Originally posted by MossyRocks While I understand your reasoning for a lot of the shooting I do, astrophotography, the need for that extra stop in wide open aperture is needed. Add in that those bigger better lenses don't need to be stopped down as much compared to other ones and instead of it being a one stop difference it may be a 2 stop difference in a very punishing situation. The 560/5.6 is one that needs a lot more stopping down for astro shots so it isn't one I would consider and am very please with my 400/2.8 for what I do with it. However as you say lugging that monster around through the woods is not something I would do and in more normal shooting to just use my 300/4 as it is plenty sharp and I can throw ISO at the problem in most cases without issue to get the shutter speed I need plus I can hand hold a shot with the 300/4 without problem.
Excellent points! DSO (deep-sky object) astrophotography really does benefit from the biggest possible chunks of glass. The only substitute for the light gathering power of a huge physical aperture is a painful amount of stacking.
Even in the astro arena, I've tended to stay non-telephoto, first with a Sigma 12-24 f/4.5-5.6 (a slow lens but higher ISO fixes that) and now with a Rokinon 14/2.8 which is so sharp that it outresolves the sensor @f/2.8. The high ISO performance of modern cameras and IBIS also opens some new frontiers such as hand-held astrophotography with a 50/1.2.