The Smartphone Camera Sensor: A Simple Introduction

In any digital camera, the sensor is the most important piece of equipment. Without it, we wouldn’t be able to capture any images with our digital cameras. In fact, if you search around the internet, you may notice how many people agree that the sensor is more important than megapixels.

And for good reason. That’s because, if you look at how a smartphone camera works, the sensor is the most vital part of the whole process.

But what exactly is the sensor? How does it work? Does the size impact picture at all? In this article, I will answer these and other questions you may have regarding the smartphone camera sensor.

Before you continue, I recommend you read my article on how smartphone cameras work just so that you have a bit of context around what happens in a phone’s camera before the light that enters the camera reaches the sensor. It might help you understand this article a bit better.

Also, this article will not cover mobile camera depth sensors (also known as depth cameras). For that, you can read this article or learn about smartphone ToF cameras here.

Now, let’s jump right in.

What is the sensor?

A smartphone camera image sensor is a device that takes the light that enters the camera through the lens and produces a digital image from it. The surface of a sensor contains millions of photosites also known as pixels which are responsible for capturing the light. The total number of these light-capturing elements is known as megapixels.

Essentially, a sensor is similar to a film frame. Back in the early days of photography before digital cameras, people used to take photos on a roll of celluloid film. This film was coated with a special chemical that produced an image when it was exposed to light.

When digital photography took over, the old film system was done away with and replaced with an electronic device– the image sensor. When the camera shutter is activated, the sensor is exposed to light and captures it in its photosites until the shutter is deactivated.

The duration for which the shutter remains activated is known as the shutter speed. The longer the shutter is activated, the more light the camera’s sensor can receive. This means your photos can come out looking bright even in low light conditions.

Sounds ideal but there is a downside to messing around with the shutter speed you need to be aware of. Ignoring it can lead to blurry photos.

On a side note: When shooting at slow shutter speeds, it’s very important that you keep your phone steady by using a camera support system such as a tripod. Personally, the Joby range of tripods for mobile phones is one of my favourites because of their small size and versatility. Definitely worth checking out.

By default, smartphone sensors do not see colour. Cameras that produce colour images have a colour filter array placed over the photosites in order to reproduce the colour information in the final digital image. If you look closely at the image above, you will see the red, green, and blue of the CFA.

Sensors play a big role in how a photo turns out in terms of size and quality. A big sensor can fit more and bigger photosites than a small one. That means a smartphone with a big sensor can produce photos of a quality good enough to print and display.

Types of sensors

There are two types of sensors that can be found in digital cameras: the CCD and CMOS sensors. They’re both responsible for converting light into electric signals but they work differently.

The CCD (Charge-Coupled Device) sensor is the more traditional sensor. It’s an analogue device that captures an image in one shot and converts it into one sequence of voltage. A CCD sensor performs well in low light and doesn’t suffer as much from digital noise as a CMOS sensor.

But because the CCD sensor is expensive and uses a lot of power, it is not as popular in smartphone cameras as the CMOS sensor.

CMOS (Complementary Metal-Oxide Semiconductor) uses less power than CCD, which makes it ideal for mobile devices. This type of sensor doesn’t capture an entire image in a single instance but rather captures images in a scanning type of way.

The downside to this is an issue known as the rolling shutter effect, where the image gets skewed when the sensor tries to interpret a moving object. This is an issue that’s especially most problematic when recording video.

Smartphones almost universally use CMOS sensors. Very few use CCD these days.

How does the sensor work?

The sensor as a device is made up of millions of light-catching cavities known as photosites (sometimes referred to as pixels, which can be confusing). When the shutter is activated, these photosites capture light for as long as the sensor remains exposed.

The light photons that are captured by each photosite are interpreted as an electrical signal. The strength of this signal will vary depending on how many photons were captured by the photosite.

The best way to understand this to imagine each photosite/pixel as a bucket catching rainwater. The rain represents the light that enters the camera and is captured by the photosites. If the bucket is filled all the way up to the top, the camera’s processor determines that it’s a white pixel. If the bucket is empty, it’s a black pixel. Anything else in between will be a varied intensity of grey.

Black, white, and grey? What about colour? This is where a colour filter array comes into play.

The colour filter array

To capture images in colour, something known as a colour filter array (CFA) is needed. There are different types of CFAs but the most common is the Bayer Filter Array. It consists of alternating rows of the three primary colours red, green, and blue.

Half the array is made up of green filters, while blue and red each take up a quarter each. The reason for this is because our eyes are naturally more sensitive to green light. So having more green filters on the CFA will produce images that look more natural to our eyes.

