Shocking insight: AF is not a closed feedback loop!

Started Jul 18, 2003 | Discussions
Mishkin™ Contributing Member • Posts: 917
Shocking insight: AF is not a closed feedback loop!

And THAT and only that can explain why some lenses consistently front/back focus even if camera is well-calibrated.

Take a look at this illustration from Canon Camera Museum ( http://www.canon.com/camera-museum/tech/room/f_index.html ):

There is a device in the lens named "Drive quantity detector". As far as I understand from the diagram, the signals from this detector and from AF processor are sent to some digital comparator (triangle).

Why would this detector be needed in closed feedback loop? In closed loop, the AF processor will send the signal to the lens until the image is perfectly in focus. There's no need to measure lens displacement and compare it to the signal from AF processor.

Now let's turn our measurbating brains to U.S. Patent 4,078,171 by Honeywell (thanks to tawen mei who pointed me to it).

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=4078171.WKU.&OS=PN/4078171&RS=PN/4078171

For those having trouble with QuickTime, here's the screenshot of the illustrations:

Here's an exerpt from the patent:

"FIG. 3 shows a digital automatic focus system which may be used in photographic equipment such as a still or movie camera. The system includes a digital focus module 50, which preferably is apparatus like that shown in FIG. 1. The output of digital focus module 50 is a first digital word which is used to control a position of a primary optical means 52 such as the taking lens of a still or movie camera. A lens position encoder 54 provides a second digital word indicative of the position of optical means 52 with respect to film 56. Digital comparator 58 compares the first and second digital words. The position of optical element 52 is determined by motive means 60, which is controlled by motive means control 62. The output of digital comparator 58 is connected to motive means control 62 so that the position of optical means 52 with respect to film 56 is controlled by the comparison of the first and second digital words.

In the system shown in FIG. 3, the first digital word from digital focus module 50 represents the desired position of optical means 52. When the position of optical element 52, as represented by the second digital word, is identical to the first digital word, the system is in focus. In the case of a still camera, the motion of optical element 52 is stopped at this point."

IOW, digital comparator (the triangle in Canon's diagram) compares "the desired position" of lens (which is determined by AF processor by measuring the shift between signals on AF array half-pairs as I described in my 1st post) to the actual position of the lens as reported by "Drive quantity detector" (or "Lens position encoder", in Honeywell's terms). When they match, the lens stops.

Bingo!

No closed feedback loop. AF just tells the lens: move 10 units closer and STOP. (what these units mean, is programmed into lens' own processor; AF processor in camera doesn't have to be concerned with these low-level lens-specific details). Actually, this comparison of the signals can be performed several or many times during focusing (to rectify the signal and make sure the first command was not a mistake), but it's still NOT A CLOSED FEEDBACK LOOP.

This explains everything. Some lenses have miscalibrated "Drive quantity detector", that makes them stop not there where they should (while camera&lens happy thinking the focus is perfect).

That also explains why some lenses jump so much when trying AF many times on the same subject. Their "Drive quantity detector" does not have steps fine enough to match the signal from AF processor precisely and consistently. When two steps are equidistant from the desired position, the lens will focus one time at one position, another time at another position. That explains why several of my 24-70's are focusing so inconsistently especially in the 24-35mm range. It's focusing group moves too little in this range to have fine steps. A small miscalibration of my camera (not noticeable with other 4 lenses, which have different optical construction) might be "helping" this, too.

That also explains the words heard many times from Canon tech guys that lenses can be "tightened up for 1.6x crop". They adjust the "Drive quantity detector" digitizer to output finer steps.

It's all by design, folks. "Learn not to worry and love the Bomb."

-- hide signature --

Mishkin

DavidP Forum Pro • Posts: 29,088
one question

The 24-70/2.8 is a brand new lens. Post-DSLRs.

So why would Canon put out a lens now that needs to be "tightened"?

Mishkin2 wrote:

That also explains why some lenses jump so much when trying AF many
times on the same subject. Their "Drive quantity detector" does not
have steps fine enough to match the signal from AF processor
precisely and consistently. When two steps are equidistant from the
desired position, the lens will focus one time at one position,
another time at another position. That explains why several of my
24-70's are focusing so inconsistently especially in the 24-35mm
range. It's focusing group moves too little in this range to have
fine steps. A small miscalibration of my camera (not noticeable
with other 4 lenses, which have different optical construction)
might be "helping" this, too.

