Fixing Front and Back Focus

Causes of Front- and Back-Focus

A DSLR uses phase detect autofocus. If the camera has live view it may also feature (the much slower) contrast detect autofocus like a point and shoot camera (more on this later). The following discussion is pertinent to phase detect auto focus only.

Phase Detect Autofocus on a Pentax 645D

On a Pentax DSLR the phase detect autofocus module is placed in the bottom of the body. The main mirror is semi-transparent and lets some light pass through to the secondary mirror which reflects the light down to the auto focus module. For each auto focus point two images are formed by some tiny lenses in the AF module. Based on the distance between these two images the camera can calculate which auto focus point to use and how much to turn the focusing mechanism to achieve "correct" focus. Camera models differ in the algorithms that determine how many cycles of calculations to perform before calling it a day.

In any event the basic principle is that the camera calculates the necessary movement of the lens's focus mechanism and then performs the movement. The result is more or less accurate and depends on the calibration of the lens and of manufacturing tolerances in lens and camera. If a given lens consistently focuses either in front of or behind the subject we have front focus or back focus, respectively and we can dial a compensating value into the camera's algorithm specific to that lens. As an example the Pentax K-7 can store corrections for 20 lenses.

If all lenses exhibit either front-focus or back-focuses, the camera may be outside the manufacturing tolerances and calibration by a service center could be considered. It could be that the position of the auto focus module needs to be adjusted.

In rare cases it may not be possible to achieve correct focus because more adjustment is required than is available in the camera's AF adjustment menu. This could indicate that both lens and camera are outside of factory tolerances, and both should be sent in to a service center for calibration.

However, if your front or back focusing lens can be brought to focus correctly with the camera's AF adjustment the lens should not be considered defective. A recent poll on PentaxForums indicates that more than 70% of front/back focus issues could be corrected by in-camera adjustment.

What About Contrast Detect Auto Focus?

Contrast detect auto focus is available in live view mode of modern DSLRs, including all recent Pentax bodies. This system analyzes the contrast of the image formed on the image sensor and only works when the mirror is raised and the shutter open. Contrast detect adjusts focus continuously until maximum contrast is achieved in the image and is therefore not susceptible to front and back focus issues (it doesn't attempt to calculate (or predict) the right amount of focus adjustment but just continues adjusting until contrast is maximized). On the down side, it is much slower than phase detect, although we do expect to see significant improvements in this technology in the near future (driven by the rise in popularity of mirrorless cameras).