lens - Why are 50mm Lenses Cheaper?

There is no doubt that 50mm lenses, if all other variables are held equal, will beat any other lens when it comes to value for price (lower f/number). I want to know why.

Possible theories I have heard:

1. 
   

   There some physical property of the lens that makes it fundamentally cheaper to produce (i.e. no real magnifying or odd wide-angle lens elements).

   
   


2. 
   

   The lens makers have had years and years to optimize the design of the lens in order to make the manufacturing process cheaper.

   
   

The implication with the first possibility is that 50mm lenses will always fundamentally be cheaper. Unfortunately this will always plague cropped sensor camera owners with slight telephoto lenses if they want great valued lenses (i.e. 35mm lenses will never be as cheap at as low an f/number).

The implication of the second possibility is that one day, cropped sensor camera owners will be able to get absurdly cheap and high quality 35mm lenses.

Cropped sensors may one day fall by the wayside with Moore's Law, but for now they are going nowhere fast.

Any insight is much appreciated!

Answer

If you are talking strictly about DSLRs based on the 35mm film format (and even most crop sensor DSLRs are designed around the 35mm lens mount their manufacturers used pre-digital), then the price/performance ratio is intrinsic to the 50mm focal length (or thereabouts).

First, there's the distance between the back of the lens and the sensor. That's pretty much set in stone for the 35mm SLR format, since the camera needs to accommodate a mirror, focusing screen and focal plane shutter. A 40mm distance between the surface of the lens mount and the film plane is just about an absolute minimum, and the rear of the lens can only protrude so far into the camera before it begins to interfere with the mirror. (Canon EF-S lenses protrude a little deeper than their EF brethren, but only by a a little—there's still a mirror to contend with, even if it's a little smaller. Some very old wide-angle lens designs require that the mirror be locked up before the lens is mounted, but for reasons I'll discuss later, they're not useful on DSLRs.)

For a lens to have an apparent focal length shorter than the distance between it and the sensor, it needs to have a retrofocus design, which consists of a telephoto lens looking through a rather large concave lens at the world. That concave lens (or group) needs to be much larger in diameter than the focal length and speed of the lens would suggest. Such a design will always be more expensive than the simpler design possible with a 50mm.

But what of the 40mm-ish range? It is indeed a simple thing to make something very simple using a Tessar-type design at or around the 40mm range. That's why the Pentax and Canon "pancake" lenses can be so small. But notice that they're also of limited maximum aperture. ("Limited", here, is relative. A speed of f/2.8 is fast except in comparison to the f/1.8 and faster lenses we're talking about.) The fact of the matter is that a 40mm lens has to bend light much more abruptly than a 50mm lens does, and to get a large image circle with little vignetting and acceptable levels of aberration and distortion, you need either to restrict the diameter of the lens or use a more complex design. That more complex design will, of necessity, be physically longer, moving the optical centre of the lens farther from the sensor, and suddenly we're back in retrofocus territory.

Once we get beyond the 50mm focal length, the lens design can remain relatively simple, at least for a while, but all of the elements need to be physically larger, so the prices go up again. And because the prices go up, the prices go up—fewer people will buy the lens, so each copy is going to cost a little more. But it doesn't take long before we run into chromatic aberration in a big way: since the light has more distance to spread out, the separation of the spectrum becomes more apparent, and the lens design needs to correct more for it. That usually involves using expensive and hard-to-work-with exotic materials to get as close as possible to apochromatic performance.

The 50mm lies in the sweet spot. It doesn't need be be retrofocus (unless the design is deliberately exotic), and doesn't need heroic levels of correction in order to acheive a large enough image circle and acceptable levels of aberration and distortion. It's also just about the shortest focal length where all of the forces of good gather, so its elements are small compared to lenses of a longer focal length. (Note that there is a reason why even the 50mm gets much more expensive as you get faster than f/1.8.) That makes it intrinsically cheap, and it helps, of course, that the lens body is usually of a somewhat lower quality than the more expensive lenses (particularly in Canon's case).

So this should all get better in the mirrorless world, right? Well, yes and no. A digital camera's sensor isn't film. In almost all cases, there is a Bayer pattern filter or something similar to distinguish colours and an array of microlenses to ensure that each of the sensor elements receives as much light as possible. The upshot of that is that there is a limited range of angles from which the sensor can receive light efficiently, and that means that shorter focal lengths still need to be retrofocus even though the rear of the lens can almost touch the sensor. (The alternative would be to force all longer focal lengths to be true telephoto lenses—lenses that have a focal length longer than their physical length, and which usually have smaller image circles and maximum apertures than simple long-focus lenses. That would get you cheaper wide-angles, but would make anything longer than your nominal "normal" much, much more expensive—there is a genuine requirement to have fast long lenses in order to keep the shutter speeds high.)