, 2006
    #5
  6. Peter

    Jim Townsend Guest

    Peter wrote:

    > I am interested to know why the canon auto-focus points are all around the
    > centre of the image and not spread out more. Also, why have a diamond shape
    > and not a square shape?


    I don't know :) but I'll guess that it's a constraint that results
    from how the focus system works.

    The focus points you actually see in the viewfinder do nothing. They
    are only etchings on the focus screen. The points you see correspond
    with areas on the mirror that allow some light to pass through.

    This light that passes through is reflected by a secondary mirror to the
    focus sensors at the bottom of the mirror box. It might be difficult,
    expensive or inefficient to position the sensors differently because of
    the shape of the mirror box.
    Jim Townsend, Jul 22, 2006
    #6
  7. Peter

    Steve Wolfe Guest

    > The focus points you actually see in the viewfinder do nothing. They
    > are only etchings on the focus screen. The points you see correspond
    > with areas on the mirror that allow some light to pass through.
    >
    > This light that passes through is reflected by a secondary mirror to the
    > focus sensors at the bottom of the mirror box. It might be difficult,
    > expensive or inefficient to position the sensors differently because of
    > the shape of the mirror box.


    If I'm not mistaken (but I certainly could be), as an AF point moves
    farther from the center, the larger aperture you need in order to keep the
    sensor at the same performance level. By the time you get far enough into
    the corners to use for "rule of thirds", I think that the majority of lenses
    would lose AF capability.

    steve
    Steve Wolfe, Jul 22, 2006
    #7
  8. Peter

    Bill Hilton Guest

    >Peter wrote:
    > I will look at the link you provided.
    >
    > The photo of the bunny is perfectly focused on the eyes in this photo, which
    > is what I am looking for, however what if you managed to get closer to the
    > bunny or you had a greater focal length lens. The focus point would need to
    > be higher and more to the left than these focus points.


    Yes, this comes up sometimes with birds where you want to focus on the
    eye but it's out of the 'zone' ... if the subject is far enough away
    you can use One-Shot mode and FLR (focus-lock-recompose, described in
    the link I gave you). Or you can find something else in the frame
    that's parallel to the eye, typically on a bird this would NOT be the
    shoulder but maybe the back or the beak ... easy to miss just a bit on
    these however, especially in One-shot mode if the bird moves his head
    just a fraction.

    Probably Jim and Steve are on the right track with their posts, this is
    a physical limitation ... on my 1D series bodies if the max aperture is
    wider than f/2.8 then I get the best AF performance (again, the cross
    sensors are explained in the link I gave) ... between f/2.8 and f/5.6
    you can use all 45 points ... at f/8 you can still AF with the center
    point (something the consumer models don't allow) ... if you fake out
    the camera and are at f/11 with two converters (say f/4 lens with 2x
    and 1.4x stacked) I can still AF about half the time with the 1D Mark
    II if there's enough contrast.

    So the obvious implication is that the wider the aperture the more you
    can do with AF ... to have AF points covering more of the frame you'd
    probably need lenses with max apertures of f/2 or f/1.4 and in this age
    of zooms there aren't enough lenses like this to make it worthwhile for
    camera makers to design AF systems with these apertures in mind.

    Just curious, but are there other dSLR cameras with wider AF coverage?
    The 45 points look pretty good to me, especially compared to the 9
    point consumer models.

    BIll



    >
    > "Bill Hilton" <> wrote in message
    > news:...
    > >
    > > The active area is 8 x 15 mm for all the 1D bodies, so with a smaller
    > > sensor like the 1D Mark II the 45 points fill more of the screen,
    > > allowing more flexibility ... here's an example to compare to the
    > > full-frame one you just posted ...
    > > http://members.aol.com/bhilton665/bunny_af.jpg
    > >
    Bill Hilton, Jul 23, 2006
    #8
  9. Peter

    JohnR66 Guest

    "Peter" <> wrote in message
    news:...
    >I am interested to know why the canon auto-focus points are all around the
    >centre of the image and not spread out more. Also, why have a diamond
    >shape and not a square shape?
    >
    > I would have thought that the AF points should be square and cover the
    > areas directly where a lot of shot are composed (for example the old rule
    > of two thirds framing)?
    >
    > Example:
    >
    > 1D Mk II 45 AF points
    > http://i5.tinypic.com/20qjp6e.jpg
    >
    > 20D 9 AF points
    > http://i5.tinypic.com/20qjq53.jpg
    >
    > My design 13 AF points
    > http://i5.tinypic.com/20qkhs6.jpg
    >
    > Granted, I am no camera designer, but why not have focusing points where I
    > have positioned them?
    >

    One problem I have if the points are too spread out is the outer point
    catching something I don't want. Typically the camera will find something in
    the foreground. I turn off the out sensors and compose with the center.
    John
    JohnR66, Jul 23, 2006
    #9
  10. Peter wrote:
    > I am interested to know why the canon auto-focus points are all
    > around the centre of the image and not spread out more. Also, why
    > have a diamond shape and not a square shape?
    >
    > I would have thought that the AF points should be square and cover
    > the areas directly where a lot of shot are composed (for example the
    > old rule of two thirds framing)?
    >
    > Example:
    >
    > 1D Mk II 45 AF points
    > http://i5.tinypic.com/20qjp6e.jpg
    >
    > 20D 9 AF points
    > http://i5.tinypic.com/20qjq53.jpg
    >
    > My design 13 AF points
    > http://i5.tinypic.com/20qkhs6.jpg
    >
    > Granted, I am no camera designer, but why not have focusing points
    > where I have positioned them?


    My guess is because they work best there for most people most of the
    time.

    From my experience they do a very good job. The grouping seems to
    perform very well for the photography I do. On occasion it does not work
    and I go manual. Manual worked back in the 60's when I worked as a pro, it
    still works today.

    --
    Joseph Meehan

    Dia duit
    Joseph Meehan, Jul 23, 2006
    #10
  11. Peter

    Neil Ellwood Guest

    On Sat, 22 Jul 2006 20:32:00 +0100, Peter wrote:

    > Fair comment, but focus lock is not so practical when the subject is moving
    > and you don't want to put the subject in the centre of the frame.
    >
    > Manual focus is difficult when the subject is unpredictable, moving quickly
    > and you are concentrating on framing the subject.


    Why not change the AF mode to the one you want?
    --
    Neil
    Delete l to reply
    Neil Ellwood, Jul 23, 2006
    #11
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