Light Good Lens

how can you tell a lens is really good in low light?
i was at the camera store today and the guy showed me a $2000 nikon lens (something - 200mm) and he said 'this is a really good lens. great in low light'.
besides the high price, how can i tell if a lens is good in low light? what number do i look at in the specs?
thanks
Yep, agree with Pooky.
Basically, most zoom lenses have a maximum aperture range of f/4.5-6.5 give or take. Some may have a max aperture of f/3.5-5.6.
There are some though that have a constant aperture of f/2.8 but they are generally high priced. But they are fast lenses that are good in available light.
You can get a 3rd party lens for half as much. Tamron and Sigma both make f/2.8 zoom lenses for Canon, Nikon, Sony, Pentax, and Olympus.
Primes get even better for low-light. You can get primes with max apertures of f/1.8, f/1.4, even f/1.2
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