When we shoot images with a digital camera (DSLR or a compact point & shoot) leaving the camera to do the focusing (auto-focus on), often the AF mechanism goes haywire. A strong word (haywire) was necessary to describe the problem of focal shift (back focusing and front focusing) but it was necessary. Even a shift of + / – 2 can still create softer and thus unacceptable results for a professional photographer; although it may not mean that much to an amateur. The final result is softening of the pictures, shifting of the focal plane and unintentional focusing areas than what was focused on during composing.
Every digital camera has an auto-focusing mechanism that aligns the image properly on to the sensor. However the inherent nature of the digital cameras and lenses (which are manufactured via an automated production facility) makes for a slight aberration. This is the inability of the camera to accurately focus on a plane that is intended. This can be further explained with an example. Say you’re trying to shoot a portrait. You set your camera to auto-focus and then focus on the subject’s left eye. When the final shot comes out you notice that not the eye but the ears are the sharpest point of the picture! What just happened is that the inability of the camera and the lens combination to focus properly on the intended focus point was highlighted. Even if you used one of the AF points the actual focusing area is slightly bigger and if the left eye and the ear happen to have fallen on the same area (while both being on different plane) it can throw the AF off. This is true with varying degrees on most digital cameras and lenses, even the high end ones. The Canon 5D Mark III which has a significantly improved AF system, compared to the older model the 5D Mark II, also has this critical issue. It consistently produces softer images while severely back-focusing.
How to correct for back focusing or focal shift? It does not matter whether you’re using a semi-professional DSLR body or an entry level one, chances are that you will have to send the camera to the store for calibration. Since every piece of photography equipment is manufactured allowing for some tolerance, it means if the camera short focuses (or front focuses) by a margin of say +/- 2 or 3 (which let’s say falls within the degree of tolerance that the manufacturer has set for the equipment) manufacturers will not consider it as an error. For the users of professional DSLRs however there is a respite; some of these higher end DSLRs have an inbuilt focus correction function. It can help the photographer to adjust the problem of focal shift without having to send the camera to the shop.
Using Auto-Focus Micro Adjustment
This is a system that is built into the higher end Canon DSLRs such as the 5D Mark III and the older Mark II and the 1DX. AF fine tuning is present in Nikon professional bodies such as the D3 as well. It allows the photographer to use fine adjustments to correct the focal shift by fine tuning depending upon the amount of correction required.
Will this change AF performance consistently? Not really. Even if the calibration and or find tuning is done using built in or external tools, the camera will always have a problem to AF perfectly on a point which has more than one planes.