There are three standard (50mm, non L) lenses in the canon line-up: Canon Ef 50mm f/1.8 MK I - MK II and f/1.4 with USM. Obviously each one has something to offer to the photographer.

Basic comparison:

1. Canon Ef 50mm f/1.8 MK II - around 90$, Cheap plastic, good image quality, okay AF performance.
2. Canon Ef 50mm f/1.4 USM - around 400$, USM, has good AF. Good image quality.
3. Canon Ef 50mm f/1.8 MK I - around 200$, Good build, mediocre AF performance in low light conditions. Very good image quality.

I went for the MK I, and I’ve several reasons for that, good pricing being one of them - This little thing costs less then 200$ on the used market, despite relative ‘cheapness’ its capable of producing some breathtaking images.

Both MK I and MK II versions of the standard lens are nearly the same in optical design, so image quality is relatively same. they are both excellent… tack sharp with beautiful bokeh. The only difference between two is the build quality, older MK I lens has metal mount, distance scale and is a bit more robust then the new MK II version. My Canon EF 50mm F/1.8 Mk I was produced in 1987, and despite age it functions flawlessly. So I recommend going with a bit more expensive choice, that will last you decades. In other words MK I is a good investment.

The only serious problem I’ve with my ‘Nifty fifty’ is the auto focus. In bright sunlight there is no problem at all. But things go wrong really fast in low light conditions - This makes AF on nifty practically useless at night or any other tricky lightning conditions. So what I do is switch to Manual, this increases keeper rate at least 2x times.

I also try not to go below f/2.0 - f/1.8 is unacceptably soft for me. And f/2.4 is a sweet spot for this lens.

Splurge on the extras by investing in current digital cameras technology used in shooting most, if not all, dvd rental and recent cd covers in retail stores. These cameras are like little computers, so take care and shoot steady.