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Author Topic: Canon 61 AF points  (Read 8646 times)
AndreG
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« on: July 25, 2012, 06:54:26 AM »
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I am trying to decipher Canon documentation on their 61 Autofocus Points.

Here’s what I comprehend:
1.   The 5 cross-points column in the center are the most precise points with a F2.8 or higher and with a F4 Lens.
2.   The other F4 points would be also accurate for a F4 lens but less with a F2.8 lens or higher.
3.   Only the vertical and horizontal lines of the F5.6 AF points would respond to a F5.6 lens.
4.   I am out of luck with any F8 lens or equivalent ex. 600mm with a 1.4 multiplier.

There must be more to it; any information will be appreciated.
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NancyP
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« Reply #1 on: July 25, 2012, 04:54:14 PM »
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Canon 1 D Mk III and 1 D Mk IV are the only ones that allow auto-focusing at f/8. All other Canons are S.O.L.
BTW, your equestrian photographs are lovely.
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AndreG
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« Reply #2 on: July 25, 2012, 05:14:07 PM »
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Thank you for your appreciation and for the technical information.

The documentation is really not clear on the subject. Yes, I could wait to do some tests when I receive my 1DX but still it could same time and mist opportunities.

Thank you for your time.
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N Walker
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« Reply #3 on: July 25, 2012, 05:55:20 PM »
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I am trying to decipher Canon documentation on their 61 Autofocus Points.

Here’s what I comprehend:
1.   The 5 cross-points column in the center are the most precise points with a F2.8 or higher and with a F4 Lens.
2.   The other F4 points would be also accurate for a F4 lens but less with a F2.8 lens or higher.
3.   Only the vertical and horizontal lines of the F5.6 AF points would respond to a F5.6 lens.
4.   I am out of luck with any F8 lens or equivalent ex. 600mm with a 1.4 multiplier.

There must be more to it; any information will be appreciated.


'Only the vertical and horizontal lines of the F5.6 AF points would respond to a F5.6 lens'.


A Lens aperture remains fully open between exposures when the AF is taking measurements. For example f/2.8 lens stopped down to f/5.6 - before exposure the lens remains fully open at f2.8, the AF takes readings at full aperture then the exposure is made using the aperture selected (in this example lens closes down to users chosen f/5.6 aperture). The operation is repeated.

F5.6 AF line sensors accommodate a plurality of lenses up to and including f/5.6 but are less efficient or wont operate with slower lenses, e.g. 600mm f/4 + 2X converter (=f/8). Some cameras which do not offer f/8 compatible AF point(s) may still auto focus with a lens/converter combination at f/8 but are 'very' slow and less accurate, other cameras may refuse to operate at f/8.

F/2.8 Af line sensors are known to be 2-3 times more accurate then f/5.6 types as they are provide almost double the baseline measurements , however due to their extra precision they are not very efficient at detecting subjects with a large amount of focus blur - F/5.6 AF line sensors are far more efficient at initially tracking large amounts of focus blur - when the subject has been brought into better focus the f/2.8 line sensor(s) take over (if you have one selected).

Contrary to widespread rumours vertical only line sensors are as accurate as cross type sensors PROVIDED they have sufficient contrast in the opposite direction (horizontal) to work with - try focusing a vertical only AF point on a vertical line (door or window frame), you will find the AF system cannot operate - to get around this you have to tilt the camera slightly to obtain focus.

AF calculations are based on mathematical tables. It's critical to configure an AF system for different subject movements, speed and the type of obstructions involved, especially the tracking sensitivity.

A slower tracking sensitivity is useful if the subject you are focusing on becomes temporarily obstructed - focus point(s) in use blocked by obstruction. For example team games such as rugby where lots of players' may cross over your subject or swimming such as the breast stroke where the subject's head routinely disappears from view under the water. If the camera detects a sudden and noticeable change in the focusing distance from its mathematical tables during tracking, a slower tracking sensitivity instructs the camera to continue tracking at the same speed as if the obstruction had not occurred.

