In the same week as AP published a second test of the troubled Canon EOS 1D Mark III, a Canadian website has published details of a planned hardware fix, involving the camera?s secondary AF mirror.

Many users have reported focusing errors and poor AF performance, especially in Ai-Servo mode, since the camera became available in early summer. Canon has since released two firmware updates to correct ?focusing inaccuracies? in certain circumstances, but in our second test of the EOS 1D Mark III (AP 27 October) we found that the camera?s AF is still worryingly inconsistent.

Chuck Westfall, Canon USA’s Director of Media and Customer Relationship has broken Canon?s long silence on the issue and told of an official hardware fix to the secondary mirror in affected cameras. The secondary, or ?sub-mirror? focuses light coming through the lens onto the AF sensor beneath the main reflex mirror. Apparently, although Canon has not identified a problem with the mirror itself, the components that make up the sub-mirror mechanism in some cameras may need to be adjusted or replaced.

Speaking to AP, a spokesperson within Canon Europe indicated that a ?potential source of problems with AF performance? had been identified in the secondary AF mirror, which can cause ?unstable focus prediction with moving subjects in Ai-Servo AF mode?. Apparently, this problem becomes more acute in ?higher than normal temperatures?, which might suggest that thermal expansion of certain key components within the mirror assembly is a contributing factor.

Apparently, only a limited number of 1D Mark IIIs within a particular production range are affected, and Canon is planning to announce a simple way of determining whether or not an individual camera needs the fix. An official announcement, and details of how to determine whether individual cameras are at fault, is expected within the next fortnight.

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