镜头图片先来几张：
选配件遮光罩：
装在350D上的样子：
基础资料：
| 佳能EF 20mm f/2.8 USM基本参数 |
| 镜头结构 |
9组11枚 |
| 对焦方式 |
自动 |
| 焦距 |
20 mm |
| APS焦距 |
32 mm |
| 卡口类型 |
佳能EF | |
 佳能EF 20mm f/2.8 USM光学参数 |
| 水平视角 |
84 度 |
| 最大光圈 |
2.8 |
| 最小光圈 |
22 |
| 对焦驱动系统 |
环形超声波马达 |
| 最近对焦距离 |
0.25 m |
| 最大放大倍率 |
0.14 倍 |
| 滤镜口径 |
72 mm |
| 镜头直径 |
77.5 mm |
| 镜头长度 |
70.6 mm |
| 镜头重量 |
405 g |
| 光圈叶片数 |
5 片 |
| 镜头类型 |
定焦 |
| 镜头用途 |
广角镜头 | |
结构图：
MTF：
参考价格：
3500~3900
淘宝参考：http://auction1.taobao.com/auction/0/item_detail-0db1-e86e24aa25e034ce51a326f46385e8b0.jhtml
amazon.com参考：
http://www.amazon.com/Canon-20mm-Angle-Lens-Cameras/dp/B00007GQLT
Introduction
The Canon EF 20mm f/2.8 USM was released back in 1992 and it's one of the older designs in the current lens lineup. On APS-C DSLRs such as the EOS 350D (used for testing) the field-of-view is equivalent to 32mm so it looses its ultra-wide character towards a moderate wide-angle lens here.
The optical construction is made of 11 elements in 9 groups without any special glass. The aperture mechanism features 5 blades. With a min. focus distance of 0.25m the max. object magnification is about 1:7. The lens feature a floating system for close focus correction. The EF 20mm f/2.8 USM has a size of 78x71mm and a weight of 405g so it is roughly comparable to an average standard zoom here. The filter size is 72mm. A lens hood is available as an option.
The lens has a decent though not stellar build quality based on good quality plastics. The focus ring operates quite smooth but it was slightly wobbly in the tested sample. The EF 20mm f/2.8 USM has a true IF design so its length remains constant regardless of the focus setting. The front element does not rotate so using a polarizer remains easily possible. Despite its design age the lens features a very fast and near silent ring-type USM drive incl. full-time manual focusing (FTM) in one-shot AF mode.
Distortions
The lens showed a slight to moderate degree of barrel distortions (~1.55%) which is quite good for this focal length class.
The chart above has a real-world size of about 120x80cm.
Vignetting
On an APS-C DSLR the full frame EF 20mm f/2.8 USM enjoys a sweet spot advantage but the usually positive effect isn't all that pronounced regarding vignetting which is rather high at 1.28EV at f/2.8. From f/4 and up the issue is already very well under control and no problem under field conditions.
MTF (resolution)
The resolution figures in the lab were something of a mixed bag. The center performance is exceptionally high straight from the max. aperture. At f/4 it is even among the highest tested to date. Unfortunately the borders can't quite follow but the quality is still good to very good here. The extreme borders (data usually only obtained for dedicated APS-C lenses) show a rather pronounced fall off at f/2.8 which recovers by f/4. At f/8 even the extreme borders are in very good territory.
The lens exhibited a rather pronounced field curvature and a slight degree of residual aberrations (focus shift when stopping down) so you need to make sure that the depth-of-field is sufficient for your scene.
Below is a simplified summary of the formal MTF findings. The chart shows in line widths per picture height (LW/PH) which can be taken as a quantity for sharpness. If you want to know more about it you may check out the corresponding Imatest Explanations.
Please note the new scale of [650,2150] which will be used for all EOS tests from now on.
Chromatic Aberrations (CAs)
CAs (visible as color shadows at harsh contrast transitions) reach an average CA pixel width well below the 1 pixel barrier which is very good for a lens in this focal length class.
Verdict
Since the release of the EOS system we have seen several updates to Canon ultra-wide zoom lenses but they didn't really care about their fix-focals in this range ever since their initial release back in the early 90s. The Canon EF 20mm f/2.8 USM feels a little dated by now - less so regarding its mechanical qualities but optically it isn't really something special anymore. Most good quality zooms can match its quality by now. The fix focal lens can still convince regarding distortions and CAs. Resolution-wise it is still a good to very good lens for sure - just not more than that. Vignetting is a little on the high side at f/2.8 which is a little disappointing for a full format lens tested on an APS-C DSLR. So unless you require an affordable, large aperture lens in this range there're few reasons left to favor this lens over e.g. the Canon EF 17-40mm f/4 USM L.
样张（来自pbase）：
资料来源：
http://product.pcpop.com
http://www.pbase.com
http://www.photozone.de/8Reviews/lenses/canon_20_28/index.htm