> This is not a mistake; IIRC, one of the early 135 camera makers found it easier to make a 50mm lens than a 40mm lens, and it's stuck ever since.

"lens coverage" is the size of the image circle that's projected by a lens. You need (or at least, generally want) an image circle that's big enough to cover your negative or sensor.

lens coverage is actually something that is determined by the formula of the lens - a Super Angulon 90/8 has more coverage (will throw a larger image circle) than an Angulon 90/6.3 despite the fact that they are both 90mm lenses. And moreover it is actually something that (if you are looking at it at a formula level) is measured in degrees. You have a triangle that is formed by the nodal point of the lens, the center of the film, and the edge of the image circle, and the lens coverage (in degrees) determines that angle.

But, if you take the same lens formula and you make it longer, that leg of the triangle gets bigger too! At infinity, a 90mm lens will need to be 90mm from the film (backfocal distance) while a 75mm would only be 75mm from the film. So a 90mm Super Angulon will have the same angle of coverage, but that means more coverage (in mm) than a 75mm Super Angulon would have, because of the longer focal length. So a 90mm Super Angulon would cover 5x7 film, but a 75mm Super Angulon would only cover 4x5 film.

https://www.graflex.org/lenses/lens-spec.html

Anyway, point being, 40mm is really tight on a Cooke Triplet-type lens (or really, even Tessars) to cover a 35mm frame of film, especially in the faster configurations with wider apertures. It works, but really you're going to get a lot of softness and maybe even vignetting wide open. But making it 50mm gives you some extra image circle to play with and increases the angle of sharp coverage as well. 50mm is generally close enough that people don't really notice the difference anyway.

Some lenses actually went even farther, early SLRs struggled to get enough distance to fit the mirror in, and the way they worked around it was to make the lenses even longer (often 58mm) to give some extra room to fit the mirror. Early in the development of SLRs it was not possible at all to do wide angle lenses because (being only let's say 35mm long) they could not focus to infinity when used with a mirror box that needed 50mm of clearance. In a rangefinder design (like a Biogon), you can have glass elements that get very close to the film but on a SLR that's where the mirror has to go. The fix was the Angénieux retrofocus design, which was a reverse-telephoto design that pushed the nodal point behind the lens, so the actual glass could be farther away, but the nodal point only had to be (eg) 35mm away. This design was directly copied by a couple companies, including Pentax, who sold it for a number of years as the Pentax/Takumar 35/3.5 and the Pentax/Takumar 6x7 75/4.5.

https://en.wikipedia.org/wiki/Angénieux_retrofocus

A few more fun tidbits:

Technically long lenses and telephoto lenses are not the same thing. A telephoto lens has a focal length shorter than its physical length (or, focal length longer than its nodal distance). Generally this is done with a specific arrangement of positive and negative elements called a "telephoto group". In contrast I have a Takumar 500mm f/5 lens that is not a telephoto lens, it's actually 500mm long and has a single doublet group at the very end, so it is like a spyglass that you mount on your camera, not actually a telephoto. The correct terminology for a lens that has a narrow field of view is a "long lens", not "telephoto".

The angle of coverage changes for many lenses as you stop them down (however this is not universally true). There is often also a "angle of sharp coverage" where the image gets soft, but is still illuminated towards the edge. On many 35mm lenses this is "cropped" internally using baffles but on large format you get to determine that yourself. Again, stopping down will usually improve the angle of sharp coverage.

The "field of view" of a lens is dependent on the coverage and the image size. If you put a 35mm film in a 4x5 image circle projected by a 90mm lens, it works fine, but you're only getting a part of the possible field of view of the lens. And it is the exact same field of view you would get with a plain old 90mm lens from the 35mm camera manufacturer's lineup. So in large format you end up with a situation where that 90mm lens could be "wide on 4x5, superwide on 5x7", because you can pick your sensor size as well. Just like a lens can be "normal on full frame, long on crop sensor" for the smaller sensors.

Most of the simpler modern lens types have been known for a long time! For example the 50mm lens on your DSLR is likely a Planar type, the Zeiss Planar was invented in 1896. However, for many of them the materials science had to catch up to really make them practical - early lens design favored fewer elements and cemented groups because every air-glass interface would lose some light and you'd end up with very low contrast. Lens coatings were developed slightly before WW2 and were a key enabler for more advanced lens types, along with the later inventions of more advanced glass compositions (particularly fluorite elements), aspheric surfaces, and computer-aided design. It's not a coincidence that there's a cambrian explosion of lens design in the 50s/60s, that's when the material science really caught up!

Anyway many of these things are tangentially observable on 35mm, but of course the intent there is to provide a streamlined experience, all of this is exposed full force in large format. Hopefully this explains some of the ideas behind quirks of lenses that people might have wondered about.