-- hide signature --

The Lowest Paid Concert Photographer Around
http://www.neonlightsimaging.com/artshow/final.htm
Photography -- just another word for compromise

'Since we can't keep crime in check, why don't we legalize it and tax it out of business?' -- Will Rogers

DavidP Forum Pro • Posts: 29,088
and yet antoehr question

Shouldn't Canon ask for the lenses EVERY TIME somebody has an AF complaint if the lenses need to be "tightened"?

Or shouldn't they at least say "if you have a lens older than x, please send it in with the camera" ?

DavidP wrote:

The 24-70/2.8 is a brand new lens. Post-DSLRs.

So why would Canon put out a lens now that needs to be "tightened"?

-- hide signature --

The Lowest Paid Concert Photographer Around
http://www.neonlightsimaging.com/artshow/final.htm
Photography -- just another word for compromise

'Since we can't keep crime in check, why don't we legalize it and tax it out of business?' -- Will Rogers

Pekka Saarinen Senior Member • Posts: 1,826
Re: Shocking insight: AF is not a closed feedback loop!

Mishkin2 wrote:

That also explains why some lenses jump so much when trying AF many
times on the same subject. Their "Drive quantity detector" does not
have steps fine enough to match the signal from AF processor
precisely and consistently. When two steps are equidistant from the
desired position, the lens will focus one time at one position,
another time at another position.

You should be able to prove this theory:

When you are on subject distance that makes lens choose between two lockpoints on every new lock attempt, moving slightly back or forth should make the lens lock all the time to same position with one attempt, right?

-- hide signature --
DavidP Forum Pro • Posts: 29,088
do I get another question?

Sometimes a lens (noteably wide angle) on a brick wall test will exhibit this "jumping" behavior in AF. Yet other times (without moving the lens) it will just focus (no jumping).

Must be slight changes in lighting? Or very slight movement of the camera between shots.

-- hide signature --

The Lowest Paid Concert Photographer Around
http://www.neonlightsimaging.com/artshow/final.htm
Photography -- just another word for compromise

'Since we can't keep crime in check, why don't we legalize it and tax it out of business?' -- Will Rogers

OP Mishkin™ Contributing Member • Posts: 917
incompatibility with 10D?

24-70 was released before 10D. Maybe there's some incompatibility issue of this lens with 10D (at least, some of somewhat miscalibrated copies?). The Canon guy at B&H confirmed the focusing problems with 24-70 twice when I told him about my problems.

The 24-70 I have now is the FIFTH copy since I first bought a dud 24-70 in November (that one was a real dud - oven plane of "sharp" focus was very mushy even at f/5.6). I bought the last copy at a local store. When a sales person mounted the lens I brought for replacement on Elan 7 and checked focus through VF (he-he, they're not trained to the Gold Measurbating Standards, eh?) and said it's just fine, I voyeristically looked over his shoulded on the lens's distant scale. Guess what? It was jumping as much as on 10D when pressing AF multiple times on the same subject. Focus lock - 10ft! Another focus lock on the same subject - 15ft! Happy camera, lens, and sales man. But we measurbators know too well what this 5ft difference translates to at f/2.8.

Think about this this way. There are steps in output from AF sensor (due to finite, and rather large pixel size of Elan/10D AF sensor). Then there are steps from the "Drive quantity detector". It's possible that the matching of these steps can result in large steps. Analogy to this is moire. Put two grids with fine, but slightly different steps one on top of the other, and you get large-step patterns - moire.

My guess is that this is what happens in 24-70 on 10D especially in 24-35mm range: it can't focus reliably especially at infinity, and at f/2.8-f/4 it shows.

Many 24-70 users are happy, though. I guess, this "moire" bug does not happen in all 24-70+10D combinations out there.

DavidP wrote:
The 24-70/2.8 is a brand new lens. Post-DSLRs.

So why would Canon put out a lens now that needs to be "tightened"?

Mishkin2 wrote:

That also explains why some lenses jump so much when trying AF many
times on the same subject. Their "Drive quantity detector" does not
have steps fine enough to match the signal from AF processor
precisely and consistently. When two steps are equidistant from the
desired position, the lens will focus one time at one position,
another time at another position. That explains why several of my
24-70's are focusing so inconsistently especially in the 24-35mm
range. It's focusing group moves too little in this range to have
fine steps. A small miscalibration of my camera (not noticeable
with other 4 lenses, which have different optical construction)
might be "helping" this, too.

