For January's monthly project, Tom Persinger is joining us again to examine how depth of field and experimenting with point of focus can add interest to your photography. Please add your photo to the...
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.
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
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 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.
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
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
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
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.