Typography

The optimal line length for readability is around 10-12 words (Source by Ruari McLean)

Telecoms, networking and IT

The ratio of peak load to average load in a service with diurnal activity variations is approximately 3 to 1. Source: my own empirical observation from Wanadoo access logs and France Telecom telephone call usage logs.

Probability of a Web page having X incoming links referring to it: P = X ^ -2.1 (Source)

When specifying computers, for balanced performance provision one gigabyte of RAM per gigahertz per core/thread.

Any standard making use of ASN.1 is a piece of junk.

You only get the benefits of statistical multiplexing or compression once, and it should only be done in one layer. Any other layers attempting to do the same only add cost, complexity, brittleness, overhead and latency.

When designing high-availability systems, fail-over is not the hard part, falling back is.

Photography

For most ordinary lenses, optimal sharpness is around f/8. For high-quality lenses, it is one or two stops below full aperture. Only the very best lenses are diffraction-limited and offer optimal performance at full aperture.

Camera light meters are calibrated for 12% gray. Common gray cards are 18% gray, so if you use one for metering, you should open up one half stop to compensate. (Source)

The human eye is a 6-7 megapixel sensor. The monocular field of view is 180 °, the binocular field of view is 120-140°, and the normal focus of attention spans a 45° field of view.

Avoid Kodak products like the plague. Those products they make that are actually decent (i.e. the engineers managed to sneak them past the bean counters) soon get adulterated (like Tri-X) or discontinued (like PhotoCD or their medium-format digital backs). Prefer Fuji, Agfa or Ilford.