Canon autofocus information

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Archive 2005 · Canon autofocus information

Steve_T90
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p.3 #1 · Canon autofocus information

Thanks guys. Lexvo says what I kind of suspected after some more time thinking about it.

The only other thing I thought would make a difference is the CF control of AF/AE, so I tested out Lexvo's statement for myself. No matter what CF4 is set to, manual exposure calculation is never locked on my 20D. Just as well for Canon, as I can imagine that they would be less than thrilled if people started saying, "The 20D can't walk and chew gum at the same time..."

Jun 28, 2005 at 10:32 PM

freelancer
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p.3 #2 · Canon autofocus information

1D-ADs AF Coverage (by Chuck Westfall)

http://www.pbase.com/chuckwestfall/image/18921329

Jun 29, 2005 at 04:09 AM

Wonotch
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p.3 #3 · Canon autofocus information

freelancer wrote:
1D-ADs AF Coverage (by Chuck Westfall)

http://www.pbase.com/chuckwestfall/image/18921329

Thanks for the link! I found the 10D CMOS sensor diagram really interesting! Id like to find out more about all those filters placed in front of the sensor.

Jun 29, 2005 at 11:20 PM

prof_fate
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p.3 #4 · Canon autofocus information

Hmmm..lots of info here to digest. I was shooting birds a few weeks back with a 70-300 lens at 300...and the point of focus seemed to not agree with the center point, so that bit about the focus screen not being exaclty aligned to the sensors is true.

The info about the direction of the sensors sensitivity is very useful. Have to remember that one.

I s'pose the triple precision with 2.8 and faster lenses is not a feature of the 300D...is it in the 350d?

Jun 29, 2005 at 11:27 PM

Schlotkins
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p.3 #5 · Canon autofocus information

OK, so if I wanted to test a 70-200 f4, what's the best mention to do it? I think it's backfocusing pretty bad at 70mm and looks OK at 200. If I download the chart at http://www.photo.net/learn/focustest/ how far away should I be at 70 and 200? This is on a 300d.

Thanks,
Chris

Jun 30, 2005 at 04:39 PM

Pixel Perfect
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p.3 #6 · Canon autofocus information

Steve_T90 wrote:
The only other thing I thought would make a difference is the CF control of AF/AE, so I tested out Lexvo's statement for myself. No matter what CF4 is set to, manual exposure calculation is never locked on my 20D. Just as well for Canon, as I can imagine that they would be less than thrilled if people started saying, "The 20D can't walk and chew gum at the same time..."

What do you mean by this. If it's a manual exposure then by definition it's a locked exposure. You've set aperture and shutter speed, and they are now fixed. Exposure lock only makes sense in the context of AE. The camera may indicate that the locked exposure is different to it's metered value as you swing the camera around, but it doen't affect your settings.

Jul 01, 2005 at 04:45 AM

damjr1
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p.3 #7 · Canon autofocus information

Folks..., would someone in the know please clarify one thing in regards to the "f2.8 or faster lens" & "increased sensor capability" feature. I have read the Westfall article several times, btw...

When a lens with a maximum aperture capability of f2.8 or greater is attached:
a) the increased sensitivity is available while using "any" aperture on that lens.

b) the increased sensitivity is available when the aperture of the attached lens is set to f2.8 or greater.

Hope I have made myself clear with this wording.

Thanks,
Doug

Jul 01, 2005 at 02:43 PM

RDKirk
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p.3 #8 · Canon autofocus information

damjr1 wrote:
Folks..., would someone in the know please clarify one thing in regards to the "f2.8 or faster lens" & "increased sensor capability" feature. I have read the Westfall article several times, btw...

When a lens with a maximum aperture capability of f2.8 or greater is attached:
a) the increased sensitivity is available while using "any" aperture on that lens.

b) the increased sensitivity is available when the aperture of the attached lens is set to f2.8 or greater.

Hope I have made myself clear with this wording.

Thanks,
Doug

When you attach a lens with a maxium aperture of f2.8 or greater, the camera turns on a second vertical line of phase detection pixels in the center sensor pixel array.

This is what Canon calls the "high precision" mode and supposedly will place the actual plane of focus within 1/3 of the depth of focus (the level of accuracy at the sensor plane).

It appears to me that switching on the additional sensor is a function of the identification information the lens provides to the camera ("My maximum aperture is f2.8") rather than the amount of light being transmitted by the lens. I believe it applies to any shooting aperture used while that lens is mounted--the camera always uses the maximum aperture depth of focus factor for all its focus solutions, not the shooting aperture.

I don't have a document stating this outright, but looking at the chain of events depicted in Canon's patent application, the camera calculates the focus solution for a shot before it calculates the exposure solution (this is definitely true when using evaluative mode autoexposure), so when it sends the movement command to the lens, it doesn't yet know what the shooting aperture will be.

