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Issues for digital artifact photo archiving [Part 1]
I'm becoming involved in a historical artifact photography project using digital images. As such, I need to develop some materials to use in describing collections of digital photos of artifacts. Here's my first cut. I'm trying an experiment here by breaking this into a multi-part series. I originally intended two parts, but it has gotten longer than this...
Background: it is easy to understand how a digital camera works. Light enters the camera, and hits a rectangular tile mosaic of microscopic sensors. Typically, each sensor is square. By measuring the light values on the mosaic of squares, one gets a mosaic of picture elements called "pixels", each one consisting of a light intensity level. Color digital photography is a bit more complicated: colored filters are put in front of some of the sensors so that they measure just red, green, or blue light.
The "resolution" of a digital image is the number of pixels horizontally and vertically over a given distance. Normally, the horizontal and vertical resolution are the same. Much confusion results from the fact that the term resolution is often used, incorrectly, to denote the count of sensors across the whole mosaic in the horizontal and vertical directions: these two numbers are typically different. Thus, photographers will talk (correctly) about "75dpi" (dots per inch, aka pixels per inch) images; they will also talk (incorrectly) about a camera having "2048x1360" resolution, or even of "2 megapixel resolution". This last is simply the total number of sensors on the mosaic, computed by multiplying the horizontal and vertical sensor count of the rectangular mosaic.
True resolution computation for an image requires knowing two things: the physical size of the image, and the number of pixels vertically and horizontally in the image. However, it turns out that the physical size of a digital image is not fixed; it can be easily scaled to a variety of sizes. More importantly, from an artifact photo point of view, the resolution of the image is not the most important thing. Instead, proper emphasis should be placed on the effective resolution on the artifact. One would like to have as many pixels covering the object itself as is possible; this makes the photo much more useful.
In part 2 of this series I will complete the discussion of digital images.