Laut TDP schauts nicht so toll aus, das alte IS USM I ist sogar geringfügig besser, das Tamron etwa auf gleichem Niveau wie das IS USM II:
".... the 70-300 IS II is optically similar to or slightly softer than the last 70-300 IS I lens ...."
"The Tamron 70-300mm f/4-5.6 Di VC USD Lens is a strong competitor to the Canon 70-300mm IS II. I'll make an image quality comparison between these two lenses once we have the Canon tested on the 1Ds III, but ... don't look for any strong differences."
".... the 70-300 IS II is optically similar to or slightly softer than the last 70-300 IS I lens ...."
"The Tamron 70-300mm f/4-5.6 Di VC USD Lens is a strong competitor to the Canon 70-300mm IS II. I'll make an image quality comparison between these two lenses once we have the Canon tested on the 1Ds III, but ... don't look for any strong differences."