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Does 20D really focus more precisely than Rebel XT? Is it a "consumer" body or "pro"?

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pentool

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According to this tutorial, http://www.photo.net/learn/focustest/

it says, "From past statements by Canon it seems that the spec for focus on "consumer" bodies (and the 10D is a $1500 "consumer" body) is that focus is within the DOF. On the "pro" bodies focus spec is within 1/3 of the DOF because they use higher precision AF sensors which require faster lenses".

Is it still true with the 20D? What about the Rebel XT's AF sensor?
 
pentool said:
Is it still true with the 20D? What about the Rebel XT's AF sensor?
Click to expand...
Why don't you simply read the specs. The 20D has a central high precision (1/3rd DOF) sensor for lenses better than f/2.8, the 350D hasn't.

--
regards
Karl Günter Wünsch
 
Hi, tool,
pentool said:
According to this tutorial, http://www.photo.net/learn/focustest/

it says, "From past statements by Canon it seems that the spec for
focus on "consumer" bodies (and the 10D is a $1500 "consumer" body)
is that focus is within the DOF. On the "pro" bodies focus spec is
within 1/3 of the DOF because they use higher precision AF sensors
which require faster lenses".

Is it still true with the 20D? What about the Rebel XT's AF sensor?
Click to expand...
All professional phtographers are consumers of their camera gear. The only non-consumer purchasers are dealers and distributors.

The AF accuracy of the general-purpose AF detectors of the EOS 20D is stated as "within the depth of focus".

There is also an auxiliary AF detector at the center location, only active with larger-aperture lenses, and sensitive only to either vertical or horizontal edges in the scene (I forget which right now), whose accuracy bogey is stated as "1/3 the depth of focus".

Note that an accuracy specification of "within the depth of focus" corresponds presicesly to "within the depth of field".

However, an accuracy specification of "within 1/3 the depth of focus" does not exactly correspond to "within 1/3 the depth of field".

Best regards,

Doug

Visit The Pumpkin, a library of my technical articles on photography, optics, and other topics:

http://doug.kerr.home.att.net/pumpkin

My EOS 20D must be a point and shoot camera - I find out that if I don't point, I get erratic results; if I don't shoot, I get no results.
 
Hi Doug,

According to page 68 of the Instruction Maunual:

"With cross-type AF, vertical-line detection is twice as senstive as horizontal-line detection."

--
Henry
 
Doug Kerr said:
Which way is that high-precision detector oriented (that is,
sensitive to vertical or horizontal edges)? I never can remember.
Click to expand...
It's a secondary cross type detector which is both sensitive to vertical and horizontal edges!
--
regards
Karl Günter Wünsch
 
Hi, Henry,
Henry Mo Yung said:
Hi Doug,

According to page 68 of the Instruction Maunual:

"With cross-type AF, vertical-line detection is twice as senstive
as horizontal-line detection."
Click to expand...
And I'm not sure that is exactly the same matter. Maybe it means that for the "normal" detectors at the center, the vertical-line sensitive compnent is more sensitive that the other component, but maybe neither of these are the "precision" detector.

Best regards,

Doug
 
Hi, Karl,
Karl Günter Wünsch said:
Doug Kerr said:
Which way is that high-precision detector oriented (that is,
sensitive to vertical or horizontal edges)? I never can remember.
Click to expand...
It's a secondary cross type detector which is both sensitive to
vertical and horizontal edges!
Click to expand...
Well, in the oine fairly detailed article I had read (which I don't seem to be able to find now!), the "high-precision" detector at the center was only oriented in one direction.

But the descriptions of these things are always so ambiguous that I wouild be uncertain even if I remembered what it said!

Best regards,

Doug
 
Doug Kerr said:
Karl Günter Wünsch said:
Doug Kerr said:
Which way is that high-precision detector oriented (that is,
sensitive to vertical or horizontal edges)? I never can remember.
Click to expand...
It's a secondary cross type detector which is both sensitive to
vertical and horizontal edges!
Click to expand...
Well, in the oine fairly detailed article I had read (which I don't
seem to be able to find now!), the "high-precision" detector at the
center was only oriented in one direction.
Click to expand...
Wrong, the manual on page 68 clearly states that the center AF point is a cross type detector although with the vertical detection being twice as sensitive as the horizontal.

--
regards
Karl Günter Wünsch
 
This is what is said in the Roib Galbraith review of the EOS 20D:

http://www.robgalbraith.com/bins/multi_page.asp?cid=7-6458-7153-7161

*************

The specifications for the new camera's autofocus system certainly talk a good game. All-new in the 20D, it includes a 32-bit RISC AF microprocessor and 9-point CMOS AF sensor (both the AF and image sensors are CMOS in this camera). The AF component is composed of a single cross-type centre sensor and 8 vertically-oriented outer sensors.

