What is... Pixels

A guide to understanding pixels in computing

Author:  Tina Landers

A pixel (short for picture element - pix being short for picture) has many definitions and meanings depending on the context it is being used. There are pixels in digital images, there are pixels in your computer monitor, there are pixels in your digital camera and there are pixels in your television. In each of them a pixel is defined differently. For the sake of us everyday computer users, we're going to generalise and try to understand the pixel rather than its technicalities.

A pixel is generally considered the smallest part of a digital image. Think of a pixel as a tiny spot of colour. An image is made up of many pixels (or tiny spots of colour). Pixels are usually square or rectangular, though you'd have to magnify one many times to notice. The number of pixels used to make-up an image is generally referred to as the image's "Resolution" (though technically it's more complex than that). Higher resolution = larger number of pixels used = greater detail in the complete image.

It's very similar to "painting by squares" which we used to do when we were kids. Remember? A picture was divided into numbered squares which you coloured-in guided by the number in each square. When you completed colouring-in all the squares and held the picture slightly away, it all blended-in and produced a work of art. The concept of pixels and images is the same.

Pixels can also refer to an image's size. For example, depending on the "resolution" that you have set your computer screen, you may be looking at an image that is 1024 pixels wide x 768 pixels high, or you may have your screen resolution set to 800 pixels wide x 600 pixels high. If you find it all a bit confusing, think of the tiny spot of colour - 800 spots wide x 600 spots high.

That, however, is not the end of the complexity (or discussion) for us non-technical people. You see, a "Pixel" has no standard defined size! So when we look at or print two different images of say 2400 pixel width, we may well get two different sized images. OK, the difference in sizes may be marginal but, nevertheless, they are different. If you take a close look at a computer monitor's packaging, you might see something in the specification referring to a "dot pitch" - that is the size of a pixel used by that manufacturer.

My very first digital camera produced 640 pixels x 480 pixels photographs. It was classed as a .3 MegaPixel camera (640 pixels multiplied by 480 pixels = a total of 307,200 pixels). These days digital cameras come with far greater resolutions (or millions of pixels). A Megapixel is 1 million pixels. My current camera is a 7 MegaPixel camera. It can produce a photograph 3072 pixels x 2304 pixels (3072 x 2307 = just over 7 million pixels). Inside the camera, each pixel of colour is captured by a "Sensor Element", so a 7 MegaPixel camera has 7 million sensor elements inside. In other words, it can capture a significant level of detail.

When a digital image is enlarged, it does not change its original pixel dimensions (in the context of quality or level of detail). Your computer software simply adds more pixels to fill the space. Those "added" pixels have no original detail so they "copy" details from the pixels around them to compensate (of course, it's far more complex than I make it sound). As you keep enlarging, you will start noticing that the image becomes "jagged". This is known as "pixelation". Look at the image below, the original image (left) is an image of an icon from my monitor - it is a 78 pixel x 53 pixel image. It's crisp, sharp and clean. The centre image is the same image enlarged two times (156 x 106) - notice it begins to look blurred? The right image has been enlarged to four times the size of the original (312 x 212) - notice the jagged edges or "pixelation"?

So, do more pixels = better quality? Not necessarily, as you've seen by the images above. If you're editing and or/printing your digital photographs, stick within the size limits that your camera captured. If you look-up your camera manual, it should tell you the optimum number of pixels to use for different sizes of photographs. My camera gives me the options to shoot at 7, 5 and 3 MegaPixels - each of them ideal for a particular size of printed photograph. You can manipulate the size of the image to suit the paper (of course) but keep the resizing to a minimum or you'll begin to lose detail.