CMOS Sensors Explained


The early days of digital cameras were dominated by CCD sensors. CCD stands for Charged Coupled Device and these types of sensors are very good. However, over time the majority of camera manufacturers shifted design to incorporate CMOS sensors. CMOS stands for Complementary Metal Oxide Semiconductor, rather a mouthful and why we use the acronym CMOS instead. 

The majority of consumer and prosumer cameras use CMOS sensors these days. But why is that, how did one technology surpass the other to become the dominant force? Today we are going to look at how CMOS sensors work and why they are in most of the devices you use. 

Photo by chuttersnap

How Sensors Work: In Simple Terms.

The goal of an image sensor is to convert the light focused by our lens into electrical signals that can be processed by a CPU into a digital image. We all understand the term pixels. It's fundamental to the image quality of our cameras. But what is a pixel in relation to a camera sensor?

Each pixel is an individual photodetector. Above every one is a tiny lens to focus the light onto that photosensor. Think of them as buckets of light. The bigger that bucket, or pixel, the more light it can gather. In order for us to have a colour image, we have to capture the light in the red, green and blue wavelengths and so each sensor has an equal amount of pixels devoted to each of these colours. Once that pixel has collected the correct amount of light, it is converted in an electronic signal. This where CCD and CMOS sensors begin to differ. 

In a CCD sensor, that analogue electronic signal is sent to the edge of the sensor, where a processor converts it to a digital code. This happens for every single pixel on the sensor simultaneously. 

The CMOS sensor differs because rather than sending the analogue electronic signal to the edge of the sensor, each individual photo-site converts the signal to digital before sending it out to be processed to an image. There are inherent advantages and disadvantages to both.

How CMOS Became The Standard.

Thos of you of a certain age will remember the Betamax v VHS technology war. Both were consumer video recording systems based on tapes. On paper, Betamax was technically superior yet VHS ran out to be the dominant format. The situation is not dissimilar between CCD and CMOS.

Technically CCD sensors create a superior image. This is because they do not need the extra electronic circuitry on each individual sensor. In turn, this means they can potentially have lower noise and higher dynamic range compared to a similar CMOS sensor. Because CMOS sensors have circuits on each pixel, they are more susceptible to noise, caused by electronic interference between those circuits. They are also much more prone to rolling shutter issues in both video and when panning is stills photography. 

However, there are two distinct advantages that have given CMOS sensors the lead in prosumer photography. Firstly they are significantly cheaper to produce compared to a CCD sensor. Secondly, they consume less power. Think back to the early days of digital photography and you will remember just how few shots you could get out of a single battery. Sometimes less than 100 shots before you needed to change. Even with modern battery advances a CCD sensor will still consume more battery life than a CMOS. This is a particular consideration when shooting video or time-lapses. 

CMOS The Current Choice

However, because CMOS has become the standard in most imaging devices, much more money has been spent on research and development for that technology. These days the individual circuits tend to be incorporated inside each pixel rather than adjacent to it. This helps with noise reduction. It also allows many more pixels to be added to the sensor. 

Rolling shutter issues have also been greatly reduced by increasing the speed at which information is sent from the sensor to the camera’s processor. 

Advantages of CMOS

  • Significantly cheaper to manufacture
  • Consume less power giving longer battery life.
  • De facto standard in consumer cameras means more R&D money

Disadvantages of CMOS

  • Rolling shutter is more prominent.
  • They are more prone to noise interference and hence grainier images at high ISO

CMOS will be the sensor standard for the foreseeable future. But like any current technology, it’s unlikely to be here forever. In research laboratories across the photographic world, engineers are no doubt working on the next generation sensor technology. With it, we will see even higher megapixel counts, much lower noise at higher ISOs and lower power consumption. In ten years time, we could well be reading an article on how this new tech replaced the CMOS. 

About Author

Jason has more than 35 years of experience as a professional photographer, videographer and stock shooter. You can get to know him better here.

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