When is a 25 watt lightbulb not a 25 watt light bulb? When it's a 7 watt energy saving bulb! It's unfortunate that we are used to talking about how bright a light bulb glows by saying how many watts it uses. I got a pretty good sense of how bright a 25 watt, 40 watt or 60 watt light bulb is. That was fine when all light bulbs were more or less the same, but it changed when technology evolved and more power efficient bulbs hit the market. Today I bought a couple of 7 watt energy saving bulbs, and as a needed service to the consumer (me), the manufacturer had to print on the packaging that this light bulb gives the same amount of light as an ordinary 25 watt bulb. So, 7 watts = 25 watts. This will become confusing when we eventually stop using old style bulbs completely. Will we continue to think of the brightness of this bulb as 25 watts, or will we get a good sense of how bright a 7 watt energy saving bulb is (which we'll have to adopt again when never bulbs that use even less power appear)?
The problem is that watt is not a unit for the amount of light a light bulb emits. It is a unit for the amount of power it uses. For the amount of light emitted, candela would be a better unit. Wouldn't we have avoided some confusion if we had used candela instead of watt from the very beginning?
Big deal, I hear you saying, but there are other examples of this "wrong unit" problem that perhaps are more important (at least to me). I'm thinking specifically of the unit we use for measuring the focal length of a lens. You can read the technical description of what focal length is on Wikipedia, but when you're working with photography you don't think of focal length that way. As any photographer would tell you, a lens with a focal length of 50 millimeters is called a normal lens. When you take a photo with such a lens, the world would be depicted more or less like how you see it with your eyes. The perspective and depth would look normal. If you take a photo with a lens with a longer focal length, such as a 90 millimeter, perspective will change. The depth will be compressed, so things that are at different distances from the camera will not be that much different in apparent size. Such a lens is called a telephoto lens. If you go in the other direction, and put a 28 millimeter on your camera, perspective will be exaggerated. Something close to the camera will become huge, while things that are just a few meters away become tiny. Such a lens is called a wide-angle lens. Instead of using different lenses, you often use one single zoom lens that can go all the way from wide angle to telephoto, for example a 28-90 millimeter lens. In any case, the most obvious difference between the wide angle and telephoto focal length is how the perspective and depth look. These are functions of how wide the field of view that the lens catches is. A telephoto lens has a small field of view, while a wide angle has a large field of view. (The difference in depth of field is also important, but that difference is far from as obvious as the field of view.)
Now all of the above is true only if you are taking your photos with a camera that uses 35 millimeter film (also known as 135 film). It would also be true if you're using a digital camera with a light sensor with the same size as a frame on a 35 millimeter negative (36 by 24 millimeters), but all affordable digital still or video cameras have light sensors that are smaller than that. Typically, the cheaper the camera, the smaller the sensor, and the size of the sensor affects the field of view. So while a 50 millimeter lens is a normal lens (neither telephoto nor wide angle) on a 35 millimeter camera, it would be a very long telephoto lens if you put it on your mobile phone's camera that has a tiny light sensor. This is the reason that camera and lens manufacturers write in their technical specifications both the actual focal length of a lens (for example 14.5-24 mm) and the focal length that would give the same field of view on a 35 millimeter camera (for example 28-135 mm). This is a bit silly, but necessary since photographers are used to thinking of field of view in terms of focal length. Unfortunately, because we are back at the light bulb problem. We should have used a better unit to begin with. In this case, the field of view measured in degrees would make more sense.
In the above discussion, I haven't even started talking about all the negative sizes that are possible when shooting motion pictures. For example, even though movies are typically shot on the same 35 millimeter film format that analog still cameras use, the frame size is different, so the same focal length numbers do not give the same field of view. And then there is 16 and 8 millimeter film. And for some still cameras, there are also huge negative sizes, on which a 90 millimeter lens would be a wide angle.
Wouldn't it just be simpler if we had used a better unit of measurement from the beginning? Now we're probably so familiar with the "35 mm equivalent" measurement that we won't change it, even after 35 millimeter as a format is dead -- just like 25 watt light bulbs.