Shakedown: AF Fine Tuning
About a year ago I heard about AF fine tuning for the first time and passed over it without much interest. I had never experienced any of the issues that proponents of it kept recounting in forums, so I thought I was safe ignoring it. Well, after assembling a good range of glass and having not too long ago upgraded to a Nikon D7000, I started to notice that sometimes things were a bit…off.
So first of all, what does it mean for a shot to be “in focus”? Well, say you have your Nikon camera set to AF-S, with the single center focus point selected. You line up a shot and go to hit the shutter release. Halfway through the press of the shutter release, the AF kicks in and focuses the lens on the object at the center focus point. As you completely depress the shutter release, the shot is taken. Looking at the shot, it is “in focus” if the subject you were focusing on, specifically the area covered by the center focus point, is sharp.
AF fine tuning comes into play due to the inherently imperfect nature of the manufacturing of lenses and camera bodies. While the design and manufacturing of lenses and camera bodies are incredibly precise in nature, fault tolerances are required to account for slight imperfections in the actual construction of a lens or camera body. For example, say a lens is constructed to be perfectly in focus at 0 (some imaginary measurement), but a fault tolerance of +/- 1 is allowed, so lenses with -1, -0.5, 0, 0.5, 1 are all acceptable. Now apply the same fault tolerances and acceptable values to a camera body. While each component can be off slightly, the kicker comes when combining the two together. These fault tolerances in combination can start to easily drift away from 0 and become noticeably unsharp.
AF fine tuning is a setting on the Nikon D7000 (and numerous other Nikon models; I believe it’s called something else for Canons) whereby you can save a “focus offset” for each lens to be used with the camera body. This offset value is used to shift the calculated focus to account for the faults unique to that particular lens and camera body combination.
Here’s a quick and dirty rundown of how to actually test your lenses and figure out what you need to dial into the AF fine tuning menu. Grab your camera, a lens, and a tripod; set up your camera facing straight down so that the sensor is parallel to the floor. Now, find a flat object with some detail in it (I used my X-ACTO cutting mat); any printed page with some text will work. Place the object on the floor and put the camera in (M)anual exposure mode. Dial in a good exposure (I stopped down every lens to f/4 to make it uniform and also ensure I wasn’t wide open on any lens) and leave AF on. For my tests, I shot tethered to make reviewing the shots easier.
Now, go into the Setup menu (wrench icon) and go down to “AF fine tune”. In that sub-menu, turn “AF fine tune” ON. Then go to the “Saved value” and dial in -20. Take a shot. Go back into the “Saved value” menu and dial in -15. Take a shot. Rinse and repeat until you get to +20 (by the way, I’m incrementing by 5 here, but you can do whatever interval you want). This will leave you with 9 final shots to compare sharpness in. Whichever one is sharpest becomes the last value you dial into “Saved value”. Exit the menu and now whenever you connect THAT lens to THAT body, it will automatically pull up this value and apply it while shooting. Perform the same process for every lens you have. If you have another camera body, repeat everything for that body.
I was surprised to find out how horribly off most of my lenses were. Keep in mind 0 is the default; in other words what my lenses were set to all along before going through this exercise. Here are my results, now get out there and figure out your own!
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