为什么全幅数码会容易引起暗角

首先， 数码成象有别于胶片， 有许多文章谈到这一点， 下面有很好的例子，我也就偷懒了。
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.”

文中讲到数码需要光线直接照到sensor上， 而sensor上部会因为无法直射而造成光量减少， 从而造成暗角。
最后说， 35mm的镜头在用DX的相机时用的sensor小， 而全幅sensor因为光线角度问题， 需要用大于35mm的镜头才能克服暗角的问题， 而DX的镜头又利用数码sensor的特点来设计的，这样， 如尼康12-24mm DX 的镜头也不会产生明显的暗角。
如果， 全幅用的35mm镜头，重新设计，也许会改进暗角的问题。

所以， 文章中最后建议的3种解决方法是：

1. 用DX sensor
2. 把35mm 镜头口径增大
3. 设计新的镜头来用于数码全幅相机

 
第二个误区是， 再同样的取景范围内，DX 和 全幅在暗角的表现会一样，或认为全幅会优于DX

以下一篇文章用D2X 和 1Ds MrkII
D2X 用 12-24mm F4， 而1DS用17-40mm F4， 取景范围实际上是 尼康18mm， 加能17mm， 稍宽一点， 由于尼康没有相当于全幅17mm的镜头焦段， 18 vs 17 也应该很接近了， 请看：

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.

再次说明了全幅要配比35mm更大的镜头， 或重新设计新的镜头来使光线能直射到sensor上， 才能根本性克服这问题。

最后， 补一句， 因为sensor的size大， 全幅在成像上有优势， 但在角上会有损失， 所以， 常言道， 有得必有失， 就看你要那头了。

另外， 用加能的摄友们， 本文无意贬低加能的全幅，加能相机的质量是有目共睹的，我这文章只是会对盲目崇拜全幅的人一个不同的考量， 也 只是纯技术探讨。