dpc said:
Hi! I am an enthusiast who does a fair bit of birding and wildlife shooting. I am trying to optimize my longer range lens collection without breaking the bank. I have used the Canon 400mm f/5.6 L. I found it a great lens in many ways. However I had difficulty handholding it and getting decent shots (I am aware of upping the shutter speed, using a tripod or monopod - I would like a lens with good image stabilization). I know others find it quite handhold able but that is not my experience. I am presently using the 300mm f/4 L with the Extender EF 1.4x III and am quite happy with it. I would like something longer, however. I had the Sigma 150-500mm for a while. It took good pictures but was a bit soft on the long end where my interest lies. I know the Tamron 150-600mm is just out, is decently priced and has some positive reviews. Does anyone have any initial thoughts on whether the Tamron at the 600mm end would be reasonably comparable to the Canon 300mm f/4 L with the 1.4 extender and is the Canon extender usable on the Tamron and, if it is, what are the likely effects on sharpness? I shoot with both a 7D and 5D Mark II. Thanks for any advice you may have. I will wait for more reviews but would appreciate any thoughts with what we now know.
It's difficult to judge how good the lens is at 600mm from published images because few have been taken under optimal conditions. The published ones of birds are mainly at f/6.3 where the lens is softest. It does seem from the MTFs and good images, that at 400mm, the Tamron is close to the 100-400L, which means similar to or better than your 300 + 1.4xTCIII. It looks like that the Tamron at 600mm and f/8 - f/11 would give a significantly better image than the 400mm lens cropped.
The use of extenders is probably problematic. The Canon extender will narrow the maximum aperture to f/8 and your 7D would not autofocus. On a camera that will, such as the 5DIII, you would have to use f/11 - f/16 to get into the sharper regions at 600mm, drawing you into high iso and extenders aren't usually good unless the naked lens is very sharp.