How Digital Camera CCDs and CMOSs Work :: Digital Photo Secrets

How Digital Camera CCDs and CMOSs Work

by David Peterson 1 comment

Today I'm going to go a bit geeky on you. I think it's important to not only understand how photography works in terms of apertures, shutter speeds, and ISOs, but to also understand how the innards of your camera work. Image sensors are a good place to start, since most people know they exist, but don't know why!


Simply put, rather than film, a digital camera has a sensor that converts light into electrical charges. The image sensor used in most digital cameras is a CCD (charge coupled device). Some cameras use CMOS (complementary metal oxide semiconductor) technology instead. Regardless, both types of image sensors convert light into electrons. Another way to think about these sensors is as a 2-D array of thousands or millions of tiny solar cells. Fun, huh?

Now for the nitty-gritty. Once the sensor converts the light received into electrons, it reads the value (the accumulated charge) of each cell in the image. This is where the differences between CCD and CMOS kick in:

A CCD sensor is essentially made from a slab of silicon. It transports the charge across the chip and reads it at one corner of the array. Once that happens, an ADC, analog-to-digital converter, turns each pixel's value into a digital value by measuring the amount of charge at each photosite and converting that measurement to binary form. I told you I was going geeky on you today! Stick with it though. We're almost there.

A CMOS sensor uses several transistors at each pixel to amplify and move the charge using more traditional wires.

As you might imagine, the differences between these sensors result in a number of pros and cons:

A CCD sensor:

Here are the comparisons:

  • The benefit of CCD sensors is that they create high-quality, low-noise images. Alternatively, CMOS sensors are generally more susceptible to noise.
<li>The light sensitivity of a CMOS chip is lower because each pixel on a CMOS sensor has several transistors located next to it. Many of the photons hit the transistors instead of the photodiode.</li>

<li>CMOS have great battery life. CMOS sensors usually consume less power than CCDs, which use a process that consumes more power. In fact, CCDs can consume as much as 100 times more power than a CMOS sensor.</li>

<li>Lastly, CCD sensors have been mass produced for a longer period of time than CMOS sensors. They tend to have higher quality pixels, and more of them, which we know you're a fan of.

One of the reasons for the falling prices of DSLRs was the introduction of the CMOS sensors. Although CCD sensors have been around longer, CMOS sensors are much less expensive to manufacture.

As you can see, CCD sensors tend to be used in cameras that focus on high-quality images with lots of pixels and excellent light sensitivity. Whereas, a CMOS sensor often has lower quality, lower resolution and lower sensitivity. That said, CMOS sensors are now improving to the point where they are reaching equality with CCD sensors.

At the end of the day, and although there are numerous differences between the two, both sensors play the same role in the camera by turning light into electricity. For our purposes here, you can think of them as nearly identical devices.

A word about cleaning these imaging sensors. Although we call it cleaning, in reality the sensor itself is never really cleaned. Instead, the low pass filter mounted in front of the sensor is what's cleaned. If you believe you're up for the task and boast a good mechanical aptitude, you may well be capable of accomplishing this task successfully. However, if you don't do it right, you can mess up your camera. If you're not comfortable with this task, please do yourself a favor and take your camera to a professional.

Simplicity and Speed

We all know that more and more people are relying on their phones as their back up, or even main, camera. Thanks to the CMOS sensor technology, imaging devices are becoming smaller, more powerful, and more versatile.

"Sensor speed by itself may not be something that people can see the great value in, but sensor speed together with processor power allows the CMOS sensor to realize features that you likely couldn't do with CCD," says Mark Weir, senior manager of technology for Sony Electronics. "When you can capture at very high rates of speed, all of a sudden, capturing a 'picture' is really about capturing many pictures. It becomes a question of 'Now that I can capture many images every time I want to take just one, how can I enhance what I've got?'"

The fact that CMOS sensors are capable of performing these tasks - image-processing, such as analog-to-digital conversion and noise reduction, gives the sensor technology a speed advantage over CCD sensors.

I'll let Chuck Westfall of Cannon explain the CMOS over CCD advantage in his words. "You can't get the data off the [CCD sensor] quickly enough, because there is a limit to the number of readout channels. With a CCD sensor, you're usually limited to two readout channels, and in the case of CMOS, it's basically up to the designer as to how many channels they want to put on there. We've got a 16-channel readout on the [CMOS-based Canon EOS] 1D X, for example. We have an 8-channel readout on some of our other cameras. And even in the compact cameras, they don't specify, but I'm pretty sure it's way more than two."

It's likely your camera has a CMOS sensor. If you're wondering, check your manual. Mostly, I wanted you to understand the differences because it makes for great dinner party conversations!

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Comments

  1. Anthony Orr says:

    Hi Cleaned my low pass filter recently bought the single use cleaning pads for a couple of quid from Hong Cong .Simple job excellent results
    Anthony

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Difficulty:
Beginner
Length:
7 minutes
About David Peterson
David Peterson is the creator of Digital Photo Secrets, and the Photography Dash and loves teaching photography to fellow photographers all around the world. You can follow him on Twitter at @dphotosecrets or on Google+.