There is a photographer in England that has a large collection of bird shots taken with various long lenses.
Very sharp photos
Check out some of her photos and see what you think of the quality.
Maybe it will give you some ideas.
http://www.suesbirdphotos.co.uk/
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I really want to get the 600mm f4, but as we know it is $9K.
Currently I am using a 70-300mm 4-5.6 with a 2x Teleconverter and the quality that I am getting is just not good enough.
I was thinking about the 300mm 2.8 and a Teleconverter, but is this going to get the crispness that I am looking for?
Anyone have any suggestions or do I need to suck it up and buy the 600mm???

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