Definition: Image Sensor

Image Sensors are the nitty gritty end of the digital camera.

Basically the digital version of film. Film camera’s took 35mm film, in digital camera’s the ‘digital film’ is called an image sensor.

But there are lots of different types (or sizes) of image sensors, and this can be important when choosing a digital camera.

What is an Image Sensor:

An Image Sensor is a device that converts an optical image to an electric signal. It is used mostly in digital cameras and other imaging devices. It is a set of charge-coupled devices (CCD) or CMOS sensors such as active-pixel sensors. (from Wiki)

So in other words a camera’s image sensor receives the light coming through the lens into the camera, and turns that light into an image. That’s a very simplified version of what actually happens and this is achieved in conjunction with the camera’s processor, but it helps in understanding the part the image sensor plays in image creation.

Why is the Image Sensor important?

Image sensors come in different sizes, and which size you have will affect the image quality of the photographs taken with it. Full frame sensors are the same size as 35mm film, but they are part of very expensive professional DSLR camera’s!

As always lets look at the simple description:

Compact digital cameras (Point and Shoot) generally have a 1/2.5″ sensor (3% of full frame sensor)

Entry level/mid range DSLR cameras generally have an APS-C sensor (around 40% of full frame sensor)

Both of these are called cropped sensors because they are smaller than a full frame (35mm) sensor. So if you used the same lens on a cropped sensor camera as a full frame camera taking the same image, the cropped sensor would make the image seem enlarged.

This doesn’t really give you a sense of the difference between the two, but see if the image below helps:

image sensor sizes

image sensor sizes

What’s the difference?

In the image above I’ve compared the sensor sizes of a full frame DSLR, entry/mid range DSLR, a normal point and shoot camera and an iPhone camera. I’ve also given examples of cameras that use these sensors.

So DSLR’s have much larger sensors than those in compact digital camera’s, therefore they have greater image quality.

Some Point and Shoots can be 12 megapixel’s, the same as some DSLR’s (or more) so how come they aren’t as good when it come to image quality?

This is where the size of the image sensor comes in. Take two 12 megapixel camera’s – one compact and one DSLR.

The manufacturer is trying to fit the same amount of pixels on the smaller compact sensor as they are on the much larger DSLR sensor. And this is the important part, the compact’s pixels have to be smaller to fit the 12 million pixels on the sensor.

Pixels are light-sensitive elements on the image sensor which record the light that hits them.

Say for instance you have a thimble and a mug, if it was raining and you put them outside which one would be able to hold the most rain?

Do you see what I mean?

The smaller 12 megapixel sensor in the point and shoot reduces the amount of sensitivity and light that the pixels can capture as opposed to the larger pixels in the larger 12 megapixel DSLR sensor, and that works out as a loss in image quality.


To be honest, digital camera’s are moving on all the time whichever type they may be. And it’s getting increasingly harder to notice the difference in image quality between images taken on different camera’s like a point and shoot and a DSLR.

For me, I have a Canon 450d DSLR and I love the colour and depth I get in my images and the creativity it lets me have.

But I hope you see now that it’s not all about who’s got the most megapixels in their camera. Image quality is about a lot more than that – I’ve only scrapped the surface here!


Comments 10

  1. Pingback: Definition: Image Sensor

  2. Ah, THANK YOU for writing this! I’ve been dabbling in digital photography for a while now – long enough that I’ve figured out most of the jargon, except for what the heck a sensor is! As someone who really should know by now, I always felt too embarrassed to ask but everything makes a lot more sense now 🙂

  3. Post

    Glad you liked it Danielle, I’ve always felt that the ‘simple’ questions are always the hardest to find an answer to, and that’s what I try to do here at The Photographer Blog – answer them…

  4. thanks for your information, but i have one more doubt about low light sensor….? then where it is used.? what is the difference between CCD and CMOS sensors…. and also i wanna details about frame rate…..

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