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Focus test chart
Copyright Tim Jackson 2004
Version 2.1 - (24 June 2004)
Download and print out this document (it's a PDF file).
Download Focus Test Chart (PDF)
Focus Testing
by Bob Atkins
Again I'm not saying there isn't a problem with some 10D bodies, but it's yet to be determined how widespread the problem really is.
Please note that focus accuracy isn't specified to be - and probably never is - absolutely perfect. From past statements by Canon it seems that the spec for focus on "consumer" bodies (and the 10D is a $1500 "consumer" body) is that focus is within the DOF. On the "pro" bodies focus spec is within 1/3 of the DOF because they use higher precision AF sensors which require faster lenses. With slower lenses (slower than f2.8 or f4, depending on the body), AF accuracy of the "pro" bodies reverts to the same as that of "consumer" bodies which don't have the high precision sensors. I don't know a reference where Canon state this in print, but I'll add one if anyone can provide one.
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.
The Chart
I've written this article so that users can test their 10D under a set of "standard" conditions and compare results - or at least see if their 10D falls outside acceptable limits for focus accuracy. First the test chart which is shown at reduced size below. The full size chart can be downloaded from http://www.photo.net/learn/focustest/scale45.jpg
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.
The Test
To use the chart you set it up at 45 degrees to the axis of the lens as shown below. 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. Then you take your shots. Take several and refocus each time. Try several manual focus shots. Use the maximum aperture of your lens so as to get minimum DOF.
The Results
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 pretty much perfect 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 3mm 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. This 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.
The Conclusion
Maybe I'm just lucky, but my 10D seems to be OK with respect to focus. This bears out field results so far which have yielded sharp images in all circumstances where they can reasonably be expected using a variety of lenses from consumer zooms to "L" series telephotos. Subject movement, slow shutter speeds with a handheld camera and choosing the wrong focus point can all lead to out of focus images of course, but they are not the fault of the camera or the lens.
EF 28-135 IS lens and 10D. 135mm @ f5.6, focus on frog's eyeball.
You can add comments or examples of your own tests via the comment link below. If you post images, please crop them down and/or reduce the size. A maximum width of 500 pixels would be good and should allow them to be displayed in-line. Larger images will just appear as a link.
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