Fine-Tuning the Focus of Your Lenses

In Chapter 3, I introduced you to the A77’s AF Micro Adjustment feature, which, I noted, that you might not ever need to use, because it is applied only when you find that a particular lens is not focusing properly. If the lens happens to focus a bit ahead or a bit behind the actual point of sharp focus, and it does that consistently, you can use the adjustment feature, found in the Setup 3 menu, to “calibrate” the lens’s focus.

Why is the focus “off” for some lenses in the first place? There are lots of factors, including the age of the lens (an older lens may focus slightly differently), temperature effects on certain types of glass, humidity, and tolerances built into a lens’s design that all add up to a slight misadjustment, even though the components themselves are, strictly speaking, within specs. A very slight variation in your lens’s mount can cause focus to vary slightly. With any luck (if you can call it that) a lens that doesn’t focus exactly right will at least be consistent. If a lens always focuses a bit behind the subject, the symptom is back focus. If it focuses in front of the subject, it’s called front focus.