Some Notes on Sizing Images: Part 2, Resolving Power

The human eye is an amazing instrument, but to distinguish between two separate points, we all need a little distance between them. But the distance is a matter of arc. If you set a compass with one leg where you are and the drawing part on one of the points, then twisted the compass so that the drawing part was on the second point, you would have just drawn an arc, or a short section of a much bigger circle. Remember that there are 360º in a circle and there are 60 minutes, or parts, to every degree.

The human eye can distinguish between two points if there is about 1.7 minutes of arc between them. However, the actual linear distance between them is the amount of arc times the distance from you to the points. The farther away the points are from you, the farther away they can be from each other and still keep the same amount of arc between them.

The idea behind images is that we don't want the eye to be able to distinguish between the points. Otherwise every picture would look like a bunch of points - like a pointillist painting by Seurat. The closer you get, the more you notice the points. But if you move away far enough, then everything blends together to look like a continuous image. So when you are resizing images, you need first to understand the resolution you want, and that depends on the medium you choose to display the image and the conditions under which an audiece will see it. (Tomorrow, something on image size.)