The centre sensor's design in particular is intriguing. It features 2 pairs of vertically-oriented pixel arrays (the 10D has a single pair), where each pair is slightly offset from the other. There are also 2 pairs of horizontally-oriented pixel arrays: one pair is more widely spaced than the other (the pair that's used is dictated by the maximum aperture of the lens).

Translated, this technical gobbledygook means that with f/5.6 or faster lenses, the fact that the centre AF sensor has twice as many vertically-oriented pixels for horizontal-line detection as the 10D should result in quicker and more precise autofocus with even consumer-grade, slower-aperture glass. In our brief experience thus far, this seems to be the case. In other words, the normal precision mode of the centre AF sensor is designed to offer better AF performance than the normal precision mode of the same sensor in the 10D.

With f/2.8 or faster lenses, the 20D's centre AF sensor still uses both sets of vertically-oriented pixel arrays, but switches from the inner horizontally-oriented pixel array pair to the more widely-spaced outer pair. This puts the centre AF sensor into high-precision mode, which Canon claims is 3x more precise than the 20D's normal precision mode. We're not sure what 3x more precise should feel like, but can attest to the fact that with both wide angle and telephoto lenses whose maximum aperture is f/2.8, the speed with which the 20D acquires focus using the centre AF sensor, even in dim light, is considerably quicker than the 10D (and gives the EOS-1D Mark II a run for its money also).

The outer 8 sensors are also of a different design than the 10D. Though all are single-axis, vertically-oriented sensors (in other words, not cross-type), they feature a baselength that is 30% longer than the outer sensors in the 10D. This, says Canon, should add up to improved AF performance from the outer sensors, relative to the 10D.

*************

This seems to say:

1. The center AF point has greater precision (for any aperture larger than f/5.6), with respect to both edge directions, that the other points.

(Karl: I suspect this is what you were referring to.)

2. With a large aperture lens, the vertical-edge sensitive component of the center detector goes into "high-precision" mode.

Thus, there are three precision levels that might be in force here. "Normal", "better-than-normal", and "high precision".

Here is a discussion of this (started by me!) on the Rob Galbraith forum:

http://forums.robgalbraith.com/showflat.php?Cat=&Board=UBB8&Number=315093&Searchpage=1&Main=315093&Words=%2B%26quot%3Bhigh+precision%26quot%3B+%2BAF&topic=&Search=true#Post315093

Best regards,

Doug
 
In my prior post, I said:
Doug Kerr said:
This seems to say:

1. The center AF point has greater precision (for any aperture
larger than f/5.6), with respect to both edge directions, that the
other points.
Click to expand...
Doug Kerr said:
2. With a large aperture lens, the vertical-edge sensitive
component of the center detector goes into "high-precision" mode.
Click to expand...
On re-reading the decription, it seems that a better summary would be:

This seems to say:

1. The horizontal-edge sensitive component of the center AF point has greater precision (for any aperture larger than f/5.6) than the other component of the center point or the other points.

2. With a with maximum aperture larger than f/2.8, the vertical-edge sensitive
component of the center detector goes into "high-precision" mode.

Thus, there are three precision levels that might be in force here,
"normal", "better-than-normal", and "high precision".

Best regards,

Doug
 
answer? The XT seems like a lot of camera for the money in comparison to the 20d and if I were buying a dslr today I would really be looking at it (I have a 20d purchased in the first shipment last september). But of all the differences between the two models, the one that means the most to me is AF accuracy. Does somebody out there have both cameras and have you noticed a real world difference in AF accuracy or speed of focus between the two? If you noticed a difference was it only on the fast glass?

Cindy

--
Doug Kerr said:
Doug Kerr said:
Doug Kerr said:
My Photo-A-Day for a Year Gallery Here: http://www.pbase.com/cindyd/2005
Click to expand...
Click to expand...
Click to expand...
Formerly half of 1ofUs or 1ofUs!
The other half of 'Us' is now 'Lensbaby'
 
And a very good one.

I have a 300D and a 20D, but I have no idea which actually provides the best AF performance and in what respect.

I guess I shouild stop parsing specifications and go take some pictures!

Best regards,

Doug
 
Hi, Karl,
Karl Günter Wünsch said:
Karl Gnter Wnsch said:
Well, in the oine fairly detailed article I had read (which I don't
seem to be able to find now!), the "high-precision" detector at the
center was only oriented in one direction.
Click to expand...
Click to expand...
Karl Gnter Wnsch said:
Wrong, the manual on page 68 clearly states that the center AF
point is a cross type detector although with the vertical detection
being twice as sensitive as the horizontal.
Click to expand...
Yes, that's what I said - it is only the "vertical edge" component of the central array that is high precision. There is of course also an "ordinary precision" component at that location, oriented in the other direction, completing the "cross-type" array. I didn't say that the whole thing wasn't cross-type.