-- hide signature --

Mishkin

Jason Hutchinson Forum Pro • Posts: 12,177
I said this a while ago :)

I've been saying it's not a true feedback loop for a while. Of course, I didn't have a whole lot of proof or fancy charts. That being said, I'll take my D60 over my true full feedback looped G2 any day.

Keep searching for info, I enjoy your posts.

Jason

Mishkin2 wrote:

And THAT and only that can explain why some lenses consistently
front/back focus even if camera is well-calibrated.

Take a look at this illustration from Canon Camera Museum
( http://www.canon.com/camera-museum/tech/room/f_index.html ):

There is a device in the lens named "Drive quantity detector". As
far as I understand from the diagram, the signals from this
detector and from AF processor are sent to some digital comparator
(triangle).

Why would this detector be needed in closed feedback loop? In
closed loop, the AF processor will send the signal to the lens
until the image is perfectly in focus. There's no need to measure
lens displacement and compare it to the signal from AF processor.

Now let's turn our measurbating brains to U.S. Patent 4,078,171 by
Honeywell (thanks to tawen mei who pointed me to it).

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=4078171.WKU.&OS=PN/4078171&RS=PN/4078171
For those having trouble with QuickTime, here's the screenshot of
the illustrations:

Here's an exerpt from the patent:
"FIG. 3 shows a digital automatic focus system which may be used in
photographic equipment such as a still or movie camera. The system
includes a digital focus module 50, which preferably is apparatus
like that shown in FIG. 1. The output of digital focus module 50 is
a first digital word which is used to control a position of a
primary optical means 52 such as the taking lens of a still or
movie camera. A lens position encoder 54 provides a second digital
word indicative of the position of optical means 52 with respect to
film 56. Digital comparator 58 compares the first and second
digital words. The position of optical element 52 is determined by
motive means 60, which is controlled by motive means control 62.
The output of digital comparator 58 is connected to motive means
control 62 so that the position of optical means 52 with respect to
film 56 is controlled by the comparison of the first and second
digital words.

In the system shown in FIG. 3, the first digital word from digital
focus module 50 represents the desired position of optical means
52. When the position of optical element 52, as represented by the
second digital word, is identical to the first digital word, the
system is in focus. In the case of a still camera, the motion of
optical element 52 is stopped at this point."

IOW, digital comparator (the triangle in Canon's diagram) compares
"the desired position" of lens (which is determined by AF processor
by measuring the shift between signals on AF array half-pairs as I
described in my 1st post) to the actual position of the lens as
reported by "Drive quantity detector" (or "Lens position encoder",
in Honeywell's terms). When they match, the lens stops.

Bingo!

No closed feedback loop. AF just tells the lens: move 10 units
closer and STOP. (what these units mean, is programmed into lens'
own processor; AF processor in camera doesn't have to be concerned
with these low-level lens-specific details). Actually, this
comparison of the signals can be performed several or many times
during focusing (to rectify the signal and make sure the first
command was not a mistake), but it's still NOT A CLOSED FEEDBACK
LOOP.

This explains everything. Some lenses have miscalibrated "Drive
quantity detector", that makes them stop not there where they
should (while camera&lens happy thinking the focus is perfect).

That also explains why some lenses jump so much when trying AF many
times on the same subject. Their "Drive quantity detector" does not
have steps fine enough to match the signal from AF processor
precisely and consistently. When two steps are equidistant from the
desired position, the lens will focus one time at one position,
another time at another position. That explains why several of my
24-70's are focusing so inconsistently especially in the 24-35mm
range. It's focusing group moves too little in this range to have
fine steps. A small miscalibration of my camera (not noticeable
with other 4 lenses, which have different optical construction)
might be "helping" this, too.

That also explains the words heard many times from Canon tech guys
that lenses can be "tightened up for 1.6x crop". They adjust the
"Drive quantity detector" digitizer to output finer steps.

It's all by design, folks. "Learn not to worry and love the Bomb."

