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this topic seems to be made much more complicated than it needs to be.. but i'm not sure that i can explain it any better than anybody else.
stars don't get spread out the same way as diffuse 'deep sky' objects, so if you're talking about the limiting magnitude of stars in your exposure, that is a function of aperture, not f-ratio.
with deep sky objects, the amount of exposure required (or how deep you will get in a given time) is a function of f-ratio, but comparing 35mm with 135mm really doesn't make much sense.
say you have a widefield shot that includes M8 in the middle. with your 35mm f/2 you'll have a nice spot in the middle of your frame where all the light from M8 has condensed in one small bright spot in a relatively short exposure time, say 10 minutes. but it will have no detail/structure at that wide focal length.
if you shoot the same object at a longer focal length, then that object is spread out over a much larger area. so to get the same surface brightness (intensity) over that much larger area, requires a lot more light gathering.. thus your f2 135mm lens has a much bigger aperture to deliver the same surface brightness in the same exposure time. but you've gained a whole lot of detail/magnication and you end up with two totally different images.. so it's not apples and apples..
Phil
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