I used to carry around a 135mm f2.8 lens for my Contax film cameras. It was a nice companion to the 85mm f2.8 lens and, when I also had the 50mm f1.4 lens along for the ride, I felt as though I could cover anything in my usual style. It's only in the age of rampant generalism that I feel the pressure to also cover the desperately wide focal lengths as well.
But something nasty happened when we ventured into the populist age of digital photography; the masses adored the idea of zoom lenses, and they love the basic 70 or 80 to 200 mm versions best of all. At some point I guess the single focal length lenses that were covered by that range just fell off the radar entirely. At one time Nikon made a 135mm lens in f3.5, f2.8 and f2.0 variants. If you needed the speed you carried the weight. If you needed the focal length without the speed you were rewarded with a choice of two very well corrected lenses at two lower price points. My favorite was the ais version of the 2.8 which was small and not too heavy. It fit nicely in a bag and compressed images well. Best of all, being a single focal length lens it was very well corrected and very sharp for the price.
My first 135mm was a Vivitar 135mm f2.8 that was in the original Canon FD mount, and though it was only $79, brand new, it was a great lens and could be shot wide open with reasonable results. The camera I mostly used it on was my first SLR, the Canon TX, a fully manual, all metal camera body with shutter speeds up to a whopping 1/500th of a second... That camera had one great feature: It was impossible to break.
I dragged the combo across Europe one Fall and came home with a number of great Tri-X images.
Lately I've been hankering (Texana) for a 135mm lens that fits those same parameters. Not too heavy and not too big, but plenty sharp and better optically than the "fast" zooms. Sadly, the only prime 135mm left in the Nikon catalog is the 135mm f2.0 DC lens, which I consider a specialty optic. Yes, it's very sharp and also has the feature of being able to dial in spherical distortion for a more pleasing bokeh, but the damn thing is three times as heavy as the old 135mm and three times as expensive. Great fashion lens --- crappy lens for walking around.
Almost every 135mm lens out there is a variation of a fairly simple optical formula so I can't think that the mid-speed ones are expensive to build, but because of the influx of people into the craft who always think, "faster is better" and "zooming is better", the choices we used to take for granted have disappeared. If you happen to have an old 135mm Nikon f2.8 ais or ai lens you'd like to get rid of you might want to drop me a line. It's the current gap in my portrait pantheon that's driving me a little nuts.
Are there focal lengths that you loved that have disappeared? Would you buy it if it re-appeared? Or are you as happy as can be with the "holy trinity" of f2.8 zoom lenses?