Mike. Senior Member • Posts: 1,471
Worth buying a 50mm 1.8 & trashing it

To scope the lens contacts and see what data is being exchanged between the lens & body.

its on my "to do list" just as soon as I finish building a piezo digitizing probe for my cnc

Check canons patents with regard to the two issues, AF and IR cut & "blur" filter.

-Mike.

Mishkin2 wrote:

And THAT and only that can explain why some lenses consistently
front/back focus even if camera is well-calibrated.

Take a look at this illustration from Canon Camera Museum
( http://www.canon.com/camera-museum/tech/room/f_index.html ):

There is a device in the lens named "Drive quantity detector". As
far as I understand from the diagram, the signals from this
detector and from AF processor are sent to some digital comparator
(triangle).

Why would this detector be needed in closed feedback loop? In
closed loop, the AF processor will send the signal to the lens
until the image is perfectly in focus. There's no need to measure
lens displacement and compare it to the signal from AF processor.

Now let's turn our measurbating brains to U.S. Patent 4,078,171 by
Honeywell (thanks to tawen mei who pointed me to it).

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=4078171.WKU.&OS=PN/4078171&RS=PN/4078171
For those having trouble with QuickTime, here's the screenshot of
the illustrations:

Here's an exerpt from the patent:
"FIG. 3 shows a digital automatic focus system which may be used in
photographic equipment such as a still or movie camera. The system
includes a digital focus module 50, which preferably is apparatus
like that shown in FIG. 1. The output of digital focus module 50 is
a first digital word which is used to control a position of a
primary optical means 52 such as the taking lens of a still or
movie camera. A lens position encoder 54 provides a second digital
word indicative of the position of optical means 52 with respect to
film 56. Digital comparator 58 compares the first and second
digital words. The position of optical element 52 is determined by
motive means 60, which is controlled by motive means control 62.
The output of digital comparator 58 is connected to motive means
control 62 so that the position of optical means 52 with respect to
film 56 is controlled by the comparison of the first and second
digital words.

In the system shown in FIG. 3, the first digital word from digital
focus module 50 represents the desired position of optical means
52. When the position of optical element 52, as represented by the
second digital word, is identical to the first digital word, the
system is in focus. In the case of a still camera, the motion of
optical element 52 is stopped at this point."

IOW, digital comparator (the triangle in Canon's diagram) compares
"the desired position" of lens (which is determined by AF processor
by measuring the shift between signals on AF array half-pairs as I
described in my 1st post) to the actual position of the lens as
reported by "Drive quantity detector" (or "Lens position encoder",
in Honeywell's terms). When they match, the lens stops.

Bingo!

No closed feedback loop. AF just tells the lens: move 10 units
closer and STOP. (what these units mean, is programmed into lens'
own processor; AF processor in camera doesn't have to be concerned
with these low-level lens-specific details). Actually, this
comparison of the signals can be performed several or many times
during focusing (to rectify the signal and make sure the first
command was not a mistake), but it's still NOT A CLOSED FEEDBACK
LOOP.

This explains everything. Some lenses have miscalibrated "Drive
quantity detector", that makes them stop not there where they
should (while camera&lens happy thinking the focus is perfect).

That also explains why some lenses jump so much when trying AF many
times on the same subject. Their "Drive quantity detector" does not
have steps fine enough to match the signal from AF processor
precisely and consistently. When two steps are equidistant from the
desired position, the lens will focus one time at one position,
another time at another position. That explains why several of my
24-70's are focusing so inconsistently especially in the 24-35mm
range. It's focusing group moves too little in this range to have
fine steps. A small miscalibration of my camera (not noticeable
with other 4 lenses, which have different optical construction)
might be "helping" this, too.

That also explains the words heard many times from Canon tech guys
that lenses can be "tightened up for 1.6x crop". They adjust the
"Drive quantity detector" digitizer to output finer steps.

It's all by design, folks. "Learn not to worry and love the Bomb."

-- hide signature --

-Mike

OP Mishkin™ Contributing Member • Posts: 917
thanks, Jason

I was too curious about how AF works not to dissect it to death

Now that I better understand the limitations of AF system (or at least, naively think so), I am trying find the ways how to deal with them. E.g. how to focus with ultrafast primes (see my reply to your 50/1.4 post: http://forums.dpreview.com/forums/read.asp?forum=1029&message=5621223 ). No longer I'm disappointed with blurry images at f/1.4-f/2.5. This is expected, that's all. Learn - or invent - the techiniques how to overcome AF limitations at such apertures.

