✔ 最佳答案
●本人用Canon 400D，想配一支超廣角鏡，例如 EF 20mm f/2.8 USM。但據說400D是APS-C DSLRs，400D的焦距轉換比率是1.6，支鏡EF 20mm f/2.8 USM是傳統菲林機年代的舊鏡，配上400D會變成32mm效果，如此便失去了超廣角鏡的效用。到底實情是否這樣？
Yes, it will be 20 x 1.6 crop factor = 32 mm in 400D :)
APS-C 代表什麼意思？是什麼英文簡稱？
Advanced Photo System type-C (APS-C) is a designation sometimes given to digital sensors that are approximately equivalent in size to the Advanced Photo System "classic" size negatives. These negatives were 25.1 x 16.7 mm and had an aspect ratio 3:2.
Sensors meeting these approximate dimensions are used in many digital single-lens reflex cameras, in addition to a few large-sensored live-preview digital cameras (such as the Sony DSC-R1 and the Sigma DP1) and a few digital rangefinders (e.g. the Epson R-D1). Such sensors exist in many different variants depending on the manufacturer and camera model[1]. All APS-C variants are considerably smaller than 35 mm standard film which measures 36×24 mm. Sensor sizes range from 20.7×13.8 mm to 28.7×19.1 mm. Each variant results in a slightly different field of view (FOV) from lenses at the same focal length and overall a much narrower field of view compared to 35 mm film.
●有其他超廣角鏡適合Canon 400D，又不會令到焦距比率改變呢？請介紹一下。
First, YOU DON'T NEED f=2.8 for wide angle... f=4.0 is enough, and you need the Depth of field.
I use Tokina 12mm-24mm f/4.0 ... very good optics and built like a TANK, sturdy.
It is one of the famous wide angle for Nikon D70-D80 also ... I don't need 10mm from Canon, and the tokina lens is around 3500 HKD ... worth every dollar of it :)
2 sets of example for you with 12mm-24mm tokina on 400D :
http://picasaweb.google.com/liuyifeng/AttractiveHongKongTsimShaTsui0108
http://picasaweb.google.com/liuyifeng/WinterPlumEarlySpring2007
good luck and hope all help la :)
2007-04-06 09:28:56 補充：
One note : Canon 10-22mm is a 6400 HKD lens, and personally 16mm is too wide and distortion is there not to my taste and EF-S mount sucks cannot mount on 5D, there is a full review for you to do more comparison :)
http://www.photo.net/equipment/wideangle-dslr.html
