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Learn Digital Photography with Sandro Dzneladze

Lens: diagnosing autofocus problems

Written on Jan 28, 2010 by Sandro Dzneladze
Lens: diagnosing autofocus problems

In the previous article (Image Sharpness 101) I mentioned the most likely cause of blurry photos – photographer’s skills… But, we don’t want to exclude possibility of defective equipment. In fact, dented glass surface, or malfunctioning AF motor can be major contributor to lack of image sharpness.

We will first start by looking at the technical problems of the camera lens, assuming photographer is capable enough not to make common mistakes.

*Before you read deep into this article, I advise you to check for obvious: Make sure AF switch is turned on. Remove any filter attached to the front part of the lens – Low quality filters can degrade image quality and sharpness, sometimes considerably. If these quick tips didn’t help you, then read on…

Front-focus / Back-focus

In recent years, the most persistent mechanical problem in lens (or camera) has been front or back-focus issues. This is because manufacturers try to cut costs, decreasing quality control and increasing tolerance levels – If lens is miss-focusing front or back by 3mm, and quality control spots this, manufacturer will still sell it as it is within the range of tolerance bracket.

How to test AF

Now it’s time for some lab work (This time no guinea pigs, sorry) – We will just check your lens for auto focus difficulties.

  1. Print this: AF Test Chart.
  2. Mount your camera on a sturdy tripod (obviously with the lens in question attached).
  3. Fix camera at 45 degree angle from the test chart and shoot center focus line.
  4. Shoot some more, this is to eliminate random error.

Now look at the results. Using this simple test, it’s easy to spot front or back-focusing problems. Center spot must be perfectly sharp, few millimeters above or below is fine, but if it’s off by a significant amount, then you have a problem.

Sharp Copy

Some lenses may miss-focus by much more than few millimeters, or less, hence the hysteria about ‘sharp copy’ going around photography forums. But what may be sharp for one camera body may end up opposite on another body – the reason is tolerance levels in digital bodies too… If lens is front-focusing by 5mm, and your camera body is doing the same, it will add up to -10 a significant bias! – This coupled with razor sharp depth of field (f/1.4 for example) will lead to considerable loss of sharpness. Think for example about a portrait photographer focusing on the eyes of a subject – problems described above will make his work impossible, as focus plain will land on the nose instead of eyes.

Note: it can also be that AF bias in lens and camera cancels each other out, and you have perfect pair. The ‘sharp copy’ doesn’t exist; it’s simply a right combination of AF misbehavior both in lens and camera (If your SLR body is front-focusing you want lens that back-focuses and so on).

Test described in the previous section should give you substantial information on how lens is behaving. The question is what to do if problem is detected…

There are three options, starting from the most obvious – you can try to exchange it; send it to canon for checkup; or use AF micro-adjustment feature of new cameras, more about this in the next post.

Is your lens clean?

Another aspect to consider is front element of the lens, is it clean? Does it have dents?
Serious surface blemishes can result in loss of sharpness and halo when photographing direct light. This is something to think about if there is a huge dent on the front element; minor scuffs don’t affect image quality. To learn exactly how much dirt will render your lens useless click here.

Feedback

Throughout the series “Image sharpness 101” your feedback is much appreciated. Please feel free to share your knowledge and experience in the comments below.

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