BTW, these limitations should put a stop for Pro photographers recommending to newbies asking what should be the 1st lens to start with fast primes. Newbies better start with f/3.5-f/5.6 lenses (28-135IS) so that AF limitations will be hidden from their eyes which get wet when they see blurry pixels.

Jason Hutchinson wrote:

I've been saying it's not a true feedback loop for a while. Of
course, I didn't have a whole lot of proof or fancy charts.
That being said, I'll take my D60 over my true full feedback looped
G2 any day.

Keep searching for info, I enjoy your posts.

Jason

-- hide signature --

Mishkin

OP Mishkin™ Contributing Member • Posts: 917
Godspeed, Mike

I gave a little food to us hungry measurbators, we need a constant supply, you're our next savior!

Thanks for the tip about Canon patents. That's where I'm looking at right now

Mike. wrote:
To scope the lens contacts and see what data is being exchanged
between the lens & body.

its on my "to do list" just as soon as I finish building a piezo
digitizing probe for my cnc

Check canons patents with regard to the two issues, AF and IR cut &
"blur" filter.

-Mike.

Mishkin2 wrote:

And THAT and only that can explain why some lenses consistently
front/back focus even if camera is well-calibrated.

Take a look at this illustration from Canon Camera Museum
( http://www.canon.com/camera-museum/tech/room/f_index.html ):

There is a device in the lens named "Drive quantity detector". As
far as I understand from the diagram, the signals from this
detector and from AF processor are sent to some digital comparator
(triangle).

Why would this detector be needed in closed feedback loop? In
closed loop, the AF processor will send the signal to the lens
until the image is perfectly in focus. There's no need to measure
lens displacement and compare it to the signal from AF processor.

Now let's turn our measurbating brains to U.S. Patent 4,078,171 by
Honeywell (thanks to tawen mei who pointed me to it).

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=4078171.WKU.&OS=PN/4078171&RS=PN/4078171
For those having trouble with QuickTime, here's the screenshot of
the illustrations:

Here's an exerpt from the patent:
"FIG. 3 shows a digital automatic focus system which may be used in
photographic equipment such as a still or movie camera. The system
includes a digital focus module 50, which preferably is apparatus
like that shown in FIG. 1. The output of digital focus module 50 is
a first digital word which is used to control a position of a
primary optical means 52 such as the taking lens of a still or
movie camera. A lens position encoder 54 provides a second digital
word indicative of the position of optical means 52 with respect to
film 56. Digital comparator 58 compares the first and second
digital words. The position of optical element 52 is determined by
motive means 60, which is controlled by motive means control 62.
The output of digital comparator 58 is connected to motive means
control 62 so that the position of optical means 52 with respect to
film 56 is controlled by the comparison of the first and second
digital words.

In the system shown in FIG. 3, the first digital word from digital
focus module 50 represents the desired position of optical means
52. When the position of optical element 52, as represented by the
second digital word, is identical to the first digital word, the
system is in focus. In the case of a still camera, the motion of
optical element 52 is stopped at this point."

IOW, digital comparator (the triangle in Canon's diagram) compares
"the desired position" of lens (which is determined by AF processor
by measuring the shift between signals on AF array half-pairs as I
described in my 1st post) to the actual position of the lens as
reported by "Drive quantity detector" (or "Lens position encoder",
in Honeywell's terms). When they match, the lens stops.

Bingo!

No closed feedback loop. AF just tells the lens: move 10 units
closer and STOP. (what these units mean, is programmed into lens'
own processor; AF processor in camera doesn't have to be concerned
with these low-level lens-specific details). Actually, this
comparison of the signals can be performed several or many times
during focusing (to rectify the signal and make sure the first
command was not a mistake), but it's still NOT A CLOSED FEEDBACK
LOOP.

This explains everything. Some lenses have miscalibrated "Drive
quantity detector", that makes them stop not there where they
should (while camera&lens happy thinking the focus is perfect).

That also explains why some lenses jump so much when trying AF many
times on the same subject. Their "Drive quantity detector" does not
have steps fine enough to match the signal from AF processor
precisely and consistently. When two steps are equidistant from the
desired position, the lens will focus one time at one position,
another time at another position. That explains why several of my
24-70's are focusing so inconsistently especially in the 24-35mm
range. It's focusing group moves too little in this range to have
fine steps. A small miscalibration of my camera (not noticeable
with other 4 lenses, which have different optical construction)
might be "helping" this, too.

