Shutter
03:19:00
The term shutter comes from still photography where it describes a mechanical ‘door’ between camera lens and the film.
When a photo is taken, the door opens for an instant the film is exposed to the incoming light. The speed at which the shutter opens and closes can be varied - the faster the speed, the shorter the period of time the shutter is open and less light falls on the film.
Shutter speed is measured in fractions of a second. A speed of 1/60 second means that the shutter is open for one sixtieth of a second. A speed of 1/500 is faster, and 1/10000 is very fast indeed.
Video camera shutters work quite differently from still film camera shutters but the result is basically the same. Technical difference is that, rather than using a mechanical device, the shutter speed is adjusted electronically varying the amount of time the CCD is allowed to build a charge.
The shutter opens and closes once for each frame of video; that is, 25 times per second for PAL and 30 times per second for NTSC. Thus, if a camera has its shutter set to 1/60, each frame will be exposed for 1/60 second.
If the speed is increased to 1/120, each frame will be exposed for 1/120 of a second. The shutter speed does not affect the frame rate, which is completely separate and in most cases always stays the same.
The main effect of higher shutter speed is that individual frames appear sharper due to the minimisation of motion blur. Motion blur occurs when the subject moves within the frame while the shutter is open. The less time the shutter is open (i.e. the faster the shutter speed), less movement will take place.
A side-effect of higher shutter speeds is that, the movement appears jerkier. This is because motion blur tends to smooth consecutive frames together.
Instead of shutter speed in seconds, rotating disc shutters use shutter angles.
The larger the shutter angle, the more the light passes through. The relationship is designed to be similar to shutter speeds – half or doubling the angle will half or double the light.
Most film movie cameras have shutter angles from 0o to 180o. The most common shutter angle used is 180o, which gives us the ‘film look’. This corresponds to exactly half the duration that each frame will take in one second.
Example: If a movie is shot at 24 frames per second, each frame takes about 0.0417 seconds. A 180o shutter will let light pass through for half this time period, which equates to 0.0208 seconds, which is roughly 1/48 seconds.
When cameras don’t allow you to choose 1/48 seconds exactly, can approximate the same effect with a shutter speed of 1/50 seconds for 24 fps.
Usually, CMOS sensors use rolling shutter systems while CCDs use global shutters. However, this is not universally common. There are CMOS sensors with global shutters and CCD sensors with rolling shutters.
CMOS sensor technology used by most of the digital cameras as a CCD alternative because of its ability to image with less power for high frame rates even when using large, high-resolution sensors. CMOS sensors require less specialized manufacturing and because of this, it often cost a fraction of the price of CCD sensors of the same size.
Because of the lower cost and ease of use, CMOS sensors have taken over the consumer imaging market. Since 2009, the market share for CCDs has been declining steadily as CMOS sensors have been standard sensor not only for cameras but also in almost all of our personal devices, from cell phones to computers.
CMOS stands for Complementary Metal Oxide Semiconductor, which actually refers to the process for making CMOS sensors.
Active Pixel Sensors read all of their pixels linearly from top left to bottom right while the shutter is open. The pixels don’t store any charge, they simply read how much light is hitting that pixel at the exact moment and convert that into an electrical signal.
A rolling shutter (as opposed to a global shutter) is always active and “rolling” through pixels from top to bottom. This can result in the now-familiar motion artefacts often referred to as moire effect.
CCD stands for Charge Coupled Device.
The pixels in a CCD store their charge until it has been depleted. A camera that has a CCD sensor almost always also has a shutter system, which can be electronic or mechanical. While this shutter system is “closed” (or off) the pixels can still be read because they store their charges. While the shutter is open, the sensor is collecting light and after the shutter closes, the AFE (Analog Front End) reads the pixel charges one by one and dumps any excess charge and gets the pixels ready for the next frame.
In other words, the CCD captures the entire image at the same time and then reads the information after the capture is completed rather than reading top to bottom during the exposure.
In today’s modern Digital cameras, there are two types of electronic shutters:
- Global Shutter
- Rolling Shutter
Global Shutter
Film cameras have rotating discs and each frame is exposed whole. Such a frame is called a progressive frame.
Global shutter captures a frame completely like a motion picture shutter disc operate, so all the pixels are read at the same point of time, so we have a whole frame.
Black magic 4k cinema camera, Sony F-55, feature global shutter.
Rolling Shutter
A rolling shutter is an electronic system that scans lines (rows) of pixels, similar to television scanning. Rolling shutter sometimes creates an artefact while recording moving images, or camera panning. This is because of the amount of time taken to read pixels from an imaging sensor which is still exposed to light.
If the subject changes during this readout, the remaining rows of pixels in the image will be exposed to the changing light.
As a result, objects moving horizontally may appear to be jerked somewhat diagonally by an amount proportional to the relationship between the speed of motion and the readout timing of the sensor. Some cameras, such as the Arri Alexa, use rolling shutter but have extremely brief readout time, minimising (though not entirely removing) the problem.
Red cameras offer motion mount shutter to solve the artefacts created pixel read out.
Rolling shutter artefacts - very common in CMOS-sensor cameras, most notably the HD video-capable DSLR still cameras.
CJ Rajkumar
Author/Cinematographer
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