Pixelization is defined as noticeable square pixels that can be seen in photos that are displayed on a computer screen or printed on paper. Without pixelization when you look at a photo you see continuous areas of different colors and intensities. Here is why it happens.
To understand pixelization you first need to understand how digital photos are created. Digital cameras use a special sensor (also known as CCD) in order to capture photos. This sensor is built from a large number of tiny light sensitive sensors each representing a pixel. The number of pixels in a sensor is also known as “mega pixel” – in other words a camera that has 2 mega pixels uses a sensor that is built of 2 million tiny sensors. When you take a photo the shutter opens and light hits the sensor. At that time light hits each tiny sensor and each such sensor captures that light intensity and color. Put together all those sensors create pixels that comprise a photo.
When you look at a photo if the pixels are small enough your eye “blends” them together to create an illusion of continuous areas of colors and intensity. When this happens photos look crisp and in “high resolution”. If the pixels are not small enough your eye sees individual discrete pixels. When that happen the photo is referred to as “pixelized” – meaning that each pixel is too big and the eye can see it individually as opposed to blending all the pixels together into one continuous photo.
So what determines pixels’ size? the size of the pixels is determined by the size of the photo you are looking at (for example measured in inches: width X height) and the number of pixels in the photo. In other words the size of the pixels is determined by how many pixels are spread over what size of a photo.
Another measure that is commonly used to describe the size of pixels is PPI – Pixels per Inch. PPI describes the number of pixels per inch in a photo. PPI is a function of the number of pixels the camera’s sensor supports and the size of the photo. To calculate a photo’s PPI simply multiply the page length by its width in inches. The result is the number of square inches on the page. Now divide the number of pixels the sensor supports by the number of square inches. The result is the number of pixels per square inch. All that is left to do is to find the square root of this number. PPI is easier number to use than the actual size of a pixel (which can be a very small number). PPI is a combination of the camera resolution (how many mega pixels it supports) and the size of the photo you are looking at (either printed or on a computer screen). Following is a table of PPI calculated for a 5 mega pixels camera and some common photo print paper sizes:
page size 4X6 - 456 PPI
page size 5X7 - 377 PPI
page size 8X10 - 250 PPI
page size 11X14 - 180 PPI
page size 16X20 - 125 PPI
page size 20X30 - 91 PPI
So how small is small enough? This depends on a few factors and sometimes creating some pixelization is wanted as an artistic effect. The pixel size that creates pixelization also depends on the distance that you are looking at the photo from. Your eye might be able to notice pixels in a photo from close by but blend all the pixels together when looking at the exact same photo from farther away. This is very easy to notice when looking at very large advertisement billboards – the printed photos look very good from tens of feet away but if you stand close to the billboard you can see that it is built from many quite big pixels. A general rule of thumb though is to always make sure your photos are at 150PPI or higher.