It can be very confusing trying to understand the significance of image size, resolution and file size, and how they relate to each other. Throw in pixel densities such as DPI and PPI as well and you could be forgiven if your head starts spinning.
It helps to start with the basic building block of any digital image – the pixel.
A pixel is a dot. It’s a single point of light that has two main properties – colour and brightness. You’ll find these pixels everywhere – on computer monitors, TVs and mobile phones, for example. When you combine these pixels in to rows and columns you can form a picture. These rows and columns of pixels can also be arranged on an image sensor in a camera to capture images. If you have an arrangement of 3000 columns and 2000 rows of pixels, you can form an image 3000×2000 pixels. We could refer to this as the image size or image dimensions. We could also describe it as a 6 megapixel image (3,000 x 2,000 = 6,000,000). This is what we call image resolution. You could also think of it as quality. A higher resolution image has more pixels, producing a higher quality image. More pixels means more picture information.
As well as image resolution, you might also come across screen resolution and print resolution. These refer to the density of display pixels on a screen or printed matter. Typically a computer display will have a pixel density of 90-100 pixels per inch (PPI). Printers on the other hand will pack 300 pixels in to an inch – and will refer to them as “dots” not “pixels” – so you will hear about print resolutions of 300DPI or 600DPI (higher and lower values may also be used). Obviously, the tighter the dots or pixels are packed together, the higher the image quality. If we display our 6 megapixel image on a monitor with a pixel density of 100 PPI, it will have a displayed size of 30 x 20 inches. However, if we print the same image on a 300DPI printer, it’ll only be 10 x 6.6 inches.
Obviously, images with more pixels are going to have a larger file size, but the relationship between the number of pixels in an image and the file size isn’t a direct one. In theory it is entirely possible to have a larger image with a smaller file size. This is because most photographic images are stored as JPG files (also known as JPEG files). This file format uses what is known as lossy compression. This means that a mathematical algorithm is used to reduce the file size by making small but imperceptible economies with the image data, such as combining almost identical tones or colours. The naked eye will rarely notice this with typically used compression levels. However, when an image is heavily compresssed the image degradation will become more obvious. If you hover you mouse over the image above you’ll see the effect that heavy compression has. If you can’t immediately see what’s happening, look at the gradual transition of the vignette at the corner of the image, or the curve of the model’s right shoulder, or the detail in the hair. The image still has the same number of pixels, it’s just that the fine detail has been discarded, often by reducing the variety of colours and shades in the image. I should also mention that the left-hand side of the image has been pixellated, which means that the resolution has been artificially reduced to show what happens when you use fewer pixels to represent an image.
Hopefully this article has helped clear up some of the misconceptions and confusion about image resolution and sizes. There are some finer points that I’ve chosen not to cover for the sake of brevity but if you still have questions then there are lots of articles on the web discussing these issues. Also, some explanations work for some people whilst others don’t. If you’re looking for an alternative take on all this then I would recommend Leo Notenboom’s article on Ask Leo.