Mordy Gilden has engineering in his blood. Tech Tricks and Tweaks is a biweekly column in which Mordy seeks out ways to hack, tweak, and otherwise just find interesting new uses for tech, often in ways not originally intended. Oh yeah, and blog about it.
When it comes to digital video cameras, traditionally high-end models sport a technology called CCD (Charge Coupled Device), whereas cell phones and other small consumer devices usually use a different tech, called CMOS (Complimentary Metal-Oxide Semiconductor — trying saying that 10 times fast). While there are many limitations to CMOS, their lower cost to produce as well as significantly lower electricity consumption make them ideal for small portable consumer devices. CMOS sensors used to be considered inferior in image quality, but they really have gotten quite a bit better over the years and have even found themselves in some prosumer grade video equipment. However, CMOS sensors do suffer from some technical limitations, and one of the most well known is called rolling shutter.
Rolling shutter refers to the way the chip reads the image one line at a time. In other words, the top of the image is recorded before the bottom is, instead of taking the entire image at once like a CCD does.
What does this mean? Well, if there is very little movement in the video, not much. Record a video out the side of a moving car window though and you notice an obvious skew/slant of the landscape.
Note the top of the train is trailing behind the bottom. Image courtesy of Sony
As you can see in the image above taken with a CMOS chip, the train is slanted, but the fence in front of it isn’t. That’s because the train car was moving towards the left and as the image is recorded line by line, the bottom of the train was further along than when we started at the top.
If you record a video with a strobe light, camera flash, or lightning bolt, you could have the white flash exposure on only a portion of your image.
On the image to the left, you can see that the exposure of the sky changes in mid-frame. That’s because the frame was recorded, line by line, as the lightning flashed. By the time the sensor was half-way through saving the image, the lighting had already changed.
Because CMOS-based DSLR video has become rather popular in music videos and independent film, avoiding a shot like this has become quite the challenge.
While a director of photography might consider this a nightmare, you can actually use this to your advantage to record some really wild and fascinating stuff.
Imagine recording a guitar string being plucked. The vibration of the string means that as the rolling shutter records, part of the string will be in a different location than the rest of it.
[yframe url=’http://www.youtube.com/watch?v=TKF6nFzpHBU’]
Or try recording a fan blade, or better yet the spinning rotors of an airplane.
[yframe url=’http://www.youtube.com/watch?v=LVwmtwZLG88′]
Keep your phone or other CMOS device as still as you can and try recording a fast-moving object. The results may just astound you. Get on out there and film your own rolling shutter oddity!
And share it with us, if you do. We love oddities.
[…] correction and shooting certain types of patterns messy, not to mention the wobble and skew from a rolling shutter CMOS sensor. In the wrong hands, DSLR footage can look downright ugly. But in the hands of a talented […]