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There is, of course, a third exposure parameter, and that's aperture. As you open the aperture in your lens, more light and pass through the lens to the sensor during an exposure. Different lenses have different maximum apertures. For example, one 50 mm lens might only be able to open to an aperture of f/3.5, while another might be able to open all the way to f/1.2. Remember, the lower the number, the wider the aperture. With a wider aperture, you don't need as long of an exposure to capture a given amount of light. In other words, a lens with a really wide maximum aperture will let you keep your shutter speeds faster when you're shooting in low light.
Now the maximum aperture of a lens is referred to as the speed of a lens. So a lens with a maximum aperture of f/2 is referred to as an f/2 lens, and that's generally considered to be a very speedy lens. Creating a lens with a wide maximum aperture requires a lot of glass, so fast lenses are usually physically larger and therefore more expensive, and they're also heavier. Obviously, for low-light shooting, a fast lens is great because the wider aperture helps you keep your shutter speed faster. If you're going to back country though, faster lenses might be prohibitively weighty, so you'll need to consider that.
Of course, as aperture gets wider, the depth of field in your scene gets shallower, so you'll need to factor depth of field needs into the equation as well. Now if you're using a zoom lens, you may not have the same maximum aperture across the entire zoom range. For example, a lens on this camera has a maximum aperture of f/3.5 when I'm at the widest or shortest focal length and f/5.6 when I am zoomed in all the way to the longest focal length, and it varies somewhere between those two. So, if you need a faster aperture with a lens like this, than you might want to consider standing in a different location and using a different focal length.
Now, of course, if you do that, the sense of depth in your scene will change. So along with motion blur and noise concerns, these are other factors that we're going to be considering and balancing while working in low light. If all of this is a complete mystery to you, if this is not review, then take a look at Foundations of Photography: Lenses, which details all of these lens issues that we're talking about here.
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