A number of photographers --largely sports shooters, who rely heavily on the 1D Mark III's AI-Servo predictive autofocus system -- have reported problems with inaccurate focus when shooting rapidly moving subjects in high-speed continuous exposure mode, under conditions of high temperatures.

It appears that some of the delay in arriving at a solution is because not all 1D Mark IIIs exhibit the issue, and even units that do show the issue only do so under fairly specific conditions. Canon representatives told us today that they've identified the underlying cause, and have determined thecorrective action necessary.

The issue apparently revolves around the dynamics of the secondary sub-mirror that directs light to the AF sensors. Without divulging the details, Canon says that the nature of the issue was to cause "unstable prediction performance" in continuous shooting modes. When the problem is manifesting, it's possible to see the focus distance setting on the lens jitter back and forth slightly when tracking a moving subject, rather than smoothly incrementing as the subject approaches or recedes.

Recent reports in online forums saying the fix will require replacement of the sub-mirror are incorrect. Without going into details, Canon says that the fix is more in the nature of an adjustment than a replacement of the mirror assembly.

Canon hasn't yet made a formal service announcement detailing the program to bring malfunctioning units back up to spec, but expects to do so by the end of this month. While the nature of the adjustment is now well-understood, Canon needs to get a system in place to quickly turn around cameras sent in for adjustment. This apparently accounts for the delay between the development of the fix and the formal announcement of the service program. Canon made clear that the service program would be done at their expense, and any cameras returned for service would be fast-tracked through their repair organization.

As to the question of what sort of improvement to expect, Canon notes that AI Servo performance is heavily dependent on a variety of parameters, including camera settings, subject movement, speed, and contrast, so it isn't possible to give a quantitative answer to the issue of performance improvement. They did say that "(We) are certain that the countermeasure we are offering at this time enables the camera to perform according to its design specifications in AI Servo AF mode." -- In other words, the cameras will be adjusted to perform as they were designed to in the first place.

Some have wondered whether Canon would be suspending shipments of the 1D Mark III, but this doesn't appear to be the case. Now that both the issue and solution are well-understood, all 1D Mark III bodies currently shipping are leaving the factory fully up to spec.

Users have also raised the question of whether the problem is one that might occur over time; that is, if it's usage-related. The answer to that is no. It's possible that an affected unit may not manifest the issue if it's never used under conditions that would manifest it, but there appears to be no likelihood that a camera that's functioning properly now would develop the problem over time. So there's no need or point to sending in your camera if it isn't showing the problem. Likewise, the issue only affects the 1D Mark III, no other current EOS model is subject to it, nor will the 1Ds Mark III be affected when it ships later this year, according to Canon.

While they're being a little mysterious about the exact details of the issue and its solution, it will no doubt be reassuring to 1D Mark III owners that the problem has been identified and solved, and to know that Canon will be standing behind the camera with a no-cost, expedited repair plan.