I have owned these;
- Sigy 150-500 OS
- Canon 300 f4 is,
My take on them, Sigy had to be calibrated twice coz once was only a general job to get it focusing properly. Second time was to sharpen it at f9 for birding. And like most reviews, it goes soft after 420-450 range. Still sharp on my 60D.
Canon was nice n sharp from f5.6..but i shot alot on f7.1-f9 in M mode incase of a sudden bird or animal appearing.
I traded and sold them resepctively for a 100-400 is and Sigy 120-300 2.8 OS (gen 2). One for girlfriend use and travel,the other for BIF n my use when we go birding and hiking. Heavy as hell,but for some reason,the Sigy's OS doesnt talk as properly on my 1d3 as how it does to my 60D. So,it is purely on Tv or M on 1d3.
Saving for bigger gun now,since 300 or 400 isnt getting me the distance i need. TC is also so-so depending on lighting as mentioned by other bros.
So, TC, the decision is entirely yours when it comes to buying a proper lens cos seriously, 300 f4 is?unless u are doing alot of walks in the jungle...but it depends on lighting. The f2.8 suggestion is better. But to shoot in open, you need longer reach...tc wont be satisfying.