Understanding Resolution

Digital vs. Analog
The traditional analog world was easy to understand. Negatives and prints were continuous tone and one simply made an enlargement from the negative or transparency to the size needed. Today, in the digital environment, many newcomers get-tripped up by the concepts of input and output resolution and terribly confused as to what settings to use, and when.
To understand these issues clearly one needs to start with an acceptance of certain physical limitations of the human eye. Our vision is incapable of discriminating detail below a certain level. This varies from individual to individual and even by the same individual on different days, but more or less this point is at about 200 dots per inch (80 dots / centimeter). (See my article, Understanding Sharpness).
When an image is composed of dots smaller than this they appear to the eye as continuous tone. This has been relied upon by the printing trade for a couple of hundred years. Every photograph and every image tha...