Note that the manual passage doesn't say anything about "twice as sensitive". In fact, I suspect that the "high precision" component there is "three times as sensitive" as the general purpose detectors. (That is, "within 1/3 the depth of focus " vs. "within the depth of focus").

Not really mentioned by the manual is that (according to Rob Galbraith), with apertures larger than f/5.6 (but not larger than f/2.8), the horiontal line sensitive component of the center array is "twice as sensitive" as the "normal" detectors.

Best regards,

Doug
 
Hi Sis-Q,
Sis-Q said:
The XT seems like a lot of camera for the money in
comparison to the 20d . But of all the differences between the
two models, the one that means the most to me is AF accuracy. Does
somebody out there have both cameras
Click to expand...
I gave the 350D a testrun last March...
Sis-Q said:
and have you noticed a real
world difference in AF accuracy or speed of focus between the two?
Click to expand...
Yes and Yes, 20D is more accurate and faster.
Sis-Q said:
If you noticed a difference was it only on the fast glass?
Click to expand...
No, on 'slower glass' the difference in AF-speed is more noticeable,
on 'faster glass' the difference is accuracy is more noticeable, IMHO...

What I do not read in this thread, IMHO the 'eye' 'keyhole' through which you focus is much smaller on 20D, you can aim at the pupil in stead of the eye, if you catch my drift:

350D:



20D:



...my €0.02 worth...

Kindest regards,

Max@Home
--
Max@Home - Castricum - The Netherlands (see profile for equipment)



http://www.pbase.com/max_at_home (mind you, use underscores!)
 
I think you mixed up sensitivity and precision, which are two different parameters of the focusing sensor. Sensitivity determines how strong the contrast has to be to be detectable while precision determines the amount the focus might be off the ideal position.And the manual clearly states that the vertical sensor for the f/2.8 AF sensor is twice as sensitive as the horizontal.
--
regards
Karl Günter Wünsch
 
pentool said:
According to this tutorial, http://www.photo.net/learn/focustest/

it says, "From past statements by Canon it seems that the spec for
focus on "consumer" bodies (and the 10D is a $1500 "consumer" body)
is that focus is within the DOF. On the "pro" bodies focus spec is
within 1/3 of the DOF because they use higher precision AF sensors
which require faster lenses".

Is it still true with the 20D? What about the Rebel XT's AF sensor?
Click to expand...
With f1.8 (85mm usm canon) and f4 (17-40, 70-200 L canon) they are much the same.

With 50mm 1.4 and 85mm 1.2 the xt is better.

With f2.8 (24-70L, 70-200L) the 20d is better.

Regards,
Kev

http://www.canon-dslr.com
 
Thanks so much. I've read that XT also has the 1/3 of the dof precison thing when you use 2.8 or wider. So that makes 20d and XT have same level of precison focus when used with 2.8 or wider? heres the image i found http://img228.imageshack.us/my.php?image=canonsensor9in.jpg
Doug Kerr said:
pentool said:
According to this tutorial, http://www.photo.net/learn/focustest/

it says, "From past statements by Canon it seems that the spec for
focus on "consumer" bodies (and the 10D is a $1500 "consumer" body)
is that focus is within the DOF. On the "pro" bodies focus spec is
within 1/3 of the DOF because they use higher precision AF sensors
which require faster lenses".

Is it still true with the 20D? What about the Rebel XT's AF sensor?
Click to expand...
All professional phtographers are consumers of their camera gear.
The only non-consumer purchasers are dealers and distributors.

The AF accuracy of the general-purpose AF detectors of the EOS 20D
is stated as "within the depth of focus".

There is also an auxiliary AF detector at the center location, only
active with larger-aperture lenses, and sensitive only to either
vertical or horizontal edges in the scene (I forget which right
now), whose accuracy bogey is stated as "1/3 the depth of focus".

Note that an accuracy specification of "within the depth of focus"
corresponds presicesly to "within the depth of field".

However, an accuracy specification of "within 1/3 the depth of
focus" does not exactly correspond to "within 1/3 the depth of
field".

Best regards,

Doug

Visit The Pumpkin, a library of my technical articles on
photography, optics, and other topics:

http://doug.kerr.home.att.net/pumpkin

My EOS 20D must be a point and shoot camera - I find out that if I
don't point, I get erratic results; if I don't shoot, I get no
results.
Click to expand...
 
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