Canon autofocus information

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Archive 2005 · Canon autofocus information

RDKirk
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p.2 #1 · Canon autofocus information

lexvo wrote:
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?

The precision in "high precision" mode should be the same between the 20D and 1D cameras--within 1/3 of the depth of focus. However, the 1D cameras use a dedicated processor to calculate the focus solution, which makes it quicker (the lesser cameras use a single processor for both exposure and AF calculations). I'm sure this would equate to better accuracy with AI Servo and continuous shutter operation because the camera would be able to gather more subject motion prediction data and run the calculations faster.

Also, the more precise focusing screen placement in the 1D cameras allows for smaller AF sensed areas (which is why there can be more of them closer together on the scree). That should eliminate some of the errors caused by the AF system focusing on something other than what the photographer intended.

Jun 21, 2005 at 08:22 PM

chetcarson
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p.2 #2 · Canon autofocus information

Indispensable information! From where I sit on the learning curve, mastering
the intricacies of getting the intended focus on moving people in difficult light,
which is most of the time rather than not, is far more a challenge than
understanding and controlling the relationship between aperture, shutter
speed, and ISO. Understanding what the instrument is trying to do and how it
does it to achieve AF allows us to adjust our technique to its known
capabilities. Thank you very much.

Jun 21, 2005 at 10:57 PM

andrew81
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p.2 #3 · Canon autofocus information

awesome stuff!!!

Jun 22, 2005 at 12:42 AM

Tom_W
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p.2 #4 · Canon autofocus information

Excellent collection of information by RDKirk. He's a great asset, full of useful information.

Jun 22, 2005 at 01:01 AM

Tom_W
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p.2 #5 · Canon autofocus information

zero2k wrote:
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

the double mirror system is not limited to the 20D - the 1D series has it also.

Jun 22, 2005 at 01:01 AM

Tom_W
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p.2 #6 · Canon autofocus information

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

Jun 22, 2005 at 01:06 AM

Tom_W
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p.2 #7 · Canon autofocus information

howgus wrote:
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.

The center is a cross-type meaning that the vertical sensor extends about 1 sensor width to either side and the horizontal sensor extends about 1 sensor's height above and below the rectangle. It isn't rectangular-shaped.

Jun 22, 2005 at 01:09 AM

Wonotch
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p.2 #8 · Canon autofocus information

Very nice info, thanks RDKirk.

Now the same for the 1 series AF system maybe?

Jun 22, 2005 at 06:06 AM

Mongrel
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p.2 #9 · Canon autofocus information

Thanks to Jeff and RDKirk for putting this together.

Question:

What are the chances of getting multiple cross-type sensors in a future Canon body?

I really would appreciate having *three* cross-type sensors aligned horizontally in landscape orientation (left-middle-right). When shooting sports (especially field sports-soccer, football, lacrosse, etc.) I shoot in portrait orientation using a single AF point. Most often you hear sports shooters say to use the center focus point for accuracy. However, I don't want the players belly-button in focus-I want their face in focus so I use the center upper AF point (in portrait orientation, this would be the 'right hand' AF point in landscape orientation). I've often wondered how many more shots would be in focus if this AF sensor were a cross-type?

I'm also curious if anyone else uses this focus point instead of the center AF point and what they're thought are.

Thanks

Jun 22, 2005 at 12:43 PM

timgangloff
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p.2 #10 · Canon autofocus information

after reading that, i gotta give RDKirk a big thank you. this should be mandatory reading for anyone shooting a canon dslr. THANKS! Also thanks to the poster for finding this and making it available.

Jun 22, 2005 at 07:57 PM

Monito
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p.2 #11 · Canon autofocus information

I wonder if there might be some interaction between continuous shooting mode and the focus system. I don't see discussion of this here. I seem to remember reading somewhere that when the camera is in continuous mode, it shifts from focus priority to shutter button priority, meaning that it will take the shot even if the lens has not achieved focus, even if the camera setting is one-shot focus mode. I'd be grateful for some definitive information about this.

Jun 23, 2005 at 02:39 AM

frag
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p.2 #12 · Canon autofocus information

AI Focus: The camera is normally in One Shot mode and the shutter will lock until it achieves focus. However, if it detects the subject moving (that is, the subject goes out of focus), it will automatically switch into AI Servo mode and try to maintain focus. If you are focusing on something that frequently stays still but could move suddenly (like a toddler) this mode comes in handy. The important point wiht AI Focus is that it does not lock the shutter. However, the camera will usually interpret "focus and recompose" as movement of the subject, and will refocus.

