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Message: <blockquote data-quote="Roy C" data-source="post: 1734829" data-attributes="member: 18798">From everything I have read the 100-400 is reckoned to be somewhere around 385mm at the long end although I believe the shooting distance could be a factor.  There is a old  thread somewhere on BF where someone posted two pics taken with the same camera on the same day from the same spot, one with the 100-400 at the long end and one with the 400/5.6 - the target was was noticeable larger with the prime. I also saw a thread on another forum where someone compared shots with the Bigma (50-500) at the long end and the 400/5.6 and they could hardly believe how little difference there appeared to be in focal lengths. The poster calculated the bigma as being around 460mm at the long end.It seems like zooms are more variable than primes probably because of the lens construction. I have also read that the focal length of zooms are calculated when shooting at infinity.Before anyone jumps on me I must emphasise that I have not personally tested any of the lens mentioned but could probably find the threads I quoted.</blockquote>