Nikkor 24-85mm f/2.8-4 vs Nikkor 24-70mm f/2.8?
Why is the 24-70mm significantly more expensive than the 24-85mm? What features make the 24-70mm more expensive? I want to invest in a good-quality FX lens, along with a decent DSLR body like the D5100. Will that work?
Being f2.8 through its range makes the 24-70mm more expensive. The 24-70mm f2.8 is an AF-S lens. The 24-85mm f2.8-4 is not. The 24-85mm f2.8-4 will not autofocus with the D5100.
Mainly constant aperture all the way through the zoom range; the mechanics of the optics are much more expensive to make than a variable aperture lens; if you think that the aperture is just a function of the focal length, then keeping one constant while changing the other is complicated. I don't know the 24-85 but probably will have more plastic, and have a lighter feel as well. Also, learn about nyquist frequency, make sure your lenses aren't limiting your body, and vice versa.
At 70 mm, the more expensive lens can open wide at f 2.8 (the actual aperture, i.e. The opening is 25 mm). The cheaper lens, at 85 mm at f 4, is at 21 mm.
Doesn't sound that much smaller, but it's more expensive to physically make a larger piece of glass.
The 24-85mm is f/2.8-4, meaning the aperture changes depending on how far you zoom in. At 85mm you lose half of the light that's coming through the lens, whereas with the 24-70mm you can stay at f/2.8 throughout the whole range.
More light = higher shutter speed and/or lower ISO = sharper and cleaner images.
Another thing is that the 24-70mm has a built in focusing motor and the 24-85mm doesn't, meaning that the 24-85mm won't autofocus on the D5100.
If you wanna go for the 24-85mm, you'll at least need a D7000 (unless you wanna focus manually) which has a focusing motor.
The 24-70mm is razor sharp, fast and probably the best 24-70mm f/2.8 out there. It's a very high quality pro lens. If you don't even have a camera yet and you're just starting out, you don't need this. You can get a D7000 and a Tamron 28-75mm f/2.8 as an alternative, which is still a pretty good lens.
On the other hand, if you have the money, then by all means go for it. You can't go wrong with the Nikkor 24-70mm f/2.8.
The 24-85 will not autofocus with your 5100. The 24-70 is a constant 2.8, while the 24-85 drops to f4 towards the middle of the zoom range. Here is the harsh but honest truth - since you don't know this, you don't need a 24-70. Just keep you 18-55.