I am an experimental film-maker working often with a 5D MKIII. I tend to work a lot in the macro and telephoto domains often with abstract imagery. I have a Canon 70-200 f2.8 lens and often use it with a Canon 2x extender essentially turning it into a 140-400 f5.6. I now want to push this telephoto thing and I'm wondering whether to buy a second 2x extender turning this lens into a 280-800mm f11.
The other option I am looking at is a 1000mm mirror lens. They are so cheap but are they absolutely junk? I can live with the fixed aperture but not crap optics.
Yes I know I am compromising optics and quality with either option but I am prepared to live with that to a degree to try some shooting ideas that I otherwise would not be able to do. However I'll only go so low on quality before it is not worthwhile for me. I haven't got the budget or wish to lug around an enormous lump of 1000mm glass in a traditional telephoto which I know will be the best quality choice.