Ideally, every camera and lens would be perfectly calibrated when it comes off the production line. In reality, that doesn’t always happen. Even the most careful manufacturing process has some level of error tolerance, and in practice, some lens and camera combinations are better calibrated than others.
By the time it gets into your hands to take photos, that means that the combination of the lens and camera you have might not be producing the sharpest photos it’s capable of.
Autofocus systems are particularly prone to it. Ideally, the autofocus system should work with a given combination of lens and camera to produce a spot-on focus point. But in many cases, there can be an ever-so-slight misalignment, making the autofocus settle on a focus point that’s slightly behind the one you want (back focus) or slightly in front (front focus).
It’s not a new problem. It has always been a potential shortcoming of autofocus systems. But modern cameras produce exceptionally high-resolution images, so it’s more noticeable than ever.
Recognizing this, some camera manufacturers have started building autofocus calibration tools into many of the new higher-end DSLRs and interchangeable lens cameras. In general, the calibration applies to a specific lens, so you can save different settings for different lenses.
But the function isn’t automatic–you basically have to dial in the calibration settings. To know what settings to dial in, you need to visually test the camera. There are several ways to do that.
The simplest and most reliable for fine-grained accuracy is to use a calibration board built specifically for the purpose, like the Datacolor SpyderLensCal.
Basically, this is a plastic board that folds out into a 3-dimensional unit.
The baseboard includes a bubble level and a tripod mount.
A backboard folds up vertically at 90 degrees to the baseboard. This forms the focusing target. And a ruler runs at 45 degrees from the baseboard, past the focusing target, to provide a visual measure of whether the system is front focusing, back focusing, or is accurate.
Using it is straightforward. The short version is that you take photos of the board and adjust the camera’s AF calibration settings based on whether the sharp focus aligns exactly with the focus point or is in front or behind it. You then take another photo and zero in on the perfect settings through trial and error. You can find the detailed instructions here [PDF].
A camera/lens combination that’s back-focusing will look something like this:
And front-focusing will look more like this:
Corrected, it looks more like this:
There are some ways to get better and more consistent results. You don’t need to use it in a studio, but bright, even lighting helps ensure that the autofocus is locking consistently. Ideally, you want the focus board to be exactly level with the center of the lens. Using a tripod will help with consistency and make the process quicker. And you’ll want to be shooting at a distance that makes sense for whatever focal length the lens is. You don’t want to be using a 50mm lens from 50 feet away or a telephoto from 2 feet away.
The Datacolor SpyderLensCal is purpose-built and works well, but for what is basically a few pieces of plastic, it’s not inexpensive.
There’s no real magic needed here, and it is entirely possible to whip up a DIY version. That can be anything from a ruler resting on a box on a block of wood, a chart pinned on the wall of an outside corner, to something you construct yourself.
In general, you want to make sure you’re using a high contrast target to ensure that the autofocus system is locking reliably and consistently, and you want to make sure that the backboard is square on to the camera. The measuring ruler doesn’t have to have specific intervals–that’s just providing a visual guide, and there’s no reason you can’t use a regular ruler. For best results, you’ll want it as close to 45 degrees as practical, although even that isn’t critical.
In the Field
As neatly as the Datacolor SpyderLensCal folds up, it’s not something I feel the need to carry with me all the time and allocate precious space in my camera bag to it. But from time to time, I find myself in the field needing to calibrate a lens I haven’t used before.
There are quick and dirty ways you can accomplish the same thing. They’re not as polished and might require a bit more trial and error, but they can get the job done.
Basically, you want a flat vertical surface with high contrast, sharp detail on it. I find surfaces with text work well. It might be a bookshelf lined with books, a wall with a text inscription, or even something like a few breakfast cereal boxes on a table. You want to position the camera at about 45 degrees, choose a letter or word as the focus point, and adjust the calibration by whether it’s back-focusing or front-focusing.
If Your Camera Doesn’t Have an AF Calibration Function
Not all cameras–or even all DSLRs–have an AF calibration function. And if your camera doesn’t, there’s not a lot you can do here.
If you find that the autofocus and consistently and egregiously misaligned, it could be the lens, or it could be the camera (or both, for that matter), and there’s not much you can do aside from sending one or the other back for a replacement in the hope of getting a better copy. To try to narrow down whether it’s the lens or the camera, the obvious first step is to try a different lens.