Share

Why Image Sensors Affect Video Quality

The heart of every modern video camera is the image sensor, a device that converts the light and color spectrum into electrical signals for the camera to convert into digital information. Since the first CCD sensor arrived in the mid-1970s, solid state sensors have been used in all video cameras.

The first sensors for visible light were video camera tubes, which date back to the 1930s. By the early 1990s, camera tubes had been replaced by solid-state CCD image sensors. The basis for sensors was metal oxide semiconductor (MOS) technology, which originated from the invention of the MOSFET by Mohamed Atalla and Dawon Kahng at Bell Labs in 1959.

Since then, sensors have steadily improved. In June, 2022, Samsung Electronics announced it had created a 200-million-pixel image sensor in a two by 1.4-inch lens.

In modern video cameras, there are now two main types of video sensors — the CCD, or charge-coupled device, and the CMOS, an active pixel sensor. Both CCD and CMOS sensors are based on metal oxide semiconductor technology. CCDs are based on MOS capacitors and CMOS sensors are based on MOSFET, which are MOS field effect transistors.

Regardless of the type of digital sensor used in a camera, all are small flat silicon panels with the same function — to capture light and convert it into electrical signals.

CMOS sensors were traditionally used in lower-cost, battery-operated consumer video and still cameras having low power consumption requirements. CCD sensors were normally used for high-end broadcast quality video cameras. In recent years, however, both types of sensors have improved so much, they can now be found on all types of video cameras at all price points.

There is a minor technical difference between CCD and CMOS sensors. With a CCD image sensor, pixels capture light and move it toward the edge of the sensor, where it’s converted into a digital signal. In a CMOS device, the light is converted in the pixel itself.

This subtle difference between the two types of sensors comes in power requirements. Since the signal doesn’t have to be transported to the edge of the chip for conversion, a CMOS sensor requires less electricity to operate.

In reviewing most camera specifications, two sets of numerical measurements are listed for the sensor: gross pixel count and effective pixel count. The gross count refers to the total number of pixels on the sensor. The effective number tells how many pixels are actually used when shooting video. Ignore the gross count and go only by effective pixel counts, which are needed to determine the actual resolution for a video camera.

The effective pixel count also helps cut through the camera’s marketing promotion. A sensor with fewer effective pixels often means interpolation is being used. This is a technique that makes an approximation of a pixel’s intensity based on the values of pixels surrounding them. Don’t let interpolation specs fool you. Use effective pixel count as the true resolution of the camera.

Another factor that determines image quality is the physical size of the sensor. Larger sensors capture more light than smaller ones, even if they have fewer pixels. The reason is these pixels, while fewer in number, are larger and can capture more light.

It is best to buy a camera with a larger image sensor with fewer pixels than a smaller sensor with more pixels. A larger sensor also improves a camera’s low light performance. An image sensor with more surface area means higher performance in picking up images in dim light and offers greater signal-to-noise performance.

Crop factor is another consideration in choosing a video camera. This refers to the field of view for an image when comparing smaller sensors against a full frame sensor. This is why an APS-C-sized sensor has a smaller field of view than the wider field on a camera with a full-frame sensor.

Also, since a smaller sensor has a greater crop factor, the focal length of a shot must be shorter to achieve the same look as with a larger sensor. In this case, the camera operator must move the camera with the smaller sensor further away from the subject to attain the same field of view. This increases the depth of field, meaning the smaller the sensor, the more difficult it can be to achieve a shallow depth of field look.

Image sensor size, pixel density, crop factor and signal-to-noise in a camera are all important factors that determine the image quality of a video camera. Consider your workflow and type of project in choosing the correct camera for the job.

Writer at Broadcast Beat
Frank Beacham is a New York-based writer, director and producer who works in print, radio, television, film and theatre.

Beacham has served as a staff reporter and editor for United Press International, the Miami Herald, Gannett Newspapers and Post-Newsweek. His articles have appeared in the Los Angeles Times, Washington Post, the Village Voice and The Oxford American.

Beacham’s books, Whitewash: A Southern Journey through Music, Mayhem & Murder and The Whole World Was Watching; My Life Under the Media
Microscope are currently in publication. Two of his stories are currently being developed for television.

In 1985, Beacham teamed with Orson Welles over a six month period to develop a one-man television special. Orson Welles Solo was canceled after Mr. Welles died on the day principal photography was to begin.

In 1999, Frank Beacham was executive producer of Tim Robbins’ Touchstone feature film, Cradle Will Rock. His play, Maverick, about video with Orson Welles, was staged off-Broadway in New York City in 2019.
Frank Beacham
Broadcast Beat - Production Industry Resource