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I will be there the last 10 days of Sep, before moving down to GTNP for 4 days. This is my normal fall vacation trip. In another 6 years or so I should exhaust this time frame and move to early spring or late winter.
Fall Colors in Yellowstone are never as brilliant (nor as widespread) as in GTNP, so if you are looking for that, you might try visiting GTNP first then moving up to Yellowstone.
Everything I have seen this year shows extremely dry, above average temps, and quite a few fires in YNP. As rprouty indicated, colors may be changing early this year.
Photographically, Many of the geysers and hot springs have mists associated with them which contain corrosives. Whenever possible in those areas I use a filter on the face of my higher priced lenses. If you keep your camera clean and dry and your lens clean using any of the better lens cleaning solutions (not just a micro-fiber cloth) you most likely won't have a problem, as steam and mists which are really bad for your camera/lens will drive you off the site pretty quickly.
I enjoy this time of year simply becuase of the weather (and its chance for cool/cold temps and occassionally snow) and the reduced "people count" as compared to earlier in the year. Lodging is tight because about 3/4 of the campsites and lodges are already closed (hint, hope you already have reservations!).
I can recomend the following:
(either the deluxe trip planner or the iphone app)
(second best photoguide I have seen, buy it as an ebook and transfer to your iphone)
(best photoguide for Yellowstone I have seen)
My working gear for daytime includes an FX camera body, a 300 f/4 with 1.7 converter (I simply can't afford one of the big 500s), a 70-200 with 1.4 teleconverter, a 24-85 f/2.8 zoom and a 17-35 f/2.8 zoom. I lug around a set of neutral density and soft edge grad filters and a polarizer, and a heavy tripod.
Hope all this helps.........post any questions you might have and someone will jump on them pretty quick, I think.