Just out of curiosity, if in AI Focus and you've focused on a subject that then begins to move. The above states the the autofocus system will automatically switch to AI Servo mode in order to attempt to keep the subject in focus. I assume this means that you must then depress and hold the * button again to attain and maintain focus, correct? (I've got parameter 14 set to 1).

Jun 23, 2005 at 02:06 PM

lexvo
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p.2 #13 · Canon autofocus information

Mongrel wrote:
Question:

What are the chances of getting multiple cross-type sensors in a future Canon body?

As far as I know, most 1-series bodies (I think some of the older models not) have more cross type sensors. For example, the 1D has 7 cross type sensors.

The center sensor is cross-type for lenses with a maximum aperture of f/4 or larger. The other 6 sensors are cross-type for an aperture of f/2.8 or larger. If the aperture is smaller, these sensors will act as horizontal sensor.

Jun 23, 2005 at 03:05 PM

oeyvind
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p.2 #14 · Canon autofocus information

1 series and EOS 3

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Jun 23, 2005 at 03:24 PM

Mongrel
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p.2 #15 · Canon autofocus information

Thanks to lexvo and Tim Chong for clarifying that. I remembered that the 1 series had multiple cross-type sensors but had forgotten where they were located. However, they are not optimized for shooting AI servo in portrait orientation-imho.

I took the liberty of copying the above gif to show the placement I was referring to in my post above.

I'd like to see Canon utilize cross-type sensors at the points circled below:

Well, I missed the uppermost one I wanted to circle

, but you get my drift

.

This would be ideal (imho) for tracking a player in American football, soccer, field hockey, lacrosse, rugby etc. It would give the option of focusing on the players face or helmet, or in the case of soccer and field hockey the ball. The way Canon has them laid out you would almost be better off shooting everything in landscape for the most accurate AF and then cropping the sides off.

If you use the center AF point and hold it on the players head you wind up with a whole lot of dead space at the top of the frame that needs to be cropped out.

I would gladly give up a most of the 45 AF sensors if I could get 10 or 12 cross type sensors where I want them

thanks again to lexvo and Tim...

Jun 23, 2005 at 03:47 PM

Steve_T90
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p.2 #16 · Canon autofocus information

Something dawned on me as I've been sitting here editing autocross shot with a 20D and 70-200 F4. Focus tracking is better than everything I've ever had before, but it isn't perfect. Now I've realized that I've been shooting in Tv and Av modes. I recall reading somewhere that a single processor in the 20D controls focus and exposure, while the 1-series has a second processor that frees up the first for AF-only tasks. This is part of the reason why the 1-series has faster AF.

Let's say I set my 20D into Manual exposure mode. Will this improve focus tracking, as I've given the processor one less thing -- autoexposure -- to compute?

Sometimes I have CF4=3 set (so * button activates AF, and there is no AE lock). Since the camera is always assessing exposure in this condition, am I compromising AF speed?

Would I have the best hopes for AF by setting CF4 so I have AE lock, and keeping the camera in M?

Jun 24, 2005 at 06:05 PM

Elías Seguí
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p.2 #17 · Canon autofocus information

I buy my camera for take photos not for read a book every week.

Elías

Jun 27, 2005 at 04:33 PM

Wonotch
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p.2 #18 · Canon autofocus information

Jun 27, 2005 at 07:39 PM

burningarrow
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p.2 #19 · Canon autofocus information

Steve_T90 wrote:
Something dawned on me as I've been sitting here editing autocross shot with a 20D and 70-200 F4. Focus tracking is better than everything I've ever had before, but it isn't perfect. Now I've realized that I've been shooting in Tv and Av modes. I recall reading somewhere that a single processor in the 20D controls focus and exposure, while the 1-series has a second processor that frees up the first for AF-only tasks. This is part of the reason why the 1-series has faster AF.

Let's say I set my 20D into Manual exposure mode. Will this improve focus tracking, as I've given the processor one less thing -- autoexposure -- to compute?

Sometimes I have CF4=3 set (so * button activates AF, and there is no AE lock). Since the camera is always assessing exposure in this condition, am I compromising AF speed?

Would I have the best hopes for AF by setting CF4 so I have AE lock, and keeping the camera in M?

Steve has a good point. Anybody?

Matt

Jun 28, 2005 at 07:28 PM

lexvo
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p.2 #20 · Canon autofocus information

I think the camera is still computing exposure if you set it to manual exposure. The light meter (on the right on my 1D) is showing over- or underexposure.

Jun 28, 2005 at 07:51 PM

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