Originally Posted by Jon Ruyle
MFD is a big issue as you mentioned. The weight I don't mind. In fact, telescopes tend to be much lighter for the same iris diameter, thanks to far fewer number of elements, so weight is one of their advantages. But size is where they lose. Many are not "telephoto" at all: their length is not shorter than their focal length.
Another big issue is image stabilization. A four stop I.S. would let me shoot a managable 1/125 instead of an impossible 1/2000 that would be needed with a refractor.
Another issue is cost/performance for daytime (close focus) photography. Even the best refractors have a hard time competing with the lowly Canon 400mm f/5.6. It blows away almost any similar-aperture refractor with a wide converter/field-flattener, especially on full frame, even compared to those that cost twice as much. The 500mm f/4, similarly, stomps any refractor of a similar price. I think it comes down to economies of scale.
I'm hoping that there are enough people out there, like me, that want massive focal lengths, quality (and bokeh) as good as the 500mm f/4, don't need autofocus or f/4, but can only afford $2,500. Then perhaps Canon will see a way to put out an 800 or 1000mm lens with a very slow f-number, and good I.S. I doubt it will happen, but I'll keep dreaming.
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