What's the advantage of buying a fixed 50mm f/1.8 lens when my camera has an 18-55mm zoom lens?
I've got a Nikon D3100 with an 18-55mm zoom lens. I'm keen to experiment with some other lenses and a friend of mine recently reccommended I purchase a fixed 50mm f/1.8 lens as he said it's good for portrait photography and capturing really sharp images. He also said that as it isn't a zoom lens it will sharpen my composition skills.
My question is - I currently have an 18-55mm zoom lens, so by purchasing a 50mm fixed lens, won't I be purchasing a spec of lens already covered by my 18-55mm lens? What are the main differences between these two lenses?
Finally - can anybody vouch for the Nikon fixed 50mm f/1.8 lens as being a good lens to go for?
This is _basically_ a duplicate of Between Nikon 18-55mm II and 50mm f/1.8, which suits my needs better?, although I must say you have phrased the question more coherently (and the answers to that one are a bit over-specific).
And a related but not duplicate question (about a prime lens of a different focal length): If I have a 18-55mm lens, is there a point in buying a 35mm prime lens?
At 50mm on your 18-55, the max aperture is f/5.6. On the 50mm f/1.8, the max aperture is - obviously - f/1.8. It is perhaps not immediately obvious, but f/1.8 lets in 10-12 times more light than f/5.6. That is the difference between shooting at 1/10 second shutter speed (which is absolutely a no-go for moving subjects) and shooting at 1/100 (which is a usable shutter speed for moving subjects). Big difference indoors at night, for example. It lets you shoot without flash, or with the flash used as mere fill flash instead of it being the main light-source.
Note that Nikon has two variants of the 50/1.8, one with a built-in autofocus motor and an older one without. Do get the new one.