Monday, October 12, 2009

Sensor (CCD/CMOS) Artifacts and CMOS Rolling Shutter

Have you tried to use a DSLR to take video and discovered that there were some strange "skewing" effect when you panned the camera and shot motion, in which the person or a vertical object is distorted? Well, here comes a nice technical article that gets those sensor artifacts mostly explained, in a yet easy-to-understand and clear way:-

http://dvxuser.com/jason/CMOS-CCD/

In the article, some pros and cons about the CCD against CMOS are also mentioned, e.g., CMOS is more energy efficient and generates less heat in general. Those do apply to digital still cameras as the different characteristics are the same. In fact, some sensor artifacts as mentioned in the article such as smear would occur for CCD and partial exposure (in case of electronic shutter of the sensor in use and with quick exposure like using flash) would occur for CMOS even for taking still pictures. It is just a very interesting read and useful article afterall.

The articles does not go too deep into explaining the Smear effect of the CCD, though. But I can make some quick explanation to amend it here. So, what're the major difference(s) between CCD and CMOS sensors actually? If I have to explain it very briefly, what I would say is that CCD and CMOS are just the same thing in basic working principle but they are different in the basic architecture of which the pixel data is amplified and extracted in a quite different way. Such different cause various differences in artifacts, noise, power consumption and so on. The smearing we can see in the illustration is just a case of charge overthrown and that the overthrown charge cannot be reset during the course of pixel data retrieval just because it is the limitation of the "architectural" design and as such that particular sensor artifact is seen, in the final produced picture.

AFAIK, there is no video-capable DSLR using a CCD sensor, but all are equipped with CMOS sensors. So, the three artifacts, namely, Skew, Wobble, and Partial Exposure, do apply to all video-capable DSLRs that use CMOS sensors (solely anyway).

Here in Hong Kong there is a local DSLR gear magazine, the DiGi Bi-weekly, do test the Skewing artifact/effect each time when they tested a video-capable DSLR in movie mode. In fact, I read their tests of the 500D, EP-1, GH1, D5000, K-7 etc. before and it was found that the Pana performed the best in showing the least artifact for the undesired effect. Maybe this is owing to the higher refresh rate of the sensor for its electronic shutter, i.e. the (much) shorter (re-)cycling time. So, I think Pana do have some edges in their sensor and electronics / video technologies whereas the others are catching up in the game. Well, I believe Canon do also have some better technologies too but they won't give the consumers so much and too many in any digital *still* picture camera! After all, Video (Cam) is (for) Video and Still is Still!

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