Several dSLR cameras have an AF Fine Tune capability, but it's not clear to most people how to best use this feature. I've seen many explanations…most involve using a special target mounted at an angle so that only a small part is in focus and then adjusting the camera via its AF Fine Tune feature so that the correct part is in focus. One problem with this approach is that the "special target" is normally quite small, thus the focus distance is close. The AF system can work quite differently at close and far distances, so this method can have problems…you adjust the AF system at a close distance but then use the camera + lens at much longer distances (and still have OOF issues).

Any camera (that I'm aware of) that has an AF Fine Tune feature also has a Live View feature. LV can be used to check the focus accuracy of the "normal" phase AF system and provide data to correct any focus errors with AF Fine Tune.

First a short description of the two types of AF systems in use.

All dSLRs use a Phase AF system (even those w/o LV). dSLRs w/ LV also have a Contrast AF system. The Phase AF system is "open loop" and quite fast. The Contrast AF system is "closed loop" and quite slow (at least as implemented in dSLRs circa 2010…Contrast AF is also used in non-dSLR cameras and some of them are quite fast…but that's another story). Being "closed loop", Contrast AF is very accurate…independent of focal length, focus distance, aperture, etc. However, Phase AF, being "open loop" can have substantial errors and these errors are variable…ie, one time it can work well and the next time it's off noticeably. With some lenses (at specific focal lengths), in spite of the variability in the whole Phase AF system, there may be an overall tendency to focus in front of or in back of the subject. It's this overall tendency that AF Fine Tune is designed to correct. There will still be variability because Phase AF is an "open loop" system.

["open loop" means that there is no feedback to tell the system if it actually did what was commanded.]

Since we have two separate AF systems, we can use the accurate/slow one to tell us how far off the inaccurate/fast one is. Here's how…

I'll use my Nikon D300 as a model…your camera may be different. Place the camera/lens on a tripod and position a subject at a "normal" distance…a distance that is typical for your pictures. For most photographers, this will be something around 10-15 feet. Be sure that the subject is accurately positioned in the center of the frame of view and that the camera is using Spot focusing. Be sure that the subject has high contrast and both vertical and horizontal features. For example, print out a big "+"…about 2" high… on a sheet of paper and use this as the subject…stick or pin it on a wall so that it is perpendicular to the lens axis. Also provide adequate light…usually EV 10 is enough.

Place the camera in LV mode and focus on the subject/target. With my D300, after rotating the Release Mode Dial to "LV" it's necessary to activate the Live View mode by fully pressing the shutter button. Focus is done by either holding a half-press of the shutter button or pressing and holding the "AF-ON" button until the focus rectangle turns green.













Most lenses have a focus distance scale visible on the top. If your lens doesn't have this feature, you won't be able to use this method. Sorry.
















Place the camera in non-LV mode. On my D300, this involves rotating the Release Mode Dial to "S".
















Be sure the camera is in single AF focus mode.

















Watch the focus distance scale as you do a "normal" Phase AF operation (generally this is done by half-pressing the shutter button). If the distance changes, there is an error in the Phase AF system. It's wise to do this whole sequence several times, because the Phase AF system has some variability. You will have to average the results.

If you have an overall Phase AF error, then attempt to correct it by changing the AF Fine Tune variable…then repeat the above dual AF sequence and AF Fine Tune adjustment until the distance doesn't change when doing the non-LV focus. Again, it will be necessary to do this procedure several times and average the results; Phase AF has a basic variability…it won't focus the same every time.

At the current time, cameras don't allow you to correct Phase AF errors at more than one focal length. With a zoom lens, you will have to select one focal length at which to perform the correction. If you use a wide range of focal lengths on a lens, then it makes sense to pick a central focal length and take your chances on the ends. This decision is most problematic with extremely wide ratio zoom lenses, like the popular 18-200+mm lenses. With narrow ratio zoom lenses, like a 24-70mm lens, it's a simple decision to perform the correction at 50mm and expect good correction at the extremes. But with an 18-270mm lens, you may have to pick either the wide angle or telephoto end to be corrected.