Originally posted by Marc Sabatella I don't see why you'd think reprogramming a ROm chip could possibly fix a focus issue. The chip juyst has some numbers on it; it doens't control the focus. The focus is mechanical. any focus adjustments to a lens would involving physically altering it - I can't see any way that programming could possibly have anything to do with it.
Marc, I tried hard to find a credible link to Canon's Service Center explaining the process I was describing, but no dice. Failing that, here is a link to a blog discussing this topic.
Gordon's Tech & Hobby Blog: Canon Auto-Focus Technical Information
The pertinent part is about a third of the way down the page:
Quote: When the camera determines how far and in what direction the lens must move to cancel the phase difference, it does so within a tolerance of "within the depth of focus" of lenses slower than f2.8 (down to f5.6) or "within 1/3 of the depth of focus" of lenses f2.8 and faster. The depth of focus is the range at the sensor plane within which the image of a point will be reproduced as a blur smaller than the manufacturer's designated "circle of confusion" (CoC). Canon's designated circle of confusion is 0.035mm for the 24x36mm format and 0.02mm for the APS-C format. The CoC is based on maintaining the appearance of sharpness in a 6x9 inch print at about an 10 inch viewing distance (as revealed by the Euro-Canon web site). There is no guarantee that images enlarged any greater than this will appear sharp.
The depth of focus increases when the aperture of the lens decreases (like depth of field at the subject plane), but it does not change with the focused distance or the focal length of the lens (according to Canon, unlike depth of field). That is why the camera interrogates the lens for that information; it calculates the depth of focus tolerance from the maximum aperture, not the set working aperture.
As a result of this tolerance (within the depth of focus or within 1/3 of the depth of focus), the camera can place the actual plane of focus at random anywhere within the tolerance range, and not necessarily at the same place each time.
It is my understanding, and
supported by Chuck Westfall's (Canon's US Technical Adviser) article here a little more than half way down the page) that each lens is calibrated at the factory and an MTF table based on a standard algorithm is flashed onto the ROM chip after it is tested. This is done under controlled conditions but often only at particular focal lengths (such as only 24 and 70mm on the 24-70mm zoom). If the lens is sent in for repair, this table can be recreated through retesting and re-programmed (re-flashed) onto the chip.