Tag Archives: Charge-coupled device

DSLRs and rolling shutter

Rolling shutter” is a video image distortion that commonly appears when using modern CMOS-based DSLRs – and also CMOS-based consumer camcorders. It manifests as a wobbly or skewed image. In the simple case, a fast pan left or right causes vertical lines to become slanted and bent. In a more typical case, portions of the image can appear wobbly if the camera – or the subject – are moving left or right.

I’ve elevated the following to its own blog post; it first appeared within the following post about the new Canon SX40IS. I also wrote about rolling shutter before.

I’ve been discovering some issues with rolling shutter on my Lumix GH-2 when using long telephoto settings that are, for me, more problematic than fast pans. Specifically, if I use the 45-200mm zoom at the 200mm setting (think 400mm full frame equivalent lens), and then switch into the ETC extended digital teleconverter mode where it isolates just 1920×1080 pixels (multiply by 2.6 times) giving a 1040mm (full frame) effective lens, the very slightest motion produces skew and wobble in the image.

Last week I shot a scene using this feature – since a 1040mm equivalent lens is compressing a huge amount of atmosphere, the thermal refraction occurring in the image made the image wobble. And sure enough, that resulted in rolling shutter issues even though the camera was locked down securely on a tripod!

Some day … an electronic global shutter will be added to CMOS sensors, I suppose. Until then, for long range video shooting, I prefer CCD imagers.

Is rolling shutter a problem? Some claim its not if you merely plan your images in advance. For those who can plan their images, this strategy may work fine.

But I shoot a lot of live events that I do not control and or which there can be only minimal planning. I am finding that DSLRs are great for shooting video:

  • For wide angle views, including handheld shots
  • For producing narrow depth of field
  • For convenience and small size (relative to a prosumer camcorder like the XH A1 or HMC-150)
  • Where you do not need real time audio monitoring and audio controls
  • Where you do not need a motor controlled zoom
  • For excellent low noise video images (especially at low ISOs)

For long telephoto shots, I find myself fighting rolling shutter far too often. I cannot control the wind. I cannot control the air temperature that causes thermals and refractions, making the image move around. As you can see in the linked articles, below, some people like all the wobbliness! I don’t!

Real video cameras have the following advantages:

  • CCD imaging (hopefully!) and no rolling shutter
  • Audio controls without adding on extras (I use a Beachtek audio mixer and external mics with my Lumix GH-2)
  • Motorized zoom control
  • Better auto focus. The GH-2 tends to hunt when shooting video so I end up using manual focus.

DSLRs (although not the GH-2) tend to suffer from false image artifacts created by aliasing and moire patterns in the images

On a typical modest screen HDTV (mine is 42 inches) it is very difficult to tell the difference between GH-2 video and XH A1 (once the A1’s noise issues are addressed – I’ll add that item to another post).

DSLRs are a fantastic tool for video, but they are not yet the be-all solution for video. But they are a wonderful additional tool to have for video shooting.

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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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