One of the first things one learns when starting to look at lenses is the concept of a "normal" lens; in 35mm terms, this is usually taken to be the 50mm lens. The reasoning behind this is that at that focal length, the perspective looks similar to what we see with our eyes. Use a wider lens on the same subject (i.e. going closer), and the closer parts of the image will look larger than normal (which is sometimes what you want, e.g. in landscapes). Use a longer lens, and the foremost things will look smaller - which turns out to be useful in portrait photography and can also give a striking effect in other cases. But many photographers like the "naturalness" of a normal lens.
But try looking into how the 50mm definition was arrived at, and things get a little blurry. First of, the "true" normal focal length for 35mm cameras is 43.3mm, since the standard definition of normal is to be equivalent to the image diagonal. 50mm was picked by Leica because it's easier to make a slightly longer lens. Choosing a 45 or 40 mm (equivalent) would be more normal, but only a little bit.
But where did the standard definition of normal as equivalent to the image diagonal come from? This is where it gets interesting. It's picked based on the idea that images are typically viewed at about the same distance as the diagonal, and *at that distance*, the perspective appears natural. View it from further away, and your eyes expect a longer "normal focal length". Stick your nose right up next to the picture, and a wide angle suddenly appears normal. But in a "free" viewing situation, people will supposedly tend to look at things at approximately the right distance, stepping back to take in all of a big picture, and leaning in to see a small one. It would be interesting in a gallery or museum to do a study of what people actually do.
However, these days most pictures are not viewed in a gallery or even on your wall at home. They're viewed on a computer screen, which gives a quite different situation. Let's take a closer look at what that means.
According to http://www.w3schools.com/browsers/browsers_display.asp roughly half of their users - which tend having towards newer and larger machines than the internet average - use a resolution of at most 1280x1024. However, panoguide - probably a better sampling place for photographers - say that 25% of their users (in 2009) used 1024x768 monitors, probably translating to roughly 20" or laptops of about 13".
The suggested viewing distance for monitors is between 30" and 40", though for laptop screens one is more or less forced to go down to about 20". Thus the viewing distance is roughly (very roughly) 150% of the diagonal, giving a "normal lens" length of 75mm.
But how often do you actually fill your entire screen with an image? Not too often. In the typical viewing environment - a browser - there is space taken by the window system and desktop, space taken by the browser, space taken by headers and footers and side information and whatnot. For horizontal pictures, there's usually not that much loss, maybe 30% of the width. Since the picture is scaled down proportionally, that means the diagonal is now down to 15", or 10" for laptops. "Normal" focal length is now 100mm equivalent. Whip out those macro lenses!
It gets even worse if we consider vertical pictures. Since most of the lost screen real estate is vertical, and the screen typically is placed horizontally, we can easily get down to 10" or less. How's a 150mm lens for normal? 200mm?
You'll notice a lot of "typical" and "average" and other assumptions in the above. Which only goes to show that there's so much variability that when it comes to viewing images online, there really is no such thing as a normal lens.
What I want to do when I win a bazillion dollars is open a gallery with plenty of viewing space, put up some sort of face recognition system that can determine where people are actually viewing various size images from, and get some solid data on the gallery situation at least.
In the meanwhile, all I can do (or hope others will do) is get some statistics from the browser and ask the user about viewing distance. That at least could give some clue as to just how normal normal is.
