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Archive 2005 · Canon autofocus information
  
 
Jeff
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p.1 #1 · p.1 #1 · Canon autofocus information


I ran across this archived post in relation to another thread (sorry I missed this first time around!), and thought it worth posting and sticking to the forum for awhile. Lots of good info here, and thanks to RDKirk for paraphrasing and taking the time to include the information in that thread. (BTW, this is in reference to the 20D, though I'd guess the basic technical details are valid across the line of dSLRs).

Jeff
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RDKirk wrote:

According to the major material in Canon's "Lens Work III," the description in their US patent application, and remarks by Chuck Westfall, to put it briefly:

The AF system sensors are located in the floor of the mirror box. They receive the image through the semi-silvered mirror, which is then reflected downward by a secondary mirror hinged to the back of the main mirror. This forms a virtual focusing plane that is supposed to be at the exact same plane as the sensor (a point of possible miscalibration).

Each AF sensor consists of a pair of short lines of pixels forming an
...Show more

http://www.fredmiranda.com/forum/topic2/228079/0#1877222

RDKirk wrote:

There are five main places this information comes from:

1. Canon's "Lens Work III" book (glossary, mostly)
2. Canon's latest application for a US patent on their autofocus system
Go to www.pat2pdf.org and enter patent # 6,603,929
3. Canon's pamphet on getting the most from EOS DSLRs
www.photoworkshop.com/canon/EOS_Digital.pdf
4. Canon Professional Services European web site FAQ
www.cps.canon-europe.com/faq/index.do
5. Lots of methodical observation and practice with the camera.

[furthermore] RDKirk wrote:

"If the camera places the actual focused plane within the depth of focus range, the intended focused plane of the subject should "look sharp" on a 6x9-inch print from a distance of 10 inches. In "high precision mode" the intended plane of focus should "look sharp" on about an 11x14-inch print at 10 inches (extrapolating from the standard size given by Canon for normal mode).

This standard is important to understand. If you use high precision mode and view the image on a monitor at 100 percent of the original pixel resolution, do not expect the actual plane of focus to
...Show more



Jun 21, 2005 at 05:02 AM
zero2k
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p.1 #2 · p.1 #2 · Canon autofocus information


And because of the use of double mirrors in the 20D, it has a higher risk of mirror damage than a simple single one... I should know, mine died (the inner mirror has a lever that controls the angle of its movement, that lever breaks and the mirror won't flip).

Leon



Jun 21, 2005 at 05:19 AM
glowrider
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p.1 #3 · p.1 #3 · Canon autofocus information


This is really great stuff, thanks for the repost. Had no idea about the vertical and rectangular sensors and their proclivities. With all the fuss over manually focusing, sometimes we may forget about the complicated AF systems in our cameras that need to be learned to use to our advantage.


Jun 21, 2005 at 06:02 AM
20Dshooter
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p.1 #4 · p.1 #4 · Canon autofocus information


Wow! Wha an incredible piece of work! A collection of data that Canon ought to put out, really, but since they leave it to the user to figure it out, and most of us don't, of course, thank you, RDKirk, for compiling this "non-exhaustive" collection of important information. I can't imagine how many pages this would go on for if it were exhaustive!

Anyway, I was having difficulty understanding this paragraph. Could anyone more more knowledgeable than I help me with this, please?

When the camera determines how far and in what direction the lens must move to cancel the phase difference, it does so within a tolerance of "within the depth of focus" of lenses slower than f2.8 (down to f5.6) or "within 1/3 of the depth of focus" of lenses f2.8 and faster. The depth of focus is the range at the sensor plane within which the image of a point will be reproduced as a blur smaller than the manufacturer's designated "circle of confusion" (CoC). Canon's designated circle of confusion is 0.035mm for the 24x36mm format and 0.02mm for the APS-C...Show more

Thanks again, and tank you, Jeff for "sticking" it.



Jun 21, 2005 at 06:24 AM
jxsq
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p.1 #5 · p.1 #5 · Canon autofocus information


Big thanks to both RDKirk and Jeff!


Jun 21, 2005 at 07:11 AM
Stoffer
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p.1 #6 · p.1 #6 · Canon autofocus information


Thanks for a really great post!

This got my attention big time:


2. The actual focus sensor arrays are three times larger than the viewfinder marks. A user could put an intended subject in the mark, but if there is a strong detail just outside the mark (but within the sensor area), the camera would focus on that strong detail. This is a source of much of the complaints of the back- or front-focusing -- especially with the "ruler tests." Also, as far as the camera is concerned, a focus lock on anything within the sensor area is good, which sometimes covers more area than the photographer intended.


Gotta remember this that next time I use my 135L at f/2!



Jun 21, 2005 at 07:12 AM
stevei
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p.1 #7 · p.1 #7 · Canon autofocus information


RDKirk wrote:
When you half-press the shutter release (or the * button, if you've used the custom function to move focusing control there), the activated AF sensor "looks" at the image projected by the lens from two different directions (each line of pixels in the array looks from the opposite direction of the other) and identifies the phase difference of the light from each direction. In one "look," it calculates the distance and direction the lens must be moved to cancel the phase differences. It then commands the lens to move the appropriate distance and direction and stops. It does not "hunt"
...Show more

I've read before that it works something like this, but this is all much more detailed than what I've seen before, so thank you very much for this info. What I've never understood, however, about the bit above is that I would expect the camera to need to know the current distance the lens is focused at, as surely the phase difference gives rise to a different change in focus distance depending on the current focus distance. Or perhaps the camera just tells the lens what phase difference to adjust for, and that calculation is internal to the lens, in fact I can't think of any other explanation given that we know not all lenses pass distance information to the camera. This is another possible source of miscalibration, if the lens has an incorrect measurement of the distance it's focused at when it performs this internal calculation, and this is why a lens can front or back focus on a perfect body, as well as bodies front or back focusing with perfect lenses (due to the AF mirror positioning as you mentioned).



