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Improvement On Canon Auto Focus Points

 
 
Peter
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      07-22-2006
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?


 
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Clive
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      07-22-2006

"Peter" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
>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


 
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Bill Hilton
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      07-22-2006
>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

 
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Peter
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      07-22-2006
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" <(E-Mail Removed)> wrote in message
news:44c27398$0$941$(E-Mail Removed)...

> 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?



 
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Peter
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      07-22-2006
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" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
>
> 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
>



 
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Jim Townsend
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      07-22-2006
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.




 
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Steve Wolfe
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      07-22-2006
> 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


 
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Bill Hilton
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      07-22-2006
>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" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed) oups.com...
> >
> > 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
> >


 
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JohnR66
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      07-23-2006
"Peter" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
>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


 
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Joseph Meehan
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      07-23-2006
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


 
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