A common complaint seen on web photo forums is that cameras (and/or lenses) aren't achieving accurate focus. Sometimes the focus point is in front of the intended subject and sometimes it's behind it. Some of these observations may be due to technical problems with AF, but some are due to user error or unreasonable user expectations.
Of course no manufacturer of cameras or lenses claims that autofocus is absolutely perfect. For example, Canon state that the spec for focus on "consumer" bodies (such as the Digital Rebels and the EOS 20D/30D/40D) is that it should be within the DOF (Depth of Field). On the "pro" bodies (EOS 1 series) focus spec for f2.8 and faster lenses (f4 on some models) is that it should be within 1/3 of the DOF. With slower lenses the AF accuracy of the "pro" bodies reverts to the same as that of "consumer" bodies which don't have the high precision sensors.
So the bottom line is that focus should be within the DOF, or to put it another way, the image should look sharp. If it doesn't look sharp, focus is not likely to be in spec.
Note also that both the camera and lens are involving in focusing. A focusing error can be the result of either a camera or a lens calibration problem.
I've written this article so that users can test their DSLR under a set of "standard" conditions. The lens testing chart which is shown at reduced size below can be downloaded (as a Zipped JPEG) HERE
You can see two sets of parallel lines which are your focus indicators. The numbers 1, 2 and 3 represent 1cm, 2cm and 3cm distances when the chart is used at 45 degrees as explained below. In the center is a single vertical line, and that's your focus point.
To use the lens testing chart you set it up at 45 degrees to the axis of the lens as shown below.The easiest way to do this is to lay the chart flat on the floor, and put the camera on a tripod looking down at it at and angle of 45 degrees. In this configuration (at 45 degrees) the 1, 2 and 3cm marks are correct. They are actually spaced at 1.41, 2.82 and 4.23cm from the focus line, but when viewed at 45 degrees these distances are modified by the Cosine of the viewing angle (Cos 45 = 0.707).
Use is easy. Once you have everything setup you select your focus point (let's use the center point) and make sure the focus zone includes only the single focus target line. It's important to note that the area of the AF sensor may not be exactly the same as the markings on the viewfinder screen, so make sure there's nothing near the AF zone but the focus line. Then you take your shots. Take several and refocus each time. Try several manual focus shots also, using Live View with magnification if your camera has that ability. Use the maximum aperture of your lens so as to get minimum DOF.
Below is an example cropped from a frame taken with a EOS 10D using 50mm f1.8 lens at f1.8 and focused manually on the single vertical line. Focus distance was about 0.45m (I have the Mark I version of this lens which has a focus scale, the newer Mark II version does not).
This image displays accurate focus. The "1", "2" and "3" characters are pretty much equally blurred both in front of and behind the focus line. There's maybe a very slight bias towards the front, but on the next manual focus shot you might see an equal bias towards the rear. There's nothing wrong with the manual focus of this this camera or lens. The lines are spaced at 2mm intervals. 1, 2 and 3 represent 1cn. 2cm and 3cm distances in front of and behind the focus line.
The next shot shows the same view, but taken using autofocus.
Here you can see a slight bias in front of the focus line. The "1 cm" mark is sharper in front of the focus line than behind it, though the focus line itself is still quite sharp. I'd estimate that focus is maybe 2mm in front if the focus line, but the line itself is still within the "sharp" zone, maybe close to the rear limit of the DOF. For a 50mm lens at f1.8 focused at a distance of 0.45mm the calculated DOF is 5mm (+/- 2.5mm). So the focus isn't "perfect" but it's pretty good and within spec for autofocus. I'd suspect that such small focus offsets are pretty common but nobody ever notices them. Below is the same shot at f5.6 and the focus offset is undetectable to my eyes due to the increased depth of field at f5.6.