I'm hoping to get up early and into the park to try and catch some of the wolves in the snow in the early morning, not to mention other park wildlife. Currently looking at this sigma: http://www.lensrentals.com/rent/sigma-50-500mm-f4-6.3-ex-hsm-xxx/for-nikon or maybe this Nikon http://www.lensrentals.com/rent/nikkor-300mm-f4-af-s/for-nikon and just pick up a 1.4 converter? How many stops do you lose with a converter? I'll need long reach to get good shots, I can't spring for the super fast lenses because I'll be renting for probably 21 days and it gets too pricey. Plan on pretty much being tied to using a tripod or monopod for these for sharp pics obviously.