There is a common issue surfacing from time to time, during discussions about photographic equipment. It goes somewhat like this: "Mirrorless cameras are supposed to be all about smaller size. But what we see is lenses getting bigger all the time. Doesn't it defeat the purpose?"
Well actually, no, it doesn't, but the argument is generally valid. So let's discuss about small(er) lenses in ILC cameras, in more detail.
It's a fact that, not having to rely on a retrofocal design, due to the presence of a mirror behind the lens, gives lens designers the freedom to come up with a smaller, very efficient lens. This is very evident in many rangefinder lenses through the years. On the other hand, the laws of optics cannot be circumvented.
It seems it's a lot easier to design smaller lenses, with large apertures, when speaking of the "normal" focal range, which means between about 35mm and 55-60mm in FF equivalence. It gets harder to make fast lenses, which are also small, when going either towards the telephoto or the wide-angle area of focal lengths. This is true regardless of the existence of a mirror; in other words, flange distance is less of a consideration in these cases.
The Tessar design has been extremely popular until our days, especially for wide and normal lenses. This "pancake" design is very simple optically, but, with today's improvements in lens element materials and design, current lenses range from good to excellent.
It seems that, from all mirrorless systems, micro four thirds is the one with the most small size lenses, which is probably to be expected, given the smaller size of the sensor. But there are also some Sony and Samsung pancakes available, while Fuji has two: the 18mm f/2 and the 27mm f/2.8. We must also not forget about the various collapsible zoom designs of recent years, from Olympus/Panasonic, Samsung and others. These lenses offer the same practical performance as a regular "kit" zoom, being almost half the length when collapsed.
It also seems curious that, among DSLR makers, Canon seems to get it: they offer a wonderful, tiny 40mm pancake, as well as a 28mm (specifically for APS-C systems). Pentax has a number of pancakes too; I guess mirrorless makers should take notes here.
The issue I thought I'd raise is that of, supposedly, "sacrificing" optical performance and aperture size with a smaller lens.
You see, there is this misconception, mainly among photographers that entered the field through digital. And it's that fast (f/1.8 and faster) prime lenses were invented so that Mr. Wedding Photographer can make dreamy, hair-thin DoF photos of the bride, where only the edge of one eyelash is in focus, obliterating the background and leaving no photographic meaning whatsoever. Actually, no.
Large diaphragms were a necessity, both for Photography and Cinematography, simply because there was a need for low light performance. Not so long ago, all you had for low light photography was ISO 800 film, which you could push two stops and, honestly, get the same image quality you now have with a decent compact camera at ISO 12800. Shallow DoF was not a consideration. In fact shallow DoF was considered a no-no in a wide variety of situations; but that's material for another post.
Bright lenses, unfortunately, have to be larger to accommodate the necessary optics. So we see, for example, Fuji making some extraordinary 16mm, 23mm and 56mm lenses, with apertures of f/1.4 or f/1.2; they are magnificent, but they are also quite large and heavy, considering the whole system size.
I propose that manufacturers should consider making versions of their more popular lenses that are somewhat slower but also smaller. Fuji announced something in this direction recently; a 35mm f/2 lens, which is quite smaller than the existing f/1.4, although it is also weather sealed. Sony already has a 35mm f/2.8 for their FE line, which is also considered a very capable lens.
There is also a precedent for such "alternate" versions. In older days, everyone lusted over the Zeiss 85mm f/1.4, but there also was a f/2.8 version, smaller and lighter... and also sharper at f/4 than its faster brother. Herein lies the secret to a potential market success of such lenses: make them optically great even while wide open. With today's mainstream ISO performance, giving up a stop or two would be ok in most shooting circumstances.
My guess is that a lot of people would be interested in such designs; e.g. Fuji could make f/2 versions of their 16mm and 23mm lenses and Sony could do the same for ~50mm and 85mm focal lengths. Keeping the whole system compact, will always be a compromise, but, if done cleverly enough, it could put all related complaints to rest.