« on: Today at 11:57:51 AM »
Time has passed this lens by - it is two years to late.
The 150-600 zoom is the new kid on the block...
Indeed, and like much of America's youth, the 'new kids' are big and overweight. The retracted 100-400L is the size of a 70-200/2.8, a very convenient size for a 400mm lens. Considering the optical improvements going from original to MkII of the 70-200/2.8 IS, a new 100-400 should be excellent. Mount a 1.4x TC behind it, you'll have a 140-560mm f/8 lens that will AF on recent higher-end bodies, deliver great IQ, and be a heck of a lot more portable than those "I'm not fat, I'm big-boned" new kids.
How quaint. You recommend using a lens 2/3 of a stop slower, thus requiring the use of a higher ISO with an f-stop that requires many autofocus compromises.
When the issue is portability, yes. The 1D X and 100-400L with 1.4xIII mounted fits in a Lowepro Toploader Pro 75 AW or even a regular backpack – it's a very portable combo. At 560mm f/8, the IQ of the 100-400 is similar to the larger, heavier Tamron at 600mm. If I want to compromise portability instead, I'd bring the 600/4L IS II which is 1.33-stops faster, has better AF and much better IQ, and can go to 840mm f/5.6 or 1200mm f/8 (still with better IQ than the Tamron at 600mm).
Or I'd buy the 300/2.8, which is about the same length as the retracted Tamron 150-600 (and shorter and lighter than the Sigma 150-600), and with the 2xIII is 1/3-stop faster, has better IQ and better AF. Sure, it's more expensive...but you always have to compromise somewhere. Personally, the higher cost isn't a big concern.
Here's a challenge for you: limit your comparisons to lens solutions that are no more than 150% of the price of the Tamron lens.
Otherwise, if you want to take the "money is no object" path...