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Message: <blockquote data-quote="Roy C" data-source="post: 1504967" data-attributes="member: 18798">You are right that when you stack converters the Camera will only ever see one but you are still effectively shooting at f8 the same as a f5.6 lens with a 1.4tc (be it taped or non reporting). There is no reason why a f4 lens with stacked 1.4tc's should focus any better than a f5.6 lens with a single 1.4tc.The same thing would apply to the 300/2.8 if you stacked a 2x and 1.4tc, sure the camera would attempt to AF as it only sees f5.6 but again you are shooting at f8 regardless of what the camera actually thinks it is.I have a 70-200/4 which I have stacked tc's on and it will attempt to AF as it only sees f5.6 but the actual AF performance is not quite as good as the 400/5.6 with a taped tc. I would expect it to be about the same but maybe the zoom does not perform as good as the prime.At the end of the day the only reason why anyone can get AF on a non 1 series Camera at f8 is because Canon have to build-in a bit of slack with the AF system to make doubly sure that it will perform well at f5.6.I would be surprised if the 300/4 with stacked converters would come near to the IQ of the 400/5.6 with a single converter. With due respect I think your second picture compared with the three that I posted kind of substantiates this, maybe others would disagree.BTW I am not in any way knocking the 300/4 as it is obviously a very fine  lens and over the 400/5.6 you have the obvious advantage of IS and if shooting at 300mm the extra stop is handy in low light conditions. Only yesterday I was reading a review from a well know professional who reckoned that the 300/4 was not that far behind the 300/2.8 in terms of IQ with the bare lens.</blockquote>

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