That also explains the words heard many times from Canon tech guys
that lenses can be "tightened up for 1.6x crop". They adjust the
"Drive quantity detector" digitizer to output finer steps.

It's all by design, folks. "Learn not to worry and love the Bomb."

-- hide signature --

Mishkin

hyslopc Senior Member • Posts: 2,129
Actually, isn't AI-Servo just that?

Unless I've misunderstood, you couldn't make the AF a closed feedback loop and still support moving targets. Since many targets move, the AF would constantly be re-adjusting itself and would never "lock" - sound familiar? That's right, I've just described Canon's AI-Servo mode, which constantly and continuously re-focusses for as long as you half-press.

Kura Regular Member • Posts: 170
Not shocking at all.

This was all covered in the 10D manual.

You could also tell if you did the "inverted secondary focus test" that was described in Chuck's post over at Rob's.

ok, all kidding aside, I appreciate this post. Now for the dumb question. What does this have to do with your later post about saying 1.4 and 2.8 would have more of an issue (newbies should get a f3.5). Is that a DOF issue or something else.

I have a headache.

But seriously, thanks.

Mishkin2 wrote:

And THAT and only that can explain why some lenses consistently
front/back focus even if camera is well-calibrated.

Take a look at this illustration from Canon Camera Museum
( http://www.canon.com/camera-museum/tech/room/f_index.html ):

There is a device in the lens named "Drive quantity detector". As
far as I understand from the diagram, the signals from this
detector and from AF processor are sent to some digital comparator
(triangle).

Why would this detector be needed in closed feedback loop? In
closed loop, the AF processor will send the signal to the lens
until the image is perfectly in focus. There's no need to measure
lens displacement and compare it to the signal from AF processor.

Now let's turn our measurbating brains to U.S. Patent 4,078,171 by
Honeywell (thanks to tawen mei who pointed me to it).

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=4078171.WKU.&OS=PN/4078171&RS=PN/4078171
For those having trouble with QuickTime, here's the screenshot of
the illustrations:

Here's an exerpt from the patent:
"FIG. 3 shows a digital automatic focus system which may be used in
photographic equipment such as a still or movie camera. The system
includes a digital focus module 50, which preferably is apparatus
like that shown in FIG. 1. The output of digital focus module 50 is
a first digital word which is used to control a position of a
primary optical means 52 such as the taking lens of a still or
movie camera. A lens position encoder 54 provides a second digital
word indicative of the position of optical means 52 with respect to
film 56. Digital comparator 58 compares the first and second
digital words. The position of optical element 52 is determined by
motive means 60, which is controlled by motive means control 62.
The output of digital comparator 58 is connected to motive means
control 62 so that the position of optical means 52 with respect to
film 56 is controlled by the comparison of the first and second
digital words.

In the system shown in FIG. 3, the first digital word from digital
focus module 50 represents the desired position of optical means
52. When the position of optical element 52, as represented by the
second digital word, is identical to the first digital word, the
system is in focus. In the case of a still camera, the motion of
optical element 52 is stopped at this point."

IOW, digital comparator (the triangle in Canon's diagram) compares
"the desired position" of lens (which is determined by AF processor
by measuring the shift between signals on AF array half-pairs as I
described in my 1st post) to the actual position of the lens as
reported by "Drive quantity detector" (or "Lens position encoder",
in Honeywell's terms). When they match, the lens stops.

Bingo!

No closed feedback loop. AF just tells the lens: move 10 units
closer and STOP. (what these units mean, is programmed into lens'
own processor; AF processor in camera doesn't have to be concerned
with these low-level lens-specific details). Actually, this
comparison of the signals can be performed several or many times
during focusing (to rectify the signal and make sure the first
command was not a mistake), but it's still NOT A CLOSED FEEDBACK
LOOP.

This explains everything. Some lenses have miscalibrated "Drive
quantity detector", that makes them stop not there where they
should (while camera&lens happy thinking the focus is perfect).

