With the advent of digital photography we have become much more critical about focus accuracy. We're now able to enlarge our photos to 100% size on our monitors and look at them pixel by pixel. Inaccuracies in focus and other flaws which no one noticed or cared about in the film era are now blatantly visible.
When we shot film and put that big enlargement up on the wall we understood that we would be looking at it from a distance, and the larger the print, the larger the viewing distance. Today, we sit close to our monitors and when an image is displayed at 100% zoom, we easily notice the many flaws introduced by our cameras and lenses.
Two of the most frequently-discussed flaws are inaccuracies in focus, commonly called front-focus and back-focus. Assuming we focus our cameras on a given subject, front-focus occurs when the area in front of our subject seems sharper than the subject itself, and back-focus is the opposite of that: when the area behind our subject seems sharper than the subject.
Most of the time, these two issues can be attributed to the lens, and not the camera. The test for this is easy: if different lenses exhibit different degrees of front- or back-focus, then they are at fault, you can usually fix the problem at home!
Often customers return a front- or back-focusing lens to their retailer and get another one hoping for the better. Their next lens could very well also be "defective". But is it truly defective? In all likelihood the answer is no, as we simply expect too much from our mass-produced equipment.
Most DSLRs today have a per-lens autofocus adjustment feature, so that in case of a front- or back- focusing lens all that is needed is an in-camera adjustment to compensate for the inaccuracy. Modern Pentax DSLRs, including the K-3, K-5, K-r, K-7, K-x, and K20D all have this feature, and you can access it via the "AF Fine Adjustment" screen in the custom function menu (for the K-x, you have to enter debug mode first). Unless your lens requires more adjustment than this feature provides, it should completely resolve any front-focus or back-focus issues that your lens may have. In case more drastic adjustments are needed, you should consider exchanging your lens or sending in both your camera and the lens for professional servicing/adjustments.
Before we look at how to adjust for front/back focus let's look at what may cause focus inaccuracy. Bear in mind that no auto focus system is 100% accurate 100% of the time, but that's another discussion which we will not get into in this article.
Keep in mind that lots of camera users are actually never be affected by front or back focus. Unless you're experiencing consistent autofocus inaccuracies with your lenses, you equipment is likely problem-free. We don't recommend making any adjustments unless your photos are out of focus.