Q and A: How does the Autofocus Adjustment function work–and should I use it?


From reading reviews, I know that some recent DSLRs, like my Pentax K-7, include an Autofocus Adjustment function that allows for modifying the AF system for more accurate focus with certain lenses. Is this function necessary? If so, how does it work? R. Webber


Yes, an increasing number of high-end DSLRs of several brands include a feature of this type. It allows for instructing the camera to focus slightly more closely, or to a slightly greater distance, than it would do at default. This function would be useful for an older AF lens-especially one that has been banged around-if it is no longer focusing accurately. You may find that it focuses slightly ahead of the subject, or slightly behind it; these issues are called “front focus” and “back focus” and they call for AF Adjustment.

Before using this in-camera feature however, be absolutely certain of the lens’s exact focusing tendencies. Conduct a thorough test by taking some photos shots of a newspaper page-taped flat against a wall-using single shot (AF.S) autofocus, with the camera on a rigid tripod. Make sure the camera back is perfectly parallel to the page.

Take photos of the page from several distances: from close to distant. Always use the widest aperture (smallest f/number) available with that lens. After taking the photos, check them at 100% magnification on a high quality computer monitor.

If you have confirmed that the lens routinely focuses too close or too far, use the camera’s AF Adjustment function exactly as described in the owner’s manual. (The procedure varies from one camera to another.) Initially, set a moderate adjustment level in the appropriate direction and repeat the test (with the newspaper page). If the first adjustment did not solve the problem, set a slightly higher adjustment level and repeat the test again. While this process is tedious, it will pay off for those who own an AF lens that regularly creates a focusing error.

Accurate focus is certainly important, particularly when shooting at a very wide aperture. That's why an increasing number of prosumer and pro calibre DSLRs include a custom function for correcting a lens's inaccurate autofocusing tendencies. This feature should rarely be required and should not be used until you're certain that adjustment is necessary. (c) 2008 Peter K. Burian
Accurate focus is certainly important particularly when shooting at a very wide aperture Thats why an increasing number of prosumer and pro calibre DSLRs include a custom function for correcting a lenss inaccurate autofocusing tendencies This feature should rarely be required and should not be used until youre certain that adjustment is necessary c 2008 Peter K Burian


This Post Has One Comment

  1. Nice clear explanation of HOW to do this adjustment, but I am still waiting for someone to come up with a good explanation of WHY it should be necessary!

    Even if a lens is “older” (lenses go out of focus as they age??) or “banged up,” through-the-lens focusing (either manual or AF) should be able to accommodate the disparity, shouldn’t it?

    I thought the whole point of SLR viewing was that what you (or the AF sensors) see through the finder exactly reflects what the sensor will see… right? When we all used manual-focus SLRs, we depended solely on this finder view to achieve precise focusing — and we didn’t need to make “manual focus adjustments” for individual lenses to get it. Early-generation AF SLRs didn’t need seem to need lens-by-lens “adjustment” either — my Minolta Maxxum 9000’s autofocus was slow, all right, but it was precise — with every lens, including demanding ones such as the 85/1.4 and 200/2.8.

    So why are things suddenly different now? (And don’t go telling me that a 12-megapixel DSLR is “more demanding” than a K25 transparency viewed through a 20x loupe!)

    Here’s one cynical answer candidate:

    — Camera manufacturers assume that most photographers are using kit-zoom lenses that don’t demand very accurate focusing because of their modest maximum apertures.

    — Taking advantage of that, they’ve loosened up the tolerances of their finder and AF system adjustments — either for the sake of cost-cutting or AF speed or both.

    — Then they’ve added this “AF adjustment” feature so that photographers who DO use lenses that demand more focusing accuracy can do the “quality control” themselves!

    If that really is the case, I feel the industry should fess up and say so. And if it’s NOT the answer, I’d still like to hear what the answer IS.

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