What is Rolling Shutter?
What is Rolling Shutter?
The ability to record video with a digital camera is nothing new, but it brings a unique problem that affects video, commonly known as the “Rolling Shutter” effect. We understand this problem and how to minimize it. When talking about cameras and shutters, most people think of the shutter on a traditional DSLR that closes quickly to prevent excess light from getting on the film, giving the image the way it should. But with the arrival of digital, image sensors have replaced film, and this has brought a series of advantages and disadvantages – some of which are especially noticeable when we shoot video.
CMOS OR CCD?
There are two main types of sensors: CMOS and CCD, each with a different approach when it comes to capturing light. CMOS sensors generally use the Rolling Shutter method which reads exposure information per line. This means that information is read line by line down the sensor for the duration of the exposure. When the entire sensor collects enough light, the rows of pixels once again stop sequentially, thereby giving a “rolling” appearance.
Meanwhile, CCD sensors tend to use the Global Shutter method, in which all sensors are turned on and off together. There is no physical shutter covering the sensor, the whole process is done in real-time. Both types of sensors have their own drawbacks of artifacts. CCD sensors tend to experience vertical smearing in bright light sources, while CMOS tend to have skew, wobble, and partial exposure due to the way the sensor reads data.
PROBLEMS AND SOLUTIONS:
Vertical Smearing Vertical smearing occurs when the image light reaches the sensor’s maximum level, thereby producing the appearance of a mass of white light. This happens when photographing the sun, or when the object is dark overall and one or two light sources are brighter than the others. This artifact problem only exists in CCD sensors. This problem is easy to solve because you can set the exposure with a higher aperture. Then you can also add additional lights or use a neutral density filter to adjust the contrast differences in objects. Another option is to replace it with a CMOR sensor camera, because this sensor is immune to smearing effects.
Skew and Wobble
The skew in the video is the same as in the photo – the subject appears to be leaning to one side. This is only noticeable when panning or with slow frame rates, but can be a serious problem if you want to stitch into other footage. The wobble effect is perhaps the most noticeable effect, the lines of the image become erratic as the camera moves. You could think of it as the image being skewed in many directions, or what you call the “jello effect,” but what’s happening is that instead of tilting to one side or the other, the image is stretched as the camera moves down, and squashed as the camera moves up. The wobble and skew effects are a more serious problem that can’t be fixed completely without some post-shoot editing, but luckily most video editing software has filters that can fix it. What you can do to reduce the effect is to shoot using a slow shutter speed and, specifically, slow down the panning shot. If not, try to make changes to the video composition so that the problem object is not in the frame, or try using a depth of field to hide the skew effect.
Partial exposure is a final problem with CMOS sensors, and something you might not see often in footage. This problem occurs when you get a sudden flash that changes the exposure. In this case, you’ll see black bands moving under the video briefly because the flash of light illuminates the image faster than the shutter moves. The same thing can happen when recording in an area with low refresh rate fluorescent lighting. The intensity of the light can vary for about 1/60 second, and this will form a line in the recording. The solution is to adjust the shutter speed to match the refresh rate of the lights. In NTSC regions use 1/60 second (or multiples thereof) to equate to a refresh rate of 60 Hz. In PAL areas use a shutter speed of 1/50 to match the refresh rate of 50 Hz. If not, simply turn off the lamp in question and replace it with a suitable one. Thus, recording videos with a digital camera sometimes gives unpredictable results, but by knowing the problems and solutions, or minimizing them, you can make videos according to your taste.