Each colour filter covers one photosite and captures light that corresponds with its colour. In other words, the red filter allows red light to be captured, the green filter captures green light and the blue filter allows blue light in. Using the Bayer filter, digital cameras can only capture one primary colour in each photosite. The others are reflected.

This begs the question: if a sensor only receives red, green, and blue colour information, how do digital images pixels have colour information such as yellow, purple, orange, magenta, or any other colour? This is done through an interpolation process known as demosaicing.

Because the Bayer filter is an RGB mosaic, every pixel is missing colour information from the other two colours of the RGB colour combo. Demosaicing happens when the camera’s processor calculates the colour values missing in each pixel by calculating the colour values of neighbouring photosites.

To better understand this process, check out How A Smartphone Camera Processes An Image. This will give you better insight into how a mobile camera’s ISP (image signal processor) creates the final image you can view and share.

Smartphone camera sensor size

The size of the sensor is usually expressed in inches as a fraction such as 1/2.55” or 1/3”. This might seem to indicate the diagonal measurement of the sensor but that is not the case, which can be confusing.

There’s a whole history behind why this method of measurement. It’s quite involved but it pretty much boils down to manufacturers trying to veer consumer attention away from how small the sensors actually were. If you want to do a deep dive into it, I found this post really insightful.

Source: gigazine.net

Different phones have different size sensors but smartphone camera sensors are notoriously small. At some point, the average sensor size on popular high-end smartphone cameras from the likes of Apple and Samsung was 1/2.55”. But recent smartphone camera trends show the size going up, especially in phones with high megapixel cameras.

The phone that holds the record of having the largest sensor to date is the 2014 Panasonic Lumix CM1 that had a 1-inch sensor. In 2019, the biggest sensor was 1/1.7” found on the Huawei P30 Pro and Mate 30 Pro. In 2020, the Huawei P40 Pro+ has the largest sensor on the market at 1/1.28”.

Smartphones with the biggest sensors of all time

 * Pixel size after pixel binning

How big is it compared to full-frame DSLR

Smartphone camera sensors have definitely increased in size over the years, and indeed have reached some amazing heights but they still pale in comparison to full-frame sensors the likes of which are found in DSLR cameras.

Full-frame image sensors are 35mm in diameter, the same size as old school celluloid film. Hence the name full-frame. There are many smaller frame sensors (known as crop sensors), and smartphone sensors are found at the tail end of the spectrum.

Source: techspot.com

A full-frame 35mm sensor measures 864mm² while a 1/1.7” smartphone sensor only measures 43mm². That means the once-praised Huawei P30 Pro’s sensor, for example, is 20 times smaller than a full-frame DSLR sensor. That’s a lot!

How does the size of a sensor impact photos?

The size of the sensor definitely has a huge impact on the quality of the images that a camera can produce. It’s one of the important factors that contribute to what makes a mobile phone photo look amazing.

Firstly, the bigger the sensor, the bigger the photosites. Big photosites mean the sensor gets to capture more light. This is especially useful in situations where the lighting is poor. You’re less likely to have issues with digital noise depending on how big the photosites are.

Also, a big sensor can pack more megapixels. The more megapixels a smartphone camera has, the higher the resolution of its images will be. If you’re into printing large prints of your mobile photos, then this is a plus.

Unfortunately, you can’t have it both ways. You can’t pack a lot of large photosites on a sensor, not on smartphone camera sensors at least. Due to the physical size of the sensor being fixed, the more pixels a camera has, the smaller they become.

This poses a challenge on mobile cameras. Because they’re so small in size, the photosites on a smartphone camera sensor are very tiny. This puts smartphone cameras at a disadvantage when it comes to how much light their sensors can capture.

Small photosites don’t perform well when there isn’t enough lighting. You’d have to increase the camera’s ISO by quite a bit to get the brightness of images captured on a sensor with small photosites to match that of an image from a sensor with larger photosites.

Conclusion

When looking to buy a new phone, a lot of people simply go for the phone with the most megapixel camera and believe it’s the best. And, honestly, there’s nothing wrong with that.

However, even though having lots of megapixels can give you prints with fine detail, don’t fall for the smartphone companies’ marketing hype and believe more megapixels means a better quality camera. In reality, the truth about megapixels is something totally different.

If you’re about to buy a new smartphone and the main camera is your biggest priority, just be sure to also lookout for a phone that has a camera with a big sensor. 

This article first appeared on my other blog thesmartphonephotographer.com. Check it out!