This is the second installment of a five part series on building an affordable kit of professional grade lenses. Here's a link back to Part 1.
Photographers are often confused when first confronted with a prime lens. What? You can't zoom? My first experience with a prime was when my wife bought me a 50mm f1.8 as a gift on the recommendation of a friend. At first I figured it was one of those gifts where it was the thought that counts. But I couldn't have been more wrong. It is on my camera most of the time.
Prime lenses are fast, compact, lightweight and affordable. The 50mm focal length approximates the field of view of the human eye so you shoot what you see. Becuase of the wide open aperture primes work great in low light or situations where an extremely shallow depth of field is required.
Using a prime will make you a better photographer. You must shoot instead of fiddle. Your eyes and your feet do the work of finding and framing your subject; all the while training yourself to see better, concentrate and involve yourself.
Before proceeding, understand the crop factor you are dealing with on your camera. The size of the camera sensor determines how much of the image coming in through the lens it sees. Think about looking through smaller and smaller rectangles. Each time you capture a little less of what you see. Top end SLRs like the Canon 5D Mark II or Nikon D700 use a sensor size equivalent to 35mm film. The sensor sizes go down from there. Most SLRs are a 1.5 or 1.6 crop factor. So to get the same 50mm field of view on a crop factor camera you need something around 30mm (30 x 1.5 = 45MM). For a full explanation, see Wikipedia. I shoot with a Canon 50D at a 1.5 crop factor and I'm going to assume most of the audience here is using a similar crop factor camera.
The competition consists of four lenses around 30mm. The Sigma 30mm 1.4, Canon 35mm 1.4L, Canon 28mm 1.8 and Tokina 35mm 2.8. For more detailed analysis of individual lenses and their characteristics see the links to reviews below.
At $1400 the Canon 35 1.4L is the best of the bunch but prices itself out of the running.
Between the Canon 28 and Sigma 30, the sigma is faster and cheaper. The Sigma also fares better in reviews evaluating image quality for flare, barrel distortion, chromatic aboration and sharpness. Overall, the Sigma has the edge.
That leaves the Sigma 30 and Tokina 35. The Tokina is an odd duck - slower at 2.8 and lacking in high speed autofocus yet much cheaper ($120 less than the Sigma) and also a macro lens. There are very few reviews of this lens - only one user review on Fred Miranda, none on Amazon and I found only one comprehensive review. On the quest for a professional grade lens the slow autofocus and less than outstanding image quality knock this lens out and leave the Sigma 30 still standing.
Recomendation: Sigma 30mm 1.4. At $440 this lens offers outstanding image quality and an extremely fast 1.4 aperture with fast, accurate autofocus. Exactly the requirements of this category. The only drawback? It cannot be used on a full frame camera.
Best Deal: Tokina 35 2.8. At $320 this lens is a steal. There is some compromise on image quality and autofocus performance but the possibilities of macro are also opened up.
Honorable Mention: The venerable 50mm f/1.8. A "true" 80mm on a crop camera it makes a great portrait lens or to get a little closer in for more intimate street photography. At $100 it should be part of everyone's kit.
This Shot
Sometimes great shots are literally at your feet. I caught this standing over a very cooperative baby using a prime lens.
Canon 50D, 50mm f1.8, f/2.2, ISO 200, 1/80s.
The Useful Bit:
Get yourself a prime lens and use it. It will make you a better photographer.
. Granted, there are compromises. If your primary source of income is photography such compromise may not be acceptable. But for myself, I decided the trade offs are worth it. Read on and decide for yourself.
and 
. The Canon 50mm is bright with good image quality but for portraits I need some zoom and the focal length is a little too close for indoor shots and street photography. The Sigma 18-200mm is versatile but not very bright and in some circumstances the image quality could be better (especially wide open at 18mm with f/3.5 or 200mm with f/6.3). Both lenses could benefit from faster auto focus, especially in low light.
This version adds another 50mm on the long end and HSM, their speedy and silent auto focus system. If you aren't ready to spend $1500 for a set of lenses, one of these lenses gets you much of what the whole kit offers for about one third the price.