as said in the title i have a sigma dg apo 70-300 F4-5.6. its not as sharp as i would like at the 300mm end. without spending 3K on a L series 300mm prime, what would be a decent step up from the sigma, perhaps something even a little faster?
The next step would probably be the Canon 70-200mm F/4L or the Canon 300mm F/4L IS or Canon 400mm f/5.6L with the 400mm probably being the sharpest. The 300/400 come in around £900 and the 70-200 for around £450 sterling in the UK.