Nikon D90 dig.camera?

Buying a standard 50 mm lens why do they vairy to much in price from £100 to £300 aprox. Is the picture quality much better with the expencive one or wouldnt it be noticeable?

Quality doesn't vary much, but the presence, or absence of an aperture ring/motor is a factor.

f1.4 is half a stop faster than f1.8 - whether you need it is up to you.

In your shoes, I'd go for the f1.8 - with the D90 the aperture ring and motor are pretty much optional, so you can get the cheapest.

50 mm lenses come in various forms, depending on the speed of the lens. A 50 mm f/1.4 is can gather more light and is more expensive than a 50 mm f/1.8. Nikon's 50 f/1.8 lenses are available in the older AF-D and newer AF-S versions.

A Nikon D90 with its built-in focus motor can use both AF-D and AF-D versions, unlike Nikon's newer entry level DSLRs such as the D3100 and D5100 which lack this feature, and must necessarily rely on the internal focus motor of Nikon's AF-S series lenses for auto focusing. Something to keep in mind if you consider upgrading your D90 in the future.

The primary differences in the 50mm lenses is whether or not they have an internal focus motor (AF-S), or requires the camera to have the focus motor (AF). The AF-S lenses are more expensive. Fortunately, your D90 can use either.

The other difference is the speed of the lenses. A f/1.4 lens will have slightly better low-light performance than a f/1.8.

The f/1.4 is generally considered the pro caliber lens, while the f/1.8 the consumer version. Even then, at f/1.8, you are not looking at exactly a poor lens. To the contrary, all of Nikon's 50mm lenses are outstanding.