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Auto focus accuracy

When we reviewed the Canon EOS 300D (Digital Rebel) I added this page which attempts to check the accuracy of auto focus both of the camera and lens used. This test is designed to check for front or back focusing defined as:

Where the camera indicates an auto focus lock but the actual focus position produced is slightly in front or slightly behind the actual ideal focus position. This can be caused by errors in the camera or a problem with the lens.

Our test


AF Test Target
For this test we are using a variation of various 45 degree tests seen elsewhere. Ours uses a simple rule print with a focusing target placed at 45 degrees from the rule and facing the camera directly. This target was designed to be easy for the camera to pick out as well as being large enough (3 x 2 cm) not to allow the AF sensors to pick up background detail. The camera is placed at exactly 45 degrees from the chart (facing the AF target) at a distance of 0.5 m (from the AF target to the focal plane).

The chart is then shot at both wide angle and telephoto at a large aperture (to keep the DOF as small as possible) with center point AF selected. It is then possible to examine the image and see whether the AF target is in focus, it is also possible to see the DOF on this chart. Each small line on the ruler is 1 mm, larger line is 10 mm. The crops below are from a 50% downsized image (to make more of the chart visible).

Canon EF-S 17 - 85 mm F4.5 - F5.6 IS USM

Wide angle, at 17 mm, F4.5: Good
Telephoto, at 85 mm, F5.6: Good (slight back focus)

Canon EF 24 - 70 mm F2.8 L USM

Wide angle, at 24 mm, F2.8: Good (slight back focus)
Telephoto, at 70 mm, F2.8: Good

Canon EF 50 mm F1.4 USM

At 50 mm, F2.8: Good
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