Jul 01, 2005 at 03:10 PM

damjr1
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p.3 #9 · Canon autofocus information

sounds logical to me...thank you for that response

as an aside, I have read posts were a few have referenced patent apps...
is that available for us all ? it looks like one may gleam a few technical insights and gain a light more of an understanding of their products...

wonder if calling Canon tech, they would happily share a little knowledge?

take care,
Doug

Jul 01, 2005 at 04:02 PM

RDKirk
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p.3 #10 · Canon autofocus information

damjr1 wrote:
sounds logical to me...thank you for that response

as an aside, I have read posts were a few have referenced patent apps...
is that available for us all ? it looks like one may gleam a few technical insights and gain a light more of an understanding of their products...

wonder if calling Canon tech, they would happily share a little knowledge?

take care,
Doug

Check in the first sticky post--I listed the patent number and a web site to pull the application as a PDF.

Jul 01, 2005 at 05:02 PM

Steve_T90
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p.3 #11 · Canon autofocus information

Pixel Perfect wrote:
Steve_T90 wrote:
The only other thing I thought would make a difference is the CF control of AF/AE, so I tested out Lexvo's statement for myself. No matter what CF4 is set to, manual exposure calculation is never locked on my 20D. Just as well for Canon, as I can imagine that they would be less than thrilled if people started saying, "The 20D can't walk and chew gum at the same time..."

What do you mean by this. If it's a manual exposure then by definition it's a locked exposure. You've set aperture and shutter speed, and they are now fixed. Exposure lock only makes sense in the context of AE. The camera may indicate that the locked exposure is different to it's metered value as you swing the camera around, but it doen't affect your settings.

Bolded sentence you wrote is exactly what I mean -- the camera never stops metering (or "calculating exposure," to use my terms above) in M mode.

Jul 01, 2005 at 06:09 PM

RDKirk
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p.3 #12 · Canon autofocus information

[What do you mean by this. If it's a manual exposure then by definition it's a locked exposure. You've set aperture and shutter speed, and they are now fixed. Exposure lock only makes sense in the context of AE. The camera may indicate that the locked exposure is different to it's metered value as you swing the camera around, but it doen't affect your settings.

Bolded sentence you wrote is exactly what I mean -- the camera never stops metering (or "calculating exposure," to use my terms above) in M mode.

Mere light level indication probably doesn't call for much in terms of calculation--remember that cameras used to do it with nothing more than a tiny galvometer. That's all that's happening with manual exposure measurement. Evaluative auto exposure mode does take some computer horsepower.

Jul 02, 2005 at 01:18 AM

Pixel Perfect
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p.3 #13 · Canon autofocus information

RDKirk wrote:
Mere light level indication probably doesn't call for much in terms of calculation--remember that cameras used to do it with nothing more than a tiny galvometer. That's all that's happening with manual exposure measurement. Evaluative auto exposure mode does take some computer horsepower.

Ah yes but you can use evaluative metering in M mode, just as you can use spot or partial or centreweighted. Just that it's recommendation is not necessarily what you think

Jul 03, 2005 at 09:10 AM

RDKirk
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p.3 #14 · Canon autofocus information

RDKirk wrote:
Mere light level indication probably doesn't call for much in terms of calculation--remember that cameras used to do it with nothing more than a tiny galvometer. That's all that's happening with manual exposure measurement. Evaluative auto exposure mode does take some computer horsepower.

Pixel Perfect wrote:
Ah yes but you can use evaluative metering in M mode, just as you can use spot or partial or centreweighted. Just that it's recommendation is not necessarily what you think

Yes, it does, and to do so, it's running comparisons between the immediate scene and the thousands of scenes in its database. Very capable computer there. But that's still more horsepower than it's using just to indicate light level in other meter modes.

As for whether it's using the maximum aperture or the shooting aperture to determine the depth of focus, I'd still say it's using the maximum aperture. It's got to use some figure to determine the focusing solution, and the focusing solution does not appear to change if you change shooting aperture.

Jul 03, 2005 at 05:45 PM

frankjay
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p.3 #15 · Canon autofocus information

Thanks to the person who posted that piece of information regarding Canon lens focusing.It took time and concern. But, I do not know how to apply it. I do not interpret writings well where I can't define half the vocabulary used. All is want is a lens that works. I read on another forum (dpreview.com) many people have this backfocus problem with 70-200 f4L lens and others. I was planning to buy a 20D, 17-40L and the 70-200L this week. But now I am having second and third thoughts. I do not want to spend about $2500, not including filters, cards etc to have to constantly send it back. If anyone can help me understand how to apply that technical posting or finding an alternative lens from Sigma to the 70-200L I would appreciate it. If this is a lot to ask, that's ok. I just might go Nikon, or film, seems to be less lens problems there. (a 30 yr canon user)
thanks
frank

