CMOS versus CCD video imaging and the “Rolling Shutter” problem

Older video cameras used CCD-based image sensors. For various reasons, that I will explain in a moment, camera makers have largely switched to using CMOS-based image sensors on low end and even some low end semi-pro video cameras (like the Panasonic HMC-40). All of the new digital SLRs (DSLR) still cameras that also shoot video using high resolution CMOS sensors too.

But there’s a nasty problem with CMOS-based sensors that can cause the video image to resemble a shaky bowl of jello, as illustrated in this video comparison between the Canon HV20 (CMOS-based) and the Panasonic SD5 (CCD-based):

The problem is that the CMOS image sensor is read “line by line” from top to bottom.  If the image changes during the read out, then one line may be slightly offset or shifted from the previous line.  This is known as a “rolling shutter” and creates the peculiar “jello effect” since not all lines (or rows) in the image are lined up with all the others.

Old movie film cameras used a physical shutter that open and shut exposing an entire frame all at once. (Although, there are some issues with that too which we can ignore for now.)

CMOS reads the image row by row while CCDs read the entire image all at once and do not use a “rolling shutter” – hence, no jello on CCD cameras.

So why use CMOS instead of CCD? The basic reason is that CMOS uses less power and produces less heat and is less expensive. As image resolutions have increased, the size (and cost and power) of the CCD imager has gone up.

Camera makers, especially in the consumer market, are in an arms race to each have cameras featuring ever more pixels. This means most have switched to CMOS because, apparently, most consumers do not care about the jello or do not encounter it often enough.

In the DSLR world, most any camera with 10 or more megapixel resolution is CMOS. CMOS works great for most still photography. But when these DSLRs are used for video, they too suffer from awful “jello”.

There are other things to consider too – CCDs may show vertical streaks in photos when there are very bright lights (sun reflected on water, stage lights, etc) in the image.  For higher resolutions, CCDs need to be physically larger – and more expensive.

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