I'll put it to you this way, the Sigma 150-500 is 200mm longer than the Canon 300 f/4L. But I'd take the 300 f/4L and a 1.4 TC over the Sigma any day. Because it produces better images.
Probably true for the 150-500, but try the 50-500; it's better. If Tamron's 150-600 is just as good as the 50-500, or a little better at the long end, I'll be all over it.
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(when I was first getting started on a budget, I used the 55-250 which to my eye was better than the Sigma in the overlapping focal ranges) I found that shooting wildlife/birds it's a rarity to shoot in those lower mm's. Even when I had the Sigma 150-500 I was always shooting at 500mm 99% of the time.




