|Using a tool like the LensAlign, you can perfectly calibrate all of your DSLRs and lenses.|
Most photographers take it for granted that their lenses and their cameras are in perfect alignment with one another, and that they'll perform with complete accuracy. With shorter focal lengths, the tolerances are such that any misalignment will be disguised, but with longer focal lengths, problems are more pronounced, and soft images will be the result.
The fact is, perfect alignment out of the box isn't as likely as you'd think. Even brand-new pro-series lenses can show some imperfections. Fortunately, these imperfections are easily corrected with some simple adjustments made through the camera's menu system if you have a higher-end DSLR.
You can test and correct for misaligned lenses with a simple homemade setup, but there are commercially made systems that are inexpensive, easy to use and very effective. The LensAlign is a favorite for many pros due to its accuracy and the way its two-part target works. The LensAlign has a target that's parallel to the image plane (the sensor) and a ruler that runs at an angle. Your DSLR focuses on the target and you shoot a photo, then you review the photo to confirm the lens alignment.
The trick with any lens alignment testing is in making sure all of the elements in the system are properly oriented with each other. The target and the image plane need to be exactly parallel to each other. Here's another area where the commercially available alignment tools have an advantage. Using the LensAlign as an example again, it has what the company calls the True Parallel Alignment feature.
With the target and the measuring tool set up, position the camera at a normal shooting distance for the lens. Some manufacturers suggest 50x the focal length. A good rule of thumb is approximately 25x the focal length, and if your lens is a zoom, do everything from the maximum focal length. For a 70-200mm, try racking out the lens to 200mm and set the target about 16 to 17 feet away (25x200mm = 5000mm, which is just under 16.5 feet). Everything should be firmly mounted on tripods. Set your lens at its sharpest aperture, or just set ƒ/8 if you aren't sure where the lens is sharpest. Photograph the target. When you review the image, you can do it on the DSLR's LCD, but it's best to do it on a larger screen. After all, we're going for precision here.
When you're looking at the image on your computer screen, the target should be sharp. If it's not, this usually means the camera didn't properly lock onto the target for some reason. Try shooting again. If the target is sharp, you'll notice one of three possibilities with the ruler component: 1) The zero is tack-sharp, and the focus trails off evenly in front and behind it, indicating that your system is perfectly aligned; 2) The zero is sharp, but the focus trails off more abruptly in front than behind the zero and 3) The zero is sharp, but the focus trails off more abruptly behind than in front. Either of the latter situations indicate you need to make an adjustment.
Higher-end cameras allow you to adjust the back focus to correct for the problem. In the menu, you'll find a custom function that controls the micro-adjustment of the focus. This varies depending on the camera. Check your camera's manual for specifics. Not all DSLRs allow for this kind of adjustment, but most OP readers are likely to have models that offer micro-adjustments.
It's a good idea to repeat this procedure with all of your lenses and bodies. Once you've gone through the test procedure with a camera and lens, it will be learned by the DSLR, and when you attach that lens to the camera in the future, the adjustment will be set automatically. However, this is true with a lens model, not a specific model. In other words, if you rent a 400mm ƒ/4 for a special trip and you test the lens, the adjustments you make will take effect anytime that 400mm is attached to your camera during the trip. If you rent a different 400mm ƒ/4 later, your DSLR will make the adjustment from the previous 400mm. In the case of rentals, make sure you test and dial in the adjustments with each lens.