"A 50mm f/1.8 lens tends to cost in the $100-150 range—the same as a kit lens—while an f/1.4 lens costs at least twice as much. At their widest apertures, these lenses produce a very narrow depth of focus and create separation between subject and background (or foreground) that adds dimensionality to your photos for flattering portraits.
There’s so little difference, spec-wise, between an f/1.4 and an f/1.8, but the difference in price is huge. How does that translate as far as image quality? All settings being equal, does one lens outperform the other?"
Read more here: http://www.adorama.com/alc/0012817/article/50mm-lens-shoot-out-f18-or-f14
(For those who don’t want to geek out, let me give you the summary and spoil the ending: For both Nikon and Canon 50mm lenses, overall image quality is generally sharper, with less vignetting and fewer aberrations over the range of apertures with the less expensive f/1.8 version. In most (but not all) cases, this makes the f/1.8 a more desirable lens. Why? Read on!)