Jul 04, 2005 at 03:51 PM

RDKirk
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p.3 #16 · Canon autofocus information

frankjay wrote:
Thanks to the person who posted that piece of information regarding Canon lens focusing.It took time and concern. But, I do not know how to apply it. I do not interpret writings well where I can't define half the vocabulary used. All is want is a lens that works. I read on another forum (dpreview.com) many people have this backfocus problem with 70-200 f4L lens and others. I was planning to buy a 20D, 17-40L and the 70-200L this week. But now I am having second and third thoughts. I do not want to spend about $2500, not including filters, cards etc to have to constantly send it back. If anyone can help me understand how to apply that technical posting or finding an alternative lens from Sigma to the 70-200L I would appreciate it. If this is a lot to ask, that's ok. I just might go Nikon, or film, seems to be less lens problems there. (a 30 yr canon user)
thanks
frank

There are nowhere near as many problems in practical usage as the complaints in the forums would have you think.

Proportionately, I don't see any fewer problems in the Nikon forums. If you'll notice, the Canon forums always have two or more times more traffic than Nikon forums--which means you'll see two to three times more complaints in absolute numbers.

Moreover, the Internet "lens effect" makes everything seem bigger than it really is. Happy people are taking pictures; unhappy people are complaining. And they tend to complain repeatedly, so you see the same single complaint repeated in many topics and over several forums.

One of the problems Canon faces with being the marketing leader is that they also have a lot more inexperienced people (for instance, Canon sells cameras in Wal-Mart--Nikon doesn't) so there are a lot more people who will have problems. A lot of those people also have computers.

Much of the complaints are because people are "pixel peeping" instead of printing. The focusing system is designed to produce a good 6x9 print viewed at 10 inches. Most people who complain are looking at their images enlarged to 40x60 and viewing it at 10 inches. Of COURSE they will see problems--the system is not designed to be that accurate. But print out a picture to your normal size and see if you have a problem. People still shooting film aren't having all these problems because they judge the system based on the prints they make.

Then there is the final factor: Automatic focusing actually takes some skill to use. Not much skill, but what is necessary is necessary. People have to do some practicing and some exercises to find out just how the system works and how to make it do what they want it it to do.

Jul 04, 2005 at 04:59 PM

maxim_me
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p.3 #17 · Canon autofocus information

10DFT wrote:
Stoffer wrote:
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!

I keep this handy image of the 10D sensor arrangement overlaid in red on top of the sensor indicators in the viewfinder to illustrate this very point:

This image is copyrighted by the owner

if this is accurate, why is it that when i tested with "larger" AF area ( those protroding from the markings, i found that the AF did not respond
ie, as i slowly move towards a contrasty object, the AF did not latch on until the boundary is within the markings

Try testing this "larger" AF area, and you will know what i mean
Test for its effect on AF

Jul 05, 2005 at 12:43 AM

RDKirk
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p.3 #18 · Canon autofocus information

I've done that testing and found the diagram to be accurate. Actually, my testing had mapped out the diagram before I ever saw the diagram.

To test, try making some bold black marks on small cards, one mark to a card: Horizontal, vertical, diagonal in both directions.

Tape the cards to a blank wall with a good amount of distance between them so that you won't have any problem getting only one into a focusing area.

Use a tripod so that you can move a focus point toward any one card in very small increments.

If you run tests moving each direction of line toward each AF point from each of the primary directions (every 45 degrees in a 360 circle), you'll wind up mapping the diagram.

Jul 05, 2005 at 11:29 AM

maxim_me
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p.3 #19 · Canon autofocus information

hi rDkirk

how many shots to do this mapping?

I did just a couple , not to map them but to evaluate
where the centre mark starts
ie background is very far away, object is 5ft away and i slowly
move the centre AF towards the object

AF focused on background right until it hits the object.
ie: object border is within the AF markings

There is plenty of contrast between object and background.

Could you try my method and see if you get something like this abnormality?

Jul 06, 2005 at 02:33 AM

RDKirk
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p.3 #20 · Canon autofocus information

maxim_me wrote:
hi rDkirk

how many shots to do this mapping?

I did just a couple , not to map them but to evaluate
where the centre mark starts
ie background is very far away, object is 5ft away and i slowly
move the centre AF towards the object

AF focused on background right until it hits the object.
ie: object border is within the AF markings

There is plenty of contrast between object and background.

Could you try my method and see if you get something like this abnormality?

I put the marked cards on a wall--thus a 2D target that is mostly unfocusably blank with a few focusable marks arrayed on it.

That way, there is nothing the camera can focus on except a specific mark as I nudge the AF point closer and closer to it from one direction. I press the focus button with each nudge, until it gets close enough to grab the mark.

Then I move the AF point away from that mark and approach it or another mark with the AF point from a different direction.

Jul 06, 2005 at 01:04 PM

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