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Message: <blockquote data-quote="alanrharris53" data-source="post: 1734174" data-attributes="member: 61894">I have had the Tamron 200-500 for a few years and got some decent shots with it.  But not being able to justify £6K for a canon lens and wanting faster AF and image stabilisation I thought the Sigma 150-500 might be better provided the IQ is similar.  I managed to get a second hand copy of the Sigma from someone on Birdforum and first impressions are quite positive.  The weather has been too dark to make any comparison of IQ  - hopefully I can do a more detailed comparison at the weekend.  But one thing has puzzled me. Taking a photos of the same object from the same position with both lenses set at 500mm on the same body the Tamron appears to be about 5-10%"longer".So is the Sigma really a 500mm lens or is the Tamron actually going beyond 500mm.Or am I missing something somewhere and being a plonker?</blockquote>