How do you calibrate zoom lenses?
Most cameras only support a single micro-adjustment setting for the whole lens at all zoom positions. What you tend to find is the ideal auto-focus calibration value may vary between zoomed in and zoomed out, with the most difference seen at each of the zoom extremes.
Whilst Reikan FoCal cannot address the lack of support in a camera for multiple microadjustment values, you can still calibrate a zoom lens and get a better result than an un-calibrated zoom. It’s easiest to explain this with a quick example (below).
Suppose we have a 24-70mm lens. If we calibrate at 24mm with a result of +10 and at 70mm and get a result of +15. Since in this example both calibration results are on the positive side and the lower value is +10, almost any positive calibration value would make the lens better at any focal length than it would have been at the default setting of 0. See directly below for a more considered approach.
Which value should we actually use given our example? Well, focus errors have a greater impact at the telephoto end of a zoom lens as the depth-of-field is more shallow, it’s therefore better to bias the calibration value toward the telephoto end result. In the example above we’d suggest a calibration value of +13 or +14. In real world shooting you probably won’t notice a difference between +13 or +14 so either will work. What you will notice is a huge improvement in auto focus compared to the previously un-calibrated lens!
Canon have implicitly acknowledged the need for multiple microadjustment points on zoom lenses by allowing users to calibrate at both ends of the zoom range. Newer Canon cameras have “two point” adjustment allowing separate values, one for fully zoomed out and one for fully zoomed in, with the camera interpolating the required mid-zoom adjustment automatically.
Since “two point adjustment" exists only on a sub-set of Canon cameras you can find which cameras support this feature in the section titled “Wide and Tele Calibration” on the Supported Cameras page. The procedure for cameras which support this feature is to calibrate the zoom lenses twice, once each at both extremes of the zoom range (for example calibrate at 24mm and then a second calibration at 70mm for a 24-70mm zoom).
The target distance required for calibration of zoom lenses will vary. Treat each end of the zoom range as if it is a separate prime lens, so with our 24-70mm example calibrate the 24mm end at a distance suitable for a 24mm lens and then a separate calibration of the 70mm end at a distance suitable for a 70mm lens. See also What distance should I use for auto focus calibration?.