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Message: <blockquote data-quote="Roy C" data-source="post: 1369118" data-attributes="member: 18798">Kevin, Canon reckon that the AF system on the xxxD and xxD is only capable of delivering good results at f5.6 or faster, which is why they turn off AF if the camera can see that the lens is slower than f5.6. They could very easily allow these camera's to attempt to AF at, say, f8 but the results would give Canon a very poor name as the camera would struggle in poor light etc...The whole point of taping a tc or using a non reporting one is that there is no way that the camera can tell that there is tc present so it attempts to AF. So if there is no way that the camera can tell if a tc is present then how can they possible programming the camera to stop people trying it ?.There are good reasons why series 1 cameras cost so much more than the other models and the AF system is one of them.</blockquote>