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Testing autofocus

Is your lens front- or back-focusing?
by Gisle Hannemyr
Published: 2010-11-12.

Below is a simple-to-follow procedure for testing the focus of a lens on a digital camera. It will help you determine whether a particular lens has consistent focusing errors.

There is a lot of recipes for testing focus on the web. Many of them suggest that you test your focus on some sort of an angled target. Don't. An angled target will not work. The camera's autofocus sensor is made up of multiple pairs of linear pixel arrays. If you attempt to autofocus on a single line in an angled focusing chart, only a few pixels from each active pixel array will “see” the target. Also, the marking that you see in the viewfinder and the actual autofocus sensor array may not line up exactly. The sensor array may “see” a different line on the angled chart than the one you think it sees.

Setting up a test target

Using a proper target is essential for getting consistent results.

Make sure you use a high contrast target with a lot of detail. A large sheet of printed text is excellent. The focus target should cover an area that is considerable larger than the camera's centre focusing point, and should be placed in the centre of the frame. The target should also be perfectly flat and parallel to the camera's focal plane. Lighting should be bright and even. Outside, in the shade, in daytime, is best.

Testing the lens itself

If your camera do not have live view, skip this section and proceed to the next.

If your camera has live view, you are able to test the lens itself by using live view in combination with manual focus to actually determine how good is when focus is exact. By using manual focus, and examining how sharp the image is on the image sensor used to capture the image, you can tell how sharp the lens will be when any errors introduced by the autofocus system in lens and/or camera body are eliminated.

Below is how to do it:

  1. Set up a target for the camera to focus on.
  2. Set the camera to its native ISO (usually ISO 200).
  3. Set the lens to its maximum aperture (use aperture priority).
  4. If the lens or body has a stabiliser, turn it off.
  5. Mount the camera on a sturdy tripod. Camera-to-subject distance should be no less than 50 times the focal length of the lens. I.e. for a 50 mm lens, that would be at least 2.5 meters.
  6. To avoid camera shake use a remote control and/or the camera's self-timer to release the shutter.
  7. Switch to live view and select maximum magnification (refer to your camera's manual for detailed instructions).
  8. Make at least three exposures of the target.
  9. Examine the resulting set of images on your computer monitor at 100% pixel magnification.

These images shows the absolute best your lens is capable of at its maximum aperture. You may use these for reference to determine how much errors in the autofocus system degrade performance.

Testing the camera's autofocus

Now is the time to test the performance of autofocus. The autofocus system consists of two components: the lens and the body. These work together and it is not possible to test them seperately without access specialised equipment.

Below is the procedure I recommend for testing autofocus:

  1. Set up a target for the camera to focus on.
  2. Set the camera to its native ISO (usually ISO 200).
  3. Set the lens to its maximum aperture (use aperture priority).
  4. Set the camera to single-shot autofocus.
  5. Select the centre focusing point.
  6. If the lens or body has a stabiliser, turn it off.
  7. Mount the camera on a sturdy tripod. Camera-to-subject distance should be no less than 50 times the focal length of the lens. I.e. for a 50 mm lens, that would be at least 2.5 meters.
  8. To avoid camera shake, if possible, use mirror lock up (MLU). Also use a remote control and/or the camera's self-timer to release the shutter. If the camera do not have MLU, use a shutter speed that is shorter than 1/60 second or longer than two seconds.
  9. Before each exposure, manually set the focus on the lens to infinity before allowing the camera to autofocus the target. Make at least three exposures of the target.
  10. Examine the resulting set of images on your computer monitor at 100% pixel magnification.

Hopefully, this test should yield sharp images and confirm that your the autofocus of your lens and camera is working correctly, and that they are well matched.

If the images are unsharp, first read the following series of essays by Roger Cicala of LensRentals.com about lens and camera calibration issues:

Fine-tuning autofocus

The next step, if your camera has the ability to fine tune autofocus or make autofocus microadjustments (e.g. Nikon D7000, D300, D600, D700, D800, D3-series, Canon 50D, 5DII, 5DIII, 6D, 7D, 1D-series), is to refer to your camera's manual for doing so.

For fine tuning there is a very well designed system for focus calibration from Michael Tapes Design called “LensAlign” (search eBay for LensAlign).

If fine tuning the autofocus does not resolve the matter, consider having the lens and camera serviced. It is best to have the workshop looking at both together, because you can not know which is outside the specifications.

Note: Expect some minor variations in focusing accuracy within each set of test images. This is completely normal, and is due to the tolerances of the camera's autofocus system.

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