Focusing 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).

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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. Each crop is produced by reducing the image by 50%* (to allow more of the image to be visible).

* Except the 18 mm crop which would have been too small if reduced

Nikkor AF-S DX 18- 70 mm F3.5 - F4.5G

At 18 mm (F3.5): Too small to tell, possible back focus *(not reduced in size)*
At 55 mm (F5.6): Slight back focus

Nikkor AF-S 28-70 mm F2.8 ED

At 28 mm (F2.8): Good (DOF more to the back)
At 70 mm (F2.8): Good

Nikkor 50 mm F1.4 D

At 50 mm (F2.8): Back focus

Nikkor 50 mm F1.4 D (Manual Focus)

At 50 mm (F2.8): Back focus

The results were interesting but not unsurprising, I had noticed the 18-70 mm back focus while taking some test shots and had switched to manual focus for some of them. The 50 mm back focus was more surprising although generally I use this lens at small apertures (F9 or smaller) so this amount of back focus would be soaked up by the depth of field. As I would also expect the 28-70 mm F2.8 performed the best, clearly showing that this isn't a necessarily camera problem.

Other lens tests (on the AF-S DX 18-70 mm)

We wouldn't normally perform these tests in our digital SLR reviews but because the new AF-S DX 18-70 mm is being bundled with the camera and it is likely to become the primary lens for many D70 owners we have chosen to test some other lens characteristics.

Macro Focus

The AF-S DX 18-70 mm isn't and doesn't really pretend to be a macro lens, its closest focus distance of 38 cm (15 in) produces a frame coverage of 133 x 88 mm. Obviously there are a wide range of dedicated macro lenses available for the Nikon lens mount. Each grid line on our macro test chart is 10 mm, the shot is taken at shortest subject distance in macro mode (if available).

Wide angle (18 mm) - 353 x 235 mm coverage
9 px/mm (216 px/in)
Distortion: High
Corner softness: n/a
Telephoto (55 mm) - 133 x 88 mm coverage
23 px/mm (572 px/in)
Distortion: None
Corner softness: None

Barrel and Pincushion Distortion

The AF-S DX 18-70 mm lens exhibited 1.4% barrel distortion at full wide angle, slightly more than I had expected although not that surprising considering the focal length at moderate cost of the lens. At telephoto pincushion distortion was very low at 0.3%, so low that it would simply not be visible in normal shots.

Wide angle (18 mm)
1.4% Barrel Distortion

Telephoto (70 mm)
0.3% Pincushion Distortion

Vignetting / Lens Shading

At full wide angle and maximum aperture (18 mm / F3.5) the AF-S DX 18-70 mm lens exhibited a surprising amount of lens shading although this was tamed by stopping down the aperture, we would consider the amount of fall off seen at maximum aperture as noticeable.

Our vignetting measurement is made by taking the average luminance value of the darkest corner of the image and comparing it to the average luminance at the center of the image, any difference greater than 15% may be visible in everyday shots.

Wide angle (18 mm) Telephoto (70 mm)
Wide angle, F3.5 (wide open)
38% maximum fall off, bottom right corner
Telephoto, F4.5 (wide open)
28% maximum fall off, top right corner
Wide angle, F8
17% maximum fall off, bottom right corner
Telephoto, F8
9% maximum fall off, top right corner