Shocking insight: AF is not a closed feedback loop!

Started Jul 18, 2003 | Discussions thread
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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 ( ):

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).

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.


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."

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