Choosing a Fast 50mm lens
50mm lenses are an absolute essential to all good photography kits. Often people ask if the expense of a 50mm 1.4 is necessary or if a 1.8 is adequate. These are the two main 50mm speeds usually available but others also exist. Pentax didn't do 1.8, they did 1.7. Most of the big Japanese cameramakers have 50/1.2's. They also used to make 50mm f2's for the super economical. For the purposes of this discussion, I'll concentrate on comparing the 1.4 and 1.8 lenses as these are most commonly at the center of question but I'll talk in generalities which apply to all 50mm and fast lenses.
The differences between 1.8 and 1.4 lenses vary from manufacturer to manufacturer and model to model. Before we even get to the photos, there are other things to consider such as construction and the affect of the lens on your camera rig.
1.4 (and faster) lenses are usually better built than 1.8 (and slower) lenses. Where a 1.8 lens may use plastic, the 1.4 will use metal, making it more durable and suitable for professional use and a long-term investment. 1.4 (and faster) lenses tend to have more aperture blades than 1.8's (and slower). The more aperture blades a lens has, the more natural the out of focus points of light look at wider apertures such as from f2 - f8. Out of focus points of light will appear more circular than polygonal with more blades. Some say more blades and circlular OOF points of light are cosmetically superior than fewer blades and polygons but that's subjective of course. Increasingly people seem to enjoy polygonal OOF points of light and flaring too.
Faster lenses will also brighten up your SLR viewfinder, making it easier to see to focus, expose and just relate to your scene. This is especially important for manual focusing. When manually focusing, you may not even need a fast lens to shoot wide open but you may want it to make focusing more accurate.
Faster lenses are also bigger and heavier than their slower counterparts. 1.4's use bigger hunks of glass and are made with the aforementioned metal components to protect this glass. This makes your camera more front heavy and can be good for balancing out a heavy camera or bad for causing a lighter camera to tip forward when hand holding. The slower lens is more compact and discreet. When mounted to a smaller camera, it's a nice set-up for candid photography or anything where size is an issue. When you compare even faster lenses such as 1.2's you may even have to use different filters and hoods for them than the rest of your lenses because the size difference is so great. This is important to be aware of if you want to be able to interchange filters and hoods.
1.4 will allow you to shoot in about 2/3 a stop less light than the 1.8. The difference is extremely slim but may be JUST enough to keep from having to drag your shutter (drop it to a speed that's not ideal for hand holding) or bump up your ISO.
The faster (and longer) the lens, the more difficult it becomes to nail your focus exactly as a result of more shallow depth of field. So where you may hit someones eyes and nose with a 1.8, you may only hit their nose with the 1.4, and that's not good. So once you get into these faster lenses, you have to know when the right tyme is and isn't to use them at full aperture based on how far you are from your subject (also affects depth of field, the closer the less DoF, the further, the more DoF) and how important it is that you nail the focus exactly or can allow for some difference.
If you are really into available light work in dim situations such as concerts, the 1.4 or faster lens is a must. But if you are hanging out in studios, the fast lens may not be necessary. If you are a bokeh junkie and like to shoot wide open and very close to your subject to get tiny tiny DoF's with massive water-colour-like out of focus areas, the 1.4 or faster lens is a must. But if nailing your focus for sure is important, the 1.8 or even an f2 is great.
Finally, there's a quality difference. MTF charts measure the performance of a lens in terms of resolution and contrast. Generally speaking, higher resolution and richer contrast lenses are more desirable. The point of highest performance of any lens is in the center of the glass (smaller apertures) and the worst performance is at the edges of the glass (wider apertures.) So even though more expensive materials are used in construction of a fast lens, faster lenses have more issues around their circumference such as vignetting, soft focus and loss of contrast and resolution. So a 1.8 lens will usually rate better at all apertures on an MTF chart than the more expensive 1.4, 1.2 or 1.0. 50mm f2 lenses are technically far superior to any f1.something lens in terms of the MTF rating which is partly why you see shooters using higher quality optics such as Leica and Hasselblad using slower lenses such as the famous Leica Summicron 50mm and 35mm f2 instead of their f1.4, 1.2 or 1.0. It's not just because of the ridiculous cost of these faster lenses.
Anyway, so you want some examples, huh?