也论50mm

1. We are talking about 50mm lens for 35mm SLR cameras.

2. The technical spec defines the distance from the lens mount to the film/sensor to be about 40mm, leaving enough clearance for the reflex mirror.

3. As a result, the distance from the lens to the sensor/film is about 50mm. Which means 50mm will be the easiest lens to make (smallest size).

If the lens has a longer focal length, it of course must be bigger/longer.

But if the lens has a shorter focal length, it will be shorter than the distance from the lens mount to the sensor, thus the designers of the lens have to do some tricks to place the lens farther away than its focal length, resulting in a bigger lens.

4. Conclusion: 50mm lens is the cheapest to make, thus included as the standard lens in the old days.

5. So using 50mm lens is because:

A. Cheaper
B. Small size, so that it is most convenient to carry around.
C. Not because it is special in other aspects (say, providing the best field of view in photography).

6. For mirrorless camera, the lens mount can be placed closer to the sensor/film, thus resulting in a shorter focal length lens as the most natural lens.

7. The above is from my own analyses. If some one already came up with the same analyses, it is totally coincidental.