The following may help

http://learn.usa.canon.com/resources/articles/2011/1dx_af_precision_crosstype_article.shtml - see diagrams

http://www.learn.usa.canon.com/galleries/galleries/tutorials/eos_1dx_tutorials.shtml - video tutorials.
« Last Edit: August 24, 2012, 11:06:01 AM by Nick Walker » Logged

AndreG
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« Reply #4 on: July 25, 2012, 08:08:55 PM »
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Hi Nick,

I cannot thank you enough for taking the time to answer my question in great detail and with precision.

It's much more clearer now and explains what I have been having trouble to photograph Horse Jumping Competitions were the camera tracks the horse but the AF locks on the post holding the poles when the horse's head passes behind and the camera does not have the time lock on again, the horse is gone...

I will follow up and read the articles and videos.

Thank you again Nick.
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N Walker
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« Reply #5 on: July 26, 2012, 02:05:35 AM »
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Andre,

I have amended the following sentence which was incorrect (strange smiley icon was being displayed next to text f/2).

A Lens aperture remains fully open between exposures when the AF is taking measurements. For example f/2.8 lens stopped down to f/5.6 - before exposure the lens remains fully open at f2.8, the AF takes readings at full aperture then the exposure is made using the aperture selected (lenses closed down to users chosen f/5.6 aperture). The operation is repeated.

« Last Edit: July 26, 2012, 02:13:45 AM by Nick Walker » Logged

AndreG
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« Reply #6 on: July 26, 2012, 06:39:36 AM »
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Hi Nick,

I re-read attentively the first article you recommended. It all came together with your information. I had completely forgot that a lens is fully opened when focussing. Now, all I need is the delivery of the 1DX. I am looking forward to the results compared to my 1DSM2 (I always used the center AF sensor with a 200mm F2 and 400mm F2.Cool. I did rent the new versions but they did not convince me to upgrade. The percentage of keepers and focused photos where about the same as the Mark2.

Thank you again, you help me quite a lot.
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NancyP
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« Reply #7 on: July 26, 2012, 05:48:48 PM »
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Thanks for the explanation of tracking sensitivity, and why variable tracking sensitivity is useful (and available only on higher-end cameras). I have the 60D, a splendid first digital SLR for a former film manual SLR photographer. Its AF options are pretty basic and I am still trying to master the relatively straightforward task of "bird in flight" shots (AI Servo), and have run across the annoying Tree Branch Problem many times, analogous to Andre's horse-jump pole and/or bar problem.
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SangRaal
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« Reply #8 on: August 23, 2012, 06:44:10 PM »
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I have been experimenting since March with the 61 auto focus points on the 5Diii and I am not sure that any of the replies you received are correct; nor am I sure that the 1DX auto focus system is exactly the same as the new 5Diii, I watched that canon tutorial listed and you can't really get it until after you read the manual pages 79 to 84 (5Diii I didn't see the 1DXup on line). Those pages of the manual break down the canon lenses into 6 groups A through F each group of lenses uses the focusing points differently some of the lenses appear in more than 1 group based on whether they are used with a telextender. After you read that part of the manual go to the part of the manual that explains the focus drive system pages 69 to 79 and read them. the tutorial will make more sense then. Even better than the canon tutorial is a 4 part blog post test conducted by Roger Cicala(sic) and his team at Lens Rental Dot com on this focusing system and numerous lenses from Canon, Zeiss, and sigma etc. Yes even the manual focus lenses with the built in focus confirm system fall into 1 of the 6 groups of differing lens focus drives; this series of test articles is way better to use in understanding this system than the Canon tutorial.   
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SangRaal
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« Reply #9 on: August 24, 2012, 08:21:38 AM »
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Let me amend my above post 8 lens groups A - through- H.
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William Walker
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« Reply #10 on: September 09, 2012, 02:51:55 PM »
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Here is a link to a guide that I made the most sense of...there are good illustrations too.

learn.usa.canon.com/resources/articles/2012/1dx_guidebook.shtml?categoryId=125

Go to bottom-left of the page and download the PDF.
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