That also explains why some lenses jump so much when trying AF many
times on the same subject. Their "Drive quantity detector" does not
have steps fine enough to match the signal from AF processor
precisely and consistently. When two steps are equidistant from the
desired position, the lens will focus one time at one position,
another time at another position. That explains why several of my
24-70's are focusing so inconsistently especially in the 24-35mm
range. It's focusing group moves too little in this range to have
fine steps. A small miscalibration of my camera (not noticeable
with other 4 lenses, which have different optical construction)
might be "helping" this, too.

That also explains the words heard many times from Canon tech guys
that lenses can be "tightened up for 1.6x crop". They adjust the
"Drive quantity detector" digitizer to output finer steps.

It's all by design, folks. "Learn not to worry and love the Bomb."

OP Mishkin™ Contributing Member • Posts: 917
Is that the reason WHY AF is not implemented as a loop?

You're right, direct link between measured lens position and a signal from AF sensor allows for lens focusing on moving subject. I guess, the loop system can do this, too, but it will require extremely fast convergence of the loop and running the loop many times per second, which is probably beyond capabilities of AF systems. This is also mentioned in that Honeywell patent.

Other than that, I don't see WHY AF is not implemented as a closed feedback loop.

hyslopc wrote:

Unless I've misunderstood, you couldn't make the AF a closed
feedback loop and still support moving targets. Since many targets
move, the AF would constantly be re-adjusting itself and would
never "lock" - sound familiar? That's right, I've just described
Canon's AI-Servo mode, which constantly and continuously
re-focusses for as long as you half-press.

-- hide signature --

Mishkin

OP Mishkin™ Contributing Member • Posts: 917
headache is curable

for the reasoning I came to this conclusion, look here:
http://forums.dpreview.com/forums/read.asp?forum=1019&message=5620566
then this correction:
http://forums.dpreview.com/forums/read.asp?forum=1019&message=5621100
and proposed way of dealing with it:
http://forums.dpreview.com/forums/read.asp?forum=1029&message=5621223

When you shoot f/4 or slower, these problems are almost invisible due to DOF large enough to cover up the inherent AF inaccuracy.

There were a lot of advices from pros in the past in this forum to start with 50/1.4 as a first lens. My point is that newbies can be disappointed by blurred pictures when shooting wide open and blame the lens or the camera. While it's actually an inherent limitation of AF system.

Proud Dad wrote:

This was all covered in the 10D manual.

You could also tell if you did the "inverted secondary focus test"
that was described in Chuck's post over at Rob's.

ok, all kidding aside, I appreciate this post. Now for the dumb
question. What does this have to do with your later post about
saying 1.4 and 2.8 would have more of an issue (newbies should get
a f3.5). Is that a DOF issue or something else.

I have a headache.

But seriously, thanks.

Jason Hutchinson Forum Pro • Posts: 12,177
loop stability might be an issue

Loop stability is the most likely one, but what about the response time of the older motors? They're clearly not designed as well as modern USM motors for the small repetitive motions that'd be required for that. Who knows.

Jason

Mishkin2 wrote:
You're right, direct link between measured lens position and a
signal from AF sensor allows for lens focusing on moving subject. I
guess, the loop system can do this, too, but it will require
extremely fast convergence of the loop and running the loop many
times per second, which is probably beyond capabilities of AF
systems. This is also mentioned in that Honeywell patent.

Other than that, I don't see WHY AF is not implemented as a closed
feedback loop.

hyslopc wrote:

Unless I've misunderstood, you couldn't make the AF a closed
feedback loop and still support moving targets. Since many targets
move, the AF would constantly be re-adjusting itself and would
never "lock" - sound familiar? That's right, I've just described
Canon's AI-Servo mode, which constantly and continuously
re-focusses for as long as you half-press.

OP Mishkin™ Contributing Member • Posts: 917
I agree

Non-loop design easily produces great results when both camera and lenses are properly calibrated, and can track moving subjects without posing any extra strain on motors.

Some of the AF problems can be thus attributed to miscalibrated bodies and lenses, while some (misfocused pictures at f

Jason Hutchinson wrote:
Loop stability is the most likely one, but what about the response
time of the older motors? They're clearly not designed as well as
modern USM motors for the small repetitive motions that'd be
required for that. Who knows.

Jason

Mishkin2 wrote:
You're right, direct link between measured lens position and a
signal from AF sensor allows for lens focusing on moving subject. I
guess, the loop system can do this, too, but it will require
extremely fast convergence of the loop and running the loop many
times per second, which is probably beyond capabilities of AF
systems. This is also mentioned in that Honeywell patent.