Jun 21, 2005 at 07:25 AM
dan
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p.1 #8 · p.1 #8 · Canon autofocus information


In addition to the links above - here is Canon's 20D Technical Report:
http://www.canon.com/camera-museum/tech/report/200409/report.html

-----
dan



Jun 21, 2005 at 09:01 AM
Claude
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p.1 #9 · p.1 #9 · Canon autofocus information


Many thanks to RDKirk! Great job! Thanks Jeff for reposting!
Claude



Jun 21, 2005 at 09:27 AM
peppue
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p.1 #10 · p.1 #10 · Canon autofocus information


Thx for the posting Jeff.
Is that true, that using fisheye lens will confuse the AF?



Jun 21, 2005 at 10:05 AM
 

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Carlo Botteghi
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p.1 #11 · p.1 #11 · Canon autofocus information


Thanks a lot Jeff and RDKirk too, of course. Really interesting Post.
Carlo



Jun 21, 2005 at 11:12 AM
pscard
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p.1 #12 · p.1 #12 · Canon autofocus information


Well done, an article well researched and thoughtfully put together.

Looks like we may have the first part of the elusive Canon D/SLR FAQ



Jun 21, 2005 at 11:32 AM
Gerry Szarek
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p.1 #13 · p.1 #13 · Canon autofocus information


Also add that when using a 70-200 (or longer) and doing a focusing test you need the lines to be about 0.050 to 0.100 inch wide (a FAT marker), this is becuase you will only get a partial line on a pixel (this info was on photo.net someplace) which will cause the autofocus to FAIL. BTW there is an article on photo.net on how to conduct a proper autofocus test that probably should be linked in with this article.

http://www.photo.net/learn/focustest/ for a 10D but will work on any autofocus lens/camera setup.

RDKirk great job.



Jun 21, 2005 at 12:33 PM
rnb2
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p.1 #14 · p.1 #14 · Canon autofocus information


Gerry Szarek wrote:
Also add that when using a 70-200 (or longer) and doing a focusing test you need the lines to be about 0.050 to 0.100 inch wide (a FAT marker), this is becuase you will only get a partial line on a pixel (this info was on photo.net someplace) which will cause the autofocus to FAIL.


Has this changed between the focus system in the 10D/300D/350D and the 20D? This sounds very much like the issue I've experienced with my 20D, where it simply won't focus on a thin branch, but I don't seem to have had the same issue with my 300D or 350D.



Jun 21, 2005 at 02:46 PM
howgus
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p.1 #15 · p.1 #15 · Canon autofocus information


Gerry - the focus target in the link at photo.net is not recommended by Canon, and others, because the focus target is not parallel to the focus sensor. I recommend the focus setup here:

http://www.canon-dslr.com/Canon_Jan05/Canon_SLR_Focus_Test.htm

In my setup, I modified the focus target by simplifying it to 2 wide (0.2") vertical and 2 wide horizontal lines, spaced 1.5 inch apart. Test at 50 x focal length and 15EV, if possible, as these are the conditions Canon uses in their focus tests.



Jun 21, 2005 at 03:41 PM
Gerry Szarek
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p.1 #16 · p.1 #16 · Canon autofocus information


Guy, if you down load some of the jpeg's in the article you will see they have similar targets to yours. Somebody actually took an autofocus sensor apart, measured the pixel widths to determine it's S/n ratio so they could figure out how the thing worked.

It's interesting to know that the [ ] in the view finder are only guides, somebody actually measured all of them in a 10D and found not only were they NOT centered but they were not aligned to each other with any particular accuracy.

The short version for everybody is KNOW how your autofocus works, what causes it to achieve appropriate focus and what doesn't (like poor lighting, etc).

For truely low light work, nothing beats a rangefinder.



Jun 21, 2005 at 03:59 PM
howgus
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p.1 #17 · p.1 #17 · Canon autofocus information


And from RDKirk's posts: the AF sensor size in the 20D is 3x the box size in the viewfinder! I interpret that to mean 3x linear, both horizontal and vertical for the center sensor, which comes to 9x the area.


Jun 21, 2005 at 04:06 PM
RDKirk
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p.1 #18 · p.1 #18 · Canon autofocus information


>>the AF sensor size in the 20D is 3x the box size in the viewfinder! I interpret that to mean 3x linear, both horizontal and vertical for the center sensor, which comes to 9x the area.
<<

Actually, it's 5x the size. That's because it's shaped like the Red Cross with the square in the center. The "arms" of the cross that extend top, bottom, left, and right of the square are the same dimensions of the square itself.



Jun 21, 2005 at 06:31 PM
lexvo
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p.1 #19 · p.1 #19 · Canon autofocus information


RDKirk and Jeff: thanks for the info!

I wonder if there are differences regarding AF between 1-series bodies and other bodies. Could it be that AF on a 1-series is more precise? Or is it just faster?



Jun 21, 2005 at 06:57 PM
Gerry Szarek
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p.1 #20 · p.1 #20 · Canon autofocus information


The 10D accuracy is +/- 1.0 DOF, the 1 Series body is +/- 1/3 DOF, somebody can correct me if I am wrong.

Gerry



Jun 21, 2005 at 07:53 PM
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