Calibrating your camera’s autofocus system for best performance with individual lenses
By Ellis Vener
Every now and then a specific autofocus lens and body combination just do not work perfectly. You may not even be aware that one or more of your lens/body combinations do not perform as well as it might if it were properly tuned, or maybe you require above average performance and want assurance that is what you are getting for your money. Several manufacturers now include a function for fine-tuning autofocus performance in their newest top-of-the-line bodies. (As of early August 2009 these include: Canon EOS-50D, 5D Mark II, 1D Mark III, and 1Ds Mark III models; Nikon D300, D300s, D700, D3, and D3X; Olympus E-30 and E-620; Pentax K20D; and Sony A900.) If your camera has that option, we now have a good reliable tool from rawworkflow.com for finding out exactly how out of whack a lens /body combo might be and what AF micro-adjustment setting is needed to maximize performance.
The LensAlign PRO kit consists of three pieces: a target to focus on, a steel Depth of Field Display Ruler, and a settings Enumerator to keep track of your settings in the photo. There are two other versions available as well; a Lite version, which is less fully featured, and the PRO PLUS, which features a larger target and a 47-inch long ruler.
Image ©Ellis Vener
The main component of the LensAlign PRO kit consists of the focusing target, a smaller target behind that to make sure the sensor and the target are in perfect alignment, and the holder for the ruler. The ruler can be positioned at five different angles from near vertical (useful for close-up photography where tolerances are very tight) to nearly flat (useful for longer distances), but start with the ruler tilted to the middle position. The ruler itself is 9.5 inches long and is marked in different sized numbers.
The enumerator on the front of the target allows you to note the camera-to-target distance expressed in magnitudes of focal length. For general use, 25x the lens focal length as the target-to-camera distance is good, but if you know that you’ll be working at closer distances, do the math and calculate the magnification factor: distance in mm / focal length = magnification. The upper half of the enumerator lets you keep track of AF adjustment settings from -20 to +20.
With the camera and lens and LensAlign unit mounted on good tripods, setting up and aligning the LensAlign PRO is easy. I used either undiffused electronic flash or bright daylight as a light source. Once attached to the tripod, slide up the “site gate” on the rear of the focusing target and peer through the center hole in the rear panel until you can see the center of your lens. Move back to the camera and carefully adjust the camera’s position on its tripod until you see the red alignment target through the central hole in the focus target. Once aligned, close the site gate and make sure the camera is in Single AF mode and set to use the center AF sensor.
With your lens wide open, shoot your first test frame with the camera set to no AF adjustment. Download the file (or shoot tethered) and then open the file at 100% magnification on your computer. If there is an error in any direction, do more shots starting with your camera's most extreme AF adjustment in one direction and moving back toward zero adjustment, being careful to make the adjustment on the camera and to note it with the Enumerator. Once you have found a camera’s ideal AF adjustment setting for a specific lens, program it into the camera’s database, and from then on your camera will automatically use that setting whenever you use that lens. Repeat with each lens. Plan on about five minutes per lens.
Zooms are understandably a bit more complicated. You should test a zoom lens at both ends of the range and in the middle of the range. If the lens is off to varying degrees at different focal length settings, you can use the resulting samples to document the problems when you communicate with the lens maker’s repair center.
The beauty of the LensAlign PRO is that it is a simple, reliable and repeatable way of verifying that you are getting all of the image quality your cameras and lenses can deliver. To make the testing process more efficient, have an assistant or buddy change the enumerator settings while while you work with the camera.
Given the price of the LensAlign PRO, it might make sense for a few photographers to go in together and buy one as a shared asset, but if your organization has multiple cameras and lenses, multiple photographers, or regularly rents lenses or extra bodies, and you want to be in a position of actually knowing that you are giving your clients the best possible results, owning or at least having access to one makes sense.
LensAlign PRO $179
LensAlign PRO Plus $249
LensAlign LITE $79.95
Long Ruler Kit $99