I've been using a Sigma 100-300mm f4 lens for years. Its been a great quality lens, even if the autofocus can be a bit slow and struggles in low light. Its starting to get a bit worn out though now and I'm looking for a replacement, but there doesn't seem to be anything in that zoom range and / or with a constant aperture. A straight f/4 like this lens would be fine, f2.8 a big bonus. Any recommendations?.