The Canon EF 20mm f/2.8 USM Lens fits in a nice wide angle focal length space below the ultra-common 24-something and 28-something zoom lenses. There is a noticeable FOV (field of view) difference between 20mm and 24/28mm.
People often look for a prime lens for their ultra wide angle needs. Many people don't use these focal lengths with great frequency and look to save some money by buying a non-zoom lens. Primes are often smaller, lighter, cheaper, sharper and faster than their more versatile zoom counterparts.
Upon upgrading from a 1.6x FOVCF body to a 1.3x FOVCF body, I was questioning whether I needed to keep the expensive Canon EF 16-35mm f/2.8 L USM Lens in my kit. The 20 f/2.8 would be as wide on my 1.3x body as the 16-35mm was on the 10D, just as fast and significantly less expensive. My L zooms cover my needs beyond 24mm.
My first out-of-the-box impression was that the 20 f/2.8 is nicely built. I would put the 20's build quality between the Canon EF 50mm f/1.4 USM Lens and the Canon EF 85mm f/1.8 USM Lens, but not up to the weather-sealed Canon EF 16-35mm f/2.8 L USM Lens' quality. Overall, I am satisfied with the 20mm on this point.
The Canon EF 20mm f/2.8 USM Lens autofocuses quickly and quietly. Another good point. The 20 f/2.8 not big, but I wouldn't call it small. It is smaller than the Canon EF 16-35mm f/2.8 L USM Lens.
Shooting tests quickly revealed the differentiating points between the two lenses.
First, the 20 f/2.8 exhibited less barrel distortion than the Canon EF 16-35mm f/2.8 L USM Lens.
Second, although the 20 f/2.8 was not as sharp as my Canon EF 16-35mm f/2.8 L USM Lens, it showed decent center sharpness. At f/2.8, the 20 was slightly soft. At f/3.2 and beyond, center sharpness was good.
Unfortunately, that is about the end of the positive aspects of this lens.
The 20 f/2.8's corner sharpness was disappointing even on a 1.3x FOVCF body.
Color and contrast on the 20 f/2.8 were well below that of the Canon EF 16-35mm f/2.8 L USM Lens. There seemed to be a strong purple hue to the 20mm's images.
Vignetting is very strong when the 20 f/2.8 is used wide open - even on a 1.3x FOVCF camera. Somewhere between f/4 and f/5.6, the vignetting became reasonable. The following photos are identical auto exposed, auto white balanced shots taken at f/2.8 with these two lenses on a Canon EOS 1D Mark II.
The photo on the left (from the 20 f/2.8) shows obvious vignetting. The purple coloration is also present.
The photo above is an f/8 comparison. The vignetting is much better, but the photo from the 20 f/2.8 is still slightly underexposed (a consistent issue) - and still purple.
The Canon EF 20mm f/2.8 USM Lens exhibits noticeably more flare than the 16-35mm L does (both lenses tested with hoods attached). CA (Chromatic Aberration) is also present in the 20 f/2.8 images.
Pictured above and below from left to right are the Canon EF 20mm f/2.8 USM Lens, Zeiss 21mm f/2.8 ZE Distagon Lens, Canon EF 24mm f/1.4 L II USM Lens, Canon TS-E 24mm f/3.5 L II Lens and Canon EF 16-35mm f/2.8 L II USM Lens.
So you need an ultra-wide angle lens? My strong recommendation to you - save until you can afford the Canon EF 17-40mm f/4.0 L USM Lens or better yet, the Canon EF 16-35mm f/2.8 L II USM Lens or one of the other primes lenses referred to in this review. Image quality at 20mm is significantly better with these two lenses. Plus, you get the versatility of a zoom lens with a significantly wider angle focal length available.
If all you can afford is the Canon EF 20mm f/2.8 USM Lens, buy it and enjoy it. It will capture memories just fine.