How Canon calibrate autofocus?

Started Nov 4, 2007 | Discussions thread
DavidSvensson Senior Member • Posts: 1,374
Canon AF calibration procedure

AF microadjustment is not a substitute for calibration. For starters the Microadjustment will affect all focal length settings of a zoom lens equally, wheras a full calibration will handle 8 or 10 different focal length settings for each zoom lens.

Proper AF calibration is a very time-consuming task, if done by the book.

If the tech does not care, work will be poor, and AF calibration on a camera and a zoom lens takes a lot of work and time to get just right.

When done properly, the first thing to check is the cameras AF sensor, its calibration at each and every single one of the AF points, including the "invisible" assistant AF points. Each of these points must be checked with both horizontal and vertical lines, where there are cross-type AF sensors.

(1/2 - 1 h to check and adjust, up to a day´s work to adjust if the AF sensor is not parallell with the image sensor.)
Next put the lens on a test projector and check the lens for optical alignment.

(20 minutes to check, hours of work if the lens needs to be pulled apart for adjustments.)

Finally put the lens on the camera and at each one of 8 or 10 different focal lengths, using both horizontal and vertical lines, check and adjust AF calibration.
(1/2 h to check, 15 min to adjust per focal length that needs adjustment)

There are two calibration tables in each lens. One for 1-series cameras, and one for non-1-series) These use different types of AF sensors, so calibration has to be made from scratch for each table.

The techs are supposed always to check calibration with two cameras, one of each category, and perform calibrations as needed for both tables.

The problem is that in most countries Canon has outsorced much of this work, and Canon will only pay a small flat-rate amount per lens that gets calibrated, irrespective of the amount of work that is actually done.

Simply put, this system more or less forces the companies involved to cut corners, and middle managment from financial necessity tend to put pressure on the techs not to spend too much time on the lens calibration work.

Having a few carefully shot test-charts clearly showing front- or backfocus, very effectively yet politely communicate you are customer who knows how to check the quality of the calibration work.

Basically put, if manual focus ever can obtain a sharper image than AF, at any focal length and with any of the AF sensors, the calibration has not been properly done.
With a camera with live view, this is extremely easy to check:

Use a tripod. AF on a detailed flat surface, at a distance about 50 times further away than the lens focal length. Switch to live view. Zoom in at the AF sensor used. Look at the sharpness. Then manually focus very slowly until the image is as sharp as possible. If this sharpness is better than the sharpness straight after AF, then there is room for improvment.

This test needs to be done several times over, to make sure a momentary glitch from the AF system has not given misleading results.

David

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