Other than that, I don't see WHY AF is not implemented as a closed
feedback loop.

hyslopc wrote:

Unless I've misunderstood, you couldn't make the AF a closed
feedback loop and still support moving targets. Since many targets
move, the AF would constantly be re-adjusting itself and would
never "lock" - sound familiar? That's right, I've just described
Canon's AI-Servo mode, which constantly and continuously
re-focusses for as long as you half-press.

-- hide signature --

Mishkin

David G Regular Member • Posts: 179
I already beat you to it...

Buying a 50mm 1.8 & trashing it that is.. It fell out of my bag and broke in two.

Replaced it the next day with a new one. Gotta love those $70 disposable lenses..

-David

-- hide signature --

David
Washington, DC
Canon D60
Canon 420EX
28mm f/2.8
50mm f/1.8
70-200mm f/4.0L

Larry H. Smith Veteran Member • Posts: 6,270
Is there a bottom line "best procedure"?

Up-front: I haven't read (and probably couldn't have followed) all the tech-discussion on the AF "issue'"in the various threads.

Does this new understanding (Thanks mishkin!)make it possible to put into some reasonable number of words , a "best" way for the user to maximize the AF potential of his rig, ...or a best way to test/adjust "whatever" for that "best" performance?

(I mean when using whatever f-stop the photog WANTS to use, for image reasons other-than-focus.)

This whole "what the camera can/can't do FOR you" thing is the main reason I won't be happy till I have a pro-level camera that will let ME do what I want, with a viewfinder that CLEARLY shows me what is happening.

Thanks,

Larry

Mishkin2 wrote:

...a lotta stuff!

yada10d Regular Member • Posts: 279
Hang on...

Mishkin, doesn't that fly in the face of getting any sort of focus in low contrast situations?

Isn't the ability to focus in low light one of the reasons to step up to fast L glass?

I would think the recomendation should be to get a fast prime and stop it down, not put a possibly less capable lens on the camera to 'hide' the problem.

My 2c

Michael

Mishkin2 wrote:
for the reasoning I came to this conclusion, look here:
http://forums.dpreview.com/forums/read.asp?forum=1019&message=5620566
then this correction:
http://forums.dpreview.com/forums/read.asp?forum=1019&message=5621100
and proposed way of dealing with it:
http://forums.dpreview.com/forums/read.asp?forum=1029&message=5621223

When you shoot f/4 or slower, these problems are almost invisible
due to DOF large enough to cover up the inherent AF inaccuracy.

There were a lot of advices from pros in the past in this forum to
start with 50/1.4 as a first lens. My point is that newbies can be
disappointed by blurred pictures when shooting wide open and blame
the lens or the camera. While it's actually an inherent limitation
of AF system.

Proud Dad wrote:

This was all covered in the 10D manual.

You could also tell if you did the "inverted secondary focus test"
that was described in Chuck's post over at Rob's.

ok, all kidding aside, I appreciate this post. Now for the dumb
question. What does this have to do with your later post about
saying 1.4 and 2.8 would have more of an issue (newbies should get
a f3.5). Is that a DOF issue or something else.

I have a headache.

But seriously, thanks.

OP Mishkin™ Contributing Member • Posts: 917
about lens adjustments

Now that we forget about closed loop, it becomes clear why Canon asks you to send ALL lenses in even if you have a problem with one lens only.

First, Canon calibrates the body (by applying a shift to sensors' signals in AF processor's firmware) by using some "perfect" lens (their well-calibrated copy of say 50/1.4).

After body is calibrated, all lenses you sent in are calibrated with the body. Their drive quantity detectors get adjusted (by updating code in lens' controller chip) to be a perfect match with the body.

The myth about lens being a dumb device is finally misspelled. AF is rather a handshake of the chips in camera and lens. Both must be properly calibrated for handshake to be a perfect focus.

If lens were a dumb device with a motor only driven by a feedback loop signal, this dumbness would produce ultimate sharpness all the time. Unfortunately, closed loop system cannot compete with "open" one in speed and moving subject tracking, while "open loop" produces excellent result when all components are properly calibrated.

Due to bad Canon's QC, users may need to send the camera and all lenses in almost EACH TIME they buy a new lens. Oh well, life is life.

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Mishkin

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