DSLR autofocus has been the Gold standard for decades but the higher accuracy and precision offered by some mirrorless cameras risks tarnishing this image. However, many modern DSLRs include an option to fine-tune the autofocus behavior to help optimize their performance. Guest writer Joey Miller has written a short guide to how to make use of this feature, over on the LensRentals blog.

You don't necessarily need any specialist equipment to fine tune your lenses. But if you're going to go to all this effort, it might be worth it. Photo: Joey Miller

The article builds on the work Roger Cicala has already done, looking at the reasons that fine tuning is needed, with one of the main reasons being to cancel-out the effect of the combined tolerances of your camera body interacting with the combined tolerances of the specific copy of the lens you're using.

As we reach higher pixel counts, this imprecision is being highlighted in ever more detail (it was always there, but your camera wasn't letting you examine the problem in such fine detail).

Miller uses a Canon setup as an example, with up to two corrections per lens being possible (a 'Wide' and 'Tele' value being available for zoom lenses). But even this is a rather blunt instrument when it comes to achieving perfect accuracy. Given the variation we encounter using off-center focus points, a more complete solution would require something more like the Olympus system for Four Thirds lenses, which allowed two values per lens, per focus point. The best correction value can also change with subject distance, which is why Sigma's USB dock offers the ability to set four different values for four different subject distances.

Even if such control over calibration were possible for the end-user, it would be so arduous as to be nearly impossible. Products such as Reikan FoCal can help, but it's still fairly involved, and the situation-to-situation, day-to-day variability we've noted with some systems means even these don't completely solve the problem. Thankfully, the process looks as if it's about to be made simpler, with Nikon's D5 and D500 gaining something we've been proposing for several years now: an automated fine tune system that checks the results of its contrast-detect AF in live view mode to calculate the corrections needed to fine tune its secondary sensor phase-detection system. It's rather rudimentary in that only one value can be entered for any lens and body combination, but it's certainly a step in the right direction.