I really like prime lenses for their simplicity, zooming with my feet. I prefer it to a zoom lens because of this simplicity.
But I do carry both a 35mm and 50mm because I do two different things with them, e.g. street and portraits. I would prefer to have one lens that is 35mm and 50mm which can be toggled.
Is it technically possible to make a toggle lens? How would you do it?
Yes it is technically possible. The question is whether this will have practically same limitations as a zoom or not.
There are two objectives from Leica with stepped focal length adjustment: the 16-18-21mm Tri-Elmar and Tri-Elmar 28-35-50 and a Canon zoom objective with built-in teleconverter which would do what you want if placed inside a fixed focal length objective: EF 200-400mm with Internal 1.4x Extender .
Yes, it is possible and a "Lens Turret" is one way of accomplishing it.
It was very common to use a "Lens Turret" on film and movie cameras in the 1950's before zoom lenses became practical.
Source: Bolex 16mm
Source: Macro lens turret
Source: 8mm film camera with lens turret
It is possible and Canon has done it, although in a different way than you'd expect - their EF 200-400mm f/4L IS USM Extender 1.4x. It's a zoom lens with a built-in teleconverter that could be toggled (instead of unmounting the lens and mounting it again with a teleconverter like you would normally do). This, however, reduces the amount of light you have when you're using the teleconverter - you only get to f/4 when not using it; it drops down to f/5.6 with it.
Although it seems theoretically possible to do the same with a short prime (35 or 50 by your example), it would not be practical - the teleconverter part itself would weigh as much as a prime lens by itself.
In recent years some fast zoom lenses appeared that are as good as or better than equivalent primes - notably the Sigma 18-35 f/1.8, the 24-35 f/2 and the 50-100 f/1.8. Weight, however, is still an issue - even the lightest of the three (the 18-35) weighs 800 g.
Yes, it's possible, and technically there are a number of different ways this is achieved.
You can have a lens turret that contains multiple lenses with different focal lengths, where you rotate the turret to switch between the desired lenses—this was commonly used on movie cameras.
You can build a "stepped" zoom lens--i.e., have a lens that changes focal length, but not continuously through a range, only to pre-specified distances for which the lens is optimized (e.g., the Tri-Elmar lenses by Leica).
Or you can use a teleconverter on the back of the lens, e.g., as an add-on like the Canon Extender EF 1.4x, or an internal element that swings up into place with the flip of a switch (e.g., the Canon 200-400L—the bulge in the barrel is where the TC element lives).
Or you can use a teleside converter on the front of the lens, e.g., the Fuji TCL-X100 that converts an X1000-series camera's 35mm-equivalent lens to make it a 50mm equivalent. For the usage scenario you're describing, this is possibly the closest match that actually exists. :) But, of course, is pretty much the same as just using two separate lenses.
It is technically achievable, outside eldritch lens turrets, with constant (not fixed) aperture zoom lenses.
The technical reason you'd go for a prime over a zoom is because it is easier to achieve wide apertures with primes (hence primes are often the "fast" lenses). The maximum aperture achievable with your typical zoom lens (like a kit lens) decreases as you zoom in.
But constant aperture zoom lenses allow you to zoom without penalizing the maximum aperture. For instance, the SAL1650 2.8 can do f/2.8 at 16mm up to 50mm. The effect of which feels like I have a f/2.8 prime from focal lengths 16mm to 50mm, toggle-able with the twist of a lens barrel.
I haven't seen much constant aperture zooms around but the SAL1650 does not feel as "simple" as primes. But I think it is definitely simpler and more manageable than lens turrets.
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