I was always impressed with photographs printed in a large format with a fine detail. My 12mp DSLR allow me to print up to 11×14 inches photographs with print quality that is pleasing to my eyes. Then I asked myself how many mega pixel camera I need to print large format photographs.
Print quality is defined by dpi (dots per inch). For example if you have 300 dpi resolution, that means you have 300 different dots on one inch of length which are one over 300th of a inch thick. So 300dpi would correspond to 90,000 dots on a square inch. From the books and my own experience, resolution 250 dpi gives very good print quality. If resolution goes to 300 dpi there is a slight quality gain, but beyond that is not noticeable. Below 250 dpi is up to the photographer to decide if the print quality works or not.
When a large format photograph is viewed, in order to see it all one has to move away from the photograph. If the viewer is away from the photograph, he or she can not notice fine resolution of the photograph. So, fine resolution becomes redundant.
Famous landscape painter Clyde Monet painted his paintings in such way when viewed from close distance, the painting looks like abstract painting. If viewer moves away landscape becomes apparent. This technique would be applicable in photography.
On the other hand renowned photographer Andreas Gursky makes his large format photographs with very fine resolution. When viewer is on a distance, he or she gets the message of the photograph, but if they come closer the fine detail of the photograph gives additional meaning. In this case fine resolution of the photograph is very crucial.
To conclude, print resolution is part of a photographer’s artistic expression not just a technical standard.
In table below I listed (column 1) image sizes in mega pixels and corresponding dpi count for different sizes of photographs (column 2 to 10) .
Example: If I have 10 mega pixel image and want to print a 8 inch x 10 inch photograph, I get (from column 2) 354 dpi, which is more than enough for a good quality photograph.