- Marek Zielinski
Resolution and the myth of dpi
David by Michelangelo; dithered by User:Gerbrant using own software (cropped from Image:Dithering algorithms.png) [Public domain], via Wikimedia Commons |
Recently, during a conversation with an archivist in London, I had a chance to argue with him about dpi of digital files. It seemed to me then that it was a simple misunderstanding, until recently, when another archivist of a large institution in Poland tried to convince me that he scans 35 mm negatives with a resolution of 600 dpi. Because it would result in a scan rather like raster images in old newspapers (produced in a process called dithering) - see picture on the right, I was a bit perplexed. This gave impetus to write this blog about, as it turns out, rather common myth of dpi. A scan of the 35 mm negative will be discussed a little further, I will begin with an introduction of the concept of resolution.
Resolution
Optical resolution is a concept of established reputation; in general it defines, for a given optical system, how far apart two objects (for example, two parallel lines) need to be to be distinguished in an image. Resolution depends on the quality and diameter of the lens, the resolution of the film, on the emulsion grain size, etc. Optical resolution is often expressed in lines per centimeter. In digitization the optical resolution is of paramount importance, as it determines which of the original fine details are faithfully rendered in a digital format.
The prevalence of digital technology resulted in introduction of another measure, also called a resolution, measuring the number of pixels into which the original image is divided in conversion to a digital file, with respect to its physical dimension. To differentiate it from the optical resolution we will call it pixel resolution (PR) and express it in pixels per inch, abbreviated dpi (letter d comes from the dot). If you scan an image that has 2 inches in width, divide this dimension into 600 elements then the scan will have PR = 300 dpi. We can do the same in the second dimension (height) and if we use the same pixel density, which is common, the height of say 3 inches will be divided into 900 elements, and the pixels will be square.