Modern lenses are small miracles. They don’t defy the laws of physics, but they sure come close. Are you getting all you can out of your lens, though?
Depth of Field, or Lack Thereof
If you’re like most of us, you probably enjoy opening your lens to its widest aperture sometimes for that razor-thin, subject-isolating depth of field and that smooth bokeh. Of course, with that thin depth of field comes a thin margin of error for focus. If you’ve been careful about your focus points, you have good light and your technique is sound, but you still have a lot of out of focus shots, your lens and body might need an autofocus microadjustment.
Often, a camera body and lens will focus precisely, but not accurately. This means there is a consistent error in which the lens focuses too far forward of the subject or too far behind (i.e., the lens front focuses or back focuses). The key that allows us to correct for this error is that it is consistent; if we can measure its size and which direction it falls in, we can compensate for it by applying an equal and opposite correction. This becomes particularly important at wide apertures, where depth of field is so small that an error of a few centimeters in either direction can be the difference between making a shot and making a mess. Autofocus microadjustment (AFMA) allows us to make that correction by programming your camera body to consistently apply a compensation whenever that lens is attached.
Which Lenses Benefit
Not all lenses need AFMA. If you’re using a lens with a maximum aperture of less than f/2.8 or you never open up wider than that, the error any modern camera and lens makes will not exceed the range of your depth of field. On the other hand, if you frequently work at wider apertures, your lens may well benefit from an adjustment. However, it takes great care to make these modifications properly, lest a greater error might be introduced.
The first thing to know about autofocus microadjustment is that not all cameras support it. Supported Canon cameras include: 1D Mark III, 1Ds Mark III, 1D Mark IV, 1D X, 5D Mark II, 5D Mark III, 6D, 7D, 7D Mark II, 50D, and 70D. Supported Nikon cameras include: D3s, D3, D3X, D4, D4S, Df, D800, D800E, D810, D300, D300s, D600, D610, D700, D750, D7000, and D7100. If your Canon or Nikon camera is not on this list, there are no options for this sort of adjustment. There are two ways to go about performing an autofocus microadjustment: by hand or with the help of software designed for the task.
The DIY Method
To calibrate by hand, you’re going to need either a specially printed test chart (LensAlign makes excellent charts), or a ruler, or something similar with regularly spaced segments. You’ll also need strong lighting on the target (they don’t need to be specialty photographic lights, just make sure there’s a lot of light hitting it) and a lot of working distance. Next, you’ll need to create about a 45-degree angle between the focal plane of the camera and the test chart. This can be done either by laying the test chart flat and using a tripod to aim the camera, or by placing the camera parallel to the ground and the test chart at a 45-degree angle. If the angle is too large, you won’t be able to distinguish the focus between segments and if the angle is too small, you’ll have trouble visually separating the segments. The camera should be about 25 times the focal length of the lens away from the test chart. For example, if you’re using a 85mm lens, you’ll want the camera to be 85*25 = 2,125 millimeters, or approximately 7 feet away from the test chart, as shown above. Keep in mind that this distance is the line of sight, not the distance along the ground. Aim the center AF point at the center of the test chart, or at the middle of the ruler. If you’re using a zoom lens, use the telephoto end to begin.
If you’re using a Canon camera, set it to either remote shutter or the two second timer and use mirror lockup. Enter the C. Fn menu and select “AF Microadjustment,” then “Adjust by lens.” If you’re using a Nikon, set the shutter and mirror lockup similarly, then enter the setup menu and turn “AF Fine Tune” to on. Be sure you're working at or near maximum aperture.
Once you’ve done this, change the AFMA to a large negative value (-15 is a good starting place) and take several shots to offset random errors (remember, we’re tweaking accuracy, not precision). Increase the adjustment to -10 and do the same, continuing until you reach +15. Using the zoom function in playback (or a monitor), examine the shots to see which value shows the segment under the center AF point to be most in focus. Once you’ve determined this, repeat the procedure by narrowing the range you test; for example, if -5 seemed the sharpest, test again from -10 to 0, this time by single values. Repeat the examination to find the value that shows the center of the frame most in focus. Save this value as the adjustment for that specific lens. Whenever you attach that lens to your camera from now on, the camera will know to compensate by the tested amount. It’s rare that a lens will need more than 10 points in either direction. Some more advanced bodies will allow a zoom lens to calibrated separately at the telephoto and wide ends; if so, repeat the test at the wide end of the lens.
If all of this seems to be a bit too much work, there is also excellent software that can help automate the task for you. Check out Austin Rogers’ review of Reikan FoCal, which I also use myself. It can not only help with the AFMA process, but can also offer some very interesting statistics about your lens, such as its sharpest aperture.
Finally, if you’re using a lens heavily, or it experiences extreme temperature differentials across seasons, consider checking and possibly recalibrating it every year or so.
Making autofocus microadjustments can be a time-intensive task, but if you routinely shoot at narrow depths of field, it can help ensure that you’re getting the best possible performance out of your body and lens.