I have long been looking for a new longer lens to add to my arsenal, primarily for getting those subjects that normal photographers would like to keep VERY far off in the distance, but be able to capture them like you were standing next to them.
The lenses that I would be interested in looking at are anything with at least a 400mm focal length. Since this is more of a hobby than a profession, at this point, I have to be conscious of the cost of such a lens and mortgaging the house is not an option.
One option that does appeal to me is the use of a lower focal length prime lens like a good quality 200mm and a 2x teleconverter. Of course there will be the trade-off of sharpness and speed. Typically you lose 2 stops with a doubler as well as some sharpness. But the combination does offer some flexibility in that you now have a 200mm and a 400mm when using the doubler. There is also additional flexibility in that the doubler could possibly be used with other lenses that may be in the camera bag.
I do not have an aversion to experimenting with lens combinations, but is the trade-off with image quality worth the cost savings and flexibility?
For me, with the options on the table, I may go the 400mm prime route because the trade-off is not worth the $100 or so that I would save going the 200mm +2x route.
I did pick up a cheaper Kenko doubler from Craigslist to experiment with and will go out and look at how its use affects the images that are produced with various lens combinations.
Of course there are many other lenses out there than the two mentioned, with a wide range of quality and I only chose the 400mm because beyond that, or an aftermarket 500mm, you really start getting into professional lenses that will break the bank if you are not careful. I mean come on! Almost $10,000 for a 600mm? OK it is an F4 L IS lens, and it comes with its own
trailer, I mean tripod mount. But hey, if you are making money from the resulting images, then why not.
Thanks to Scott Bourne for his constant, practical advice and insight on photofocus.com and for his @ScottBourne tweets.