Camera Sensors

Camera sensors are the crucial part of a digital camera. It is also a vastly misunderstood part of the camera. When you go to buy a camera from a store, the ads and salesmen often talk about megapixels and zoom. But it’s the image sensor that determines the quality of each pixel. For the next few minutes, I’ll go into the specifics of how camera sensors work, and why they are so important.

Digital pictures are made up of pixels. Each pixel is made up of a color defined by three primary colors, red, green, and blue. When these three colors combine, they make up all the colors we see in digital images. This is called the RGB color mode.

Camera’s image sensors take light, and convert it into this RGB color spectrum as a digital image.

A sensor is simply a grid of cavities that light can enter, and be measured. When your camera takes a picture, it removes a cover (shutter) from in front of the image sensor letting light (the white balls) enter and collect inside the cavities. Then the shutter closes. Each cavity’s content’s are measured and then recorded to a file as a pixel.

The result in this circumstance will be a black and white image. This is because there is no way to distinguish between colors. It is only recording light intensity. So, in order to create a color image we need to be able to distinguish between different light colors.

In the video below, each cavity has a filter above it. These filters take the white light coming into the camera and filters out the red green and blue. Notice that there are two cavities filtering out all colors except green for every one filtering out all red and blue. This is because our eyes are more sensitive to green light then to blue or red light, so the camera sensor simulates our eyes sensitivity to green by adding extra green cavities.

Now, there is still an issue because in the example, each pixel is made up of red, green, or blue, not a combination of the three. So instead of generating a pixel from each cavity, we need to generate a pixel from a grid of four cavities. This process is called Bayer Demosacing – each pixel being made up of mini 2×2 grids of red, green and blue cavities. This allows us to generate proper colors for each pixel, but we also cut the image size (megapixels) in half. To solve this, the camera shares some of the cavities between pixels. So now, the camera creates nine pixels from where there was originally only four.

This is essentially how cameras convert the light you see in the viewfinder into a digital image. So what makes this so important? Why does it make such a difference within a camera? Well, the first thing we need to realize is that we’ve been talking about how camera sensors are designed to work, not how they actually work. Electronics are never perfect, and noise (small malfunctions in the signal) infiltrate the image. You may see the result of noise in pictures especially of dark environments.

Low Noise

High Noise

This is because somewhere within the circuit from cavity to image, it mistakenly added red, blue, green or any combination of the three to the image. This happens more when you have low quality electronics and wiring.  Cheap cameras will have a tendency to have more noise in images. It also happens more when you have a smaller image sensor. This is because larger sensors typically have larger light capacity than smaller sensors do. The more light you have, the stronger the signal will be, the less noise will affect the image. In other words, the higher the numbers are, the less a change of one or two will affect the final number. This is a huge advantage of SLR’s over your regular point and shoot cameras because SLR’s have significantly larger sensors than you typical point and shoot. Larger sensors also affect other things about the image like depth of field and focal length, but I’ll talk about those in another post.

The point is, you can have a very noisy 12 megapixel camera for $250 or you could have a 9 megapixel SLR for $500 and be much happier with the image results even though the image will be a little smaller. It all has to do with the image sensor, and making sure that you get the most quality for every pixel, rather than just having more pixels.

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