Digital cameras are technological marvels. Manufacturers are continually updating their products with new features. In the last few years, one of the features added to many digital SLR cameras is autofocus (AF) micro-adjustment. AF micro-adjustment allows the camera owner to make small—but noticeable—changes to the depth of field (also referred to as the depth of focus) for a given camera/lens combo. As tight as manufacturing tolerances may be, experienced photographers know that autofocus precision can vary from lens to lens, even within a given lens model. Manufacturers provide a service allowing camera owners to send in a lens for professional calibration, usually for a fee. Unfortunately, manufacturers sometimes ask that the photographer send in his camera along with the lens so the two can be calibrated as a pair. In rare cases, that is what is ultimately necessary. But thanks to the AF micro-adjustment feature, photographers can frequently make the necessary adjustments on their own.
A Suitable Target
To use the AF micro-adjustment feature you need an appropriate target. This could be just about anything but, ideally, you want to use a target that has high contrast so the camera's autofocus lock will be reliable and repeatable. In addition, you want a target that allows you to measure the depth of focus in the image. The Datacolor SpyderLensCal is one such target.
The SpyderLensCal features a proper contrasting target, an angled rule (to measure the depth of focus), a bullseye bubble level, and a standard 1/4-20 tripod socket. It is constructed of rigid and reasonably strong plastic. The focus target and the marked rule are quality paper stickers attached directly to the plastic. Using the SpyderLensCal is easy but you need to follow a few basic steps to ensure good results.
On The Level
Throughout the autofocus testing, you want to be sure that both the SpyderLensCal and your camera are level. Thanks to the bullseye bubble level on the SpyderLensCal, this is an easy process. I used a light stand with an adjustable swivel to support the SpyderLensCal, but you can also use a tripod with a ball head or just use a level tabletop.
For ease of positioning and stability, it's best to mount the camera to a tripod with a ball head. To level the camera, I recommend using a two-axis hot shoe bubble level.
Once both the SpyderLensCal and the camera are perfectly level, you need to adjust the distance between the lens and the SpyderLensCal. Datacolor recommends a distance of 5 to 10 times the focal length of the lens. If it is a zoom lens, Datacolor recommends you use the longest zoom position. Using a 24-105mm lens as an example, you would set the lens to 105mm and calculate the distance. 105mm x 5 = 525mm, or about 20.7 inches. So, you would position the lens between 20.7 to 41.4 inches from the SpyderLensCal. Also, it's a good idea to position the target in a spot with good lighting. I chose a spot next to a window.
The next step is to raise or lower either the tripod or light stand so the center focus point in the camera's viewfinder falls on the smaller contrasting squares on the SpyderLensCal (next to the zero line on the rule). You also want to be sure that the back of the SpyderLensCal and the plane of the camera's image sensor are parallel. However, since the appropriate focus point is right next to the rule and the lens is relatively close to the target, this is not as critical as it would be with a larger target at a greater distance.
Note: The settings menu structure varies from camera to camera. Please refer to your camera's user manual. The example screens shown here are for a Canon 5D Mark II.
To ensure the sharpest images, I recommend turning on the camera's mirror lockup feature.
Next, check the AF micro-adjustment setting to be sure it is enabled and set to "Adjust by lens". Start with the adjustment set to zero (0).
Ready To Shoot
Now it's time to take the first test photo. To help eliminate vibration, I recommend using a cable release or the camera's self-timer.
You can attempt to review the image on the camera's rear LCD screen, but I recommend viewing the image on your computer. Look at the rule to check where focus begins and ends. As examples, here are two image crops demonstrating the AF micro-adjustment range from -20 (front focus) to +20 (back focus).
Ideally, you want the center of the rule (0) to be in the center of the depth of focus. Here's a crop of the first test image, shot with the 24-105mm lens and AF micro-adjustment set to zero (0).
My 24-105mm wasn't off by much, but it was front-focusing slightly. Click the image for a better look. The forward .5 mark was in better focus than the rear .5 mark. The AF micro-adjustment setting needed to be moved in the backwards direction. After some experimentation with different settings, I decided an AF micro-adjustment of +2 was about right.
Use Photoshop's Emboss Filter
Tip: You can use the emboss filter in Photoshop to improve the visualization of the in-focus and out-of-focus areas of your test shots. Here are those two 24-105mm lens crops again with the emboss filter applied.
After you've repeated the process for each lens using the appropriate distances (also being certain that you keep the SpyderLensCal and camera level and square), you're all done. The camera will remember the AF micro-adjustment setting for each lens and set it automatically when you attach the lens.
Mine All Needed Some Adjustment
I have four lenses. Each of them needed a slightly different amount AF micro-adjustment. My 17-40mm lens was back-focusing slightly and needed a setting of -2. My 100-400mm lens needed the least amount of AF micro-adjustment, which is not surprising because I sent the 100-400mm to Canon for professional calibration about two years ago. The setting for the 100-400 ended up at -1. Lastly, my 100mm macro lens needed the most AF micro-adjustment. I've been suspicious that it was front-focusing, which was occasionally noticeable when I used it for portrait photography. Sometimes I'd have a nose in focus with an eye that was slightly out of focus, yet, I was aiming at the eye. Sure enough, the testing proved the 100mm macro lens was front-focusing and needed an AF micro-adjustment of +5.
Datacolor provides a really sparse set of instructions with the SpyderLensCal. It was next to worthless. Fortunately, I found much more thorough documentation on Datacolor's European site in the form of a PDF file: SpyderLensCal User's Guide
I'm happy with the results I obtained by using the SpyderLensCal focus target. I've fiddled with homemade focus targets in the past with mixed results. My results with the SpyderLensCal were consistent and easily repeatable. The SpyderLensCal costs more than a homemade target, but as digital camera accessories go, it's not exactly expensive. The included tripod socket and bullseye bubble level make for a nice package.
The Datacolor SpyderLensCal earns my recommendation for those wishing to take advantage of the AF micro-adjustment feature available on their digital SLR. If you found this review useful and are interested in purchasing the SpyderLensCal, please consider supporting Mark's Hangout by using the Amazon link below. Thank you.
Update: Datacolor has informed me that they are updating the documentation to be included with some of their products. The process is taking longer than they expected. In the meantime, just use the PDF file I linked above.