Sensors matter because everything you do, when choosing settings for each photo, is about getting the right amount of light to hit the sensor - and so expose the image correctly.
In pre-digital days, the photo was captured on a piece of film, which was exposed to light when the shot was taken, creating the picture as a ‘negative’ pattern of light and dark tones. From here you could then print a positive version of your image – or have a processing lab print it for you.
Today, the image is captured on the camera’s sensor electronically, and that ‘data’ is then transferred to the memory card as an image. All digital cameras have a sensor, including phone cameras, and they vary in size and quality depending on the type of device.
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This is what a sensor looks like, but you will only be able to see it when you take the lens off if you're using a mirrorless camera, as on DSLRs the sensor is hidden behind the mirror mechanism.
Be very careful with your sensor as it attracts dust easily, and this will cause 'dust spots' on your images. Sensors are not easy to clean, and professional cleaning is usually recommended, which isn't cheap - so try not to change your lens in a dusty environment. Do it as quickly as you can, holding your camera towards the ground, to minimise the chance of dust getting onto the sensor.
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Sensors and image quality
Quality is relevant when it comes to sensors, as the sensor is made up of millions of ‘pixels’ which capture the image. So if your camera is described as being ‘20mp’ or ‘20 megapixel’ for example, this means it has 20 million pixels on its sensor.
This relates to the amount of information which the sensor is able to capture, and hence how large the final image could be printed, or displayed digitally, if you wanted to. It also affects how small a section you could crop out of an image without the quality deteriorating too much. Without going into too much technical detail, it’s probably enough to say that most modern cameras have far more pixels on their sensor than you could ever need!
Sensor sizes
Different types of cameras have different sized sensors, depending on the market they are aimed at. The largest sensor in the type of mainstream cameras typically available today is what is known as a ‘Full Frame’ sensor, and this is exactly the same size as the negative on a 35mm film strip (36mm wide x 24mm deep) - which you may remember, depending on your age! In case you wondered, the 35mm measurement relates to the external width of the film strip.
The most common sensor size in consumer-level DSLR or mirrorless cameras today is known as ‘APS-C’ and then the next most common is ‘Micro Four Thirds’ (MFT). Some high end compact and bridge cameras now use what are known as '1” sensors', or even MFT in a few cases - but the majority still have the very small 1/2.3” sensors.
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This diagram shows the relative sizes of each of the sensors mentioned above. You may notice that on the smallest two, their name bears no resemblance to their size, and this is an anomaly that dates back to the early days of video cameras!
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​​​​​​​​​​Both APS-C sensors and MFT sensors give excellent image quality, and you can comfortably enlarge an image from one of these cameras to as big as you could possibly want to put on the wall in your house! So unless you are planning to take photography up professionally, a full frame sensor is not actually necessary - but it is of course also a great option, if you don't mind the extra cost, the bigger and heavier lenses, and having much larger image files to store and manage.
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