為什麽全幅數碼會容易引起暗角

首先， 數碼成象有別於膠片， 有許多文章談到這一點， 下麵有很好的例子，我也就偷懶了。
http://www.nikonians.org/nikon/full-frame/review.html

“The 35mm "Full-Frame" vs. DX size digital sensor controversy is caused by a general lack of "digital sensor" knowledge on the part of many users, and strong marketing propaganda on the part of some sellers.

Many camera users haven't investigated the design of lenses and how light must fall on a digital sensor, compared to film. Sound physical evidence shows that, due to the micro lenses over the sensor, a 35mm lens cannot fully cover current 35mm sized sensor chips! The light fall-off from 35mm sized lenses can be as much as two-stops on the frame edges, simply because a sensor chip requires light to hit it almost directly.”

文中講到數碼需要光線直接照到sensor上， 而sensor上部會因為無法直射而造成光量減少， 從而造成暗角。
最後說， 35mm的鏡頭在用DX的相機時用的sensor小， 而全幅sensor因為光線角度問題， 需要用大於35mm的鏡頭才能克服暗角的問題， 而DX的鏡頭又利用數碼sensor的特點來設計的，這樣， 如尼康12-24mm DX 的鏡頭也不會產生明顯的暗角。
如果， 全幅用的35mm鏡頭，重新設計，也許會改進暗角的問題。

所以， 文章中最後建議的3種解決方法是：

1. 用DX sensor
2. 把35mm 鏡頭口徑增大
3. 設計新的鏡頭來用於數碼全幅相機

 
第二個誤區是， 再同樣的取景範圍內，DX 和 全幅在暗角的表現會一樣，或認為全幅會優於DX

以下一篇文章用D2X 和 1Ds MrkII
D2X 用 12-24mm F4， 而1DS用17-40mm F4， 取景範圍實際上是 尼康18mm， 加能17mm， 稍寬一點， 由於尼康沒有相當於全幅17mm的鏡頭焦段， 18 vs 17 也應該很接近了， 請看：

http://www.naturfotograf.com/D2X_rev06.html#top_page

Full-frame cameras are known to exhibit nasty light fall-off into the corners when wide-angle lenses are deployed on them. This results from light rays striking the imager at an angle, leading to vignetting in the periphery of the recorded image. Colour fringing also may be troublesome, but this issue wasn't elaborated here. 

The images show very clearly that "DX" lenses do not display the fall-off found on "FF" systems. Testing at full aperture would exacerate any tendency of vignetting, by the way. When the lens is stopped down, better corner behaviour can be expected, but any colour issues would not be alleviated just by stopping down.

再次說明了全幅要配比35mm更大的鏡頭， 或重新設計新的鏡頭來使光線能直射到sensor上， 才能根本性克服這問題。

最後， 補一句， 因為sensor的size大， 全幅在成像上有優勢， 但在角上會有損失， 所以， 常言道， 有得必有失， 就看你要那頭了。

另外， 用加能的攝友們， 本文無意貶低加能的全幅，加能相機的質量是有目共睹的，我這文章隻是會對盲目崇拜全幅的人一個不同的考量， 也 隻是純技術探討。