This more of a photography question than astronomy but everyone knows IIS is the best source of info on almost anything.
I have a Canon 400D with the EFS 17-85 IS lens. I'm very happy with this setup but I'm thinking adding telphoto lens(es), with an L in the name. The obvious choice is a 70-200 but I'm interested in wildlife photography, especially birds, and so perhaps the extra length of the 100-400 might be more appropriate - certainly with my old film camera I used the 600mm fairly often and it is about equivalent to 400mm with the smaller sensor. Buying both is out of the question but perhaps a tele-extender could give me the best of both worlds. If I got the 70-200 in the f/2.8 then it can be converted to a 140-400 f/5.6. The question is: how much does the extender degrade image quality? How would the image through the 70-200 plus expender compare to the 100-400, especially at the longer focal lengths? In a similar vein, how do the 1.4x extender and 2x compare?
Thanks for any comments you care to make.
