E-mount lens longevity

When I was working with film and view cameras, I ran into many who loved to use old lenses — in some cases, 19^th century lenses with no shutter mechanisms at all. While not good performers by the standards of the day, these uncoated, museum-worthy relics had what I now call “endearing flaws”, and their users prized them.

Today, especially since the advent of full frame mirrorless cameras, old lenses – but not 100-ear-old ones – are enjoying new currency. Screw-mount Leica lenses, early M-mount ones, rangefinder Zeiss and Nikon lenses are all in vogue, and for the same reasons, although the quality of some of the older 35mm lenses can in some respects be quite high.

At the same time that Sony was changing the marketplace for old lenses, first with the APS-C EVIL (electronic viewfinder, interchangeable lens) cameras, and now with the a7x and a6x00 cameras, they were introducing a new line of lenses with novel focusing mechanisms. For the most part, although not universally, the Sony E-mount lenses use a focusing mechanism borrowed from moving head disks, who borrowed it from audio speakers and headphones.

The technique, called voice-coil actuation, involves immersing a coil of wire (the voice coil) in a permanent or otherwise static magnetic field. Causing current to flow in the voice coil causes it to generate its own magnetic field, which interacts with the static field to move whatever the voice coil is attached to in the desired direction. Especially if using strong permanent magnets for eh static field, very rapid motion can be achieved. When the voice coil actuator is placed inside a feedback loop, high accuracy is also possible.

Lenses that employ this technique don’t need a focusing helicoid, whose purpose is to translate rotational into linear motion; a voice coil naturally produced the linear motion directly. Sony has designed the focusing elements of lenses employing voice coil technology largely to just move along tracks that parallel the lens axis.

Sony E-mount lenses employing this technology have focusing rings. However, with one exception, those rings do not directly control the position of the focusing elements in the lens; they merely serve to indicate the photographer’s preferences to software in the camera and the lens which then activated the voice coil to – hopefully – achieve the photographer’s intent.

There are good things that happen because of the use of voice coil techniques: lenses can be smaller lighter, and faster-focusing (and probably cheaper to manufacture) than would otherwise be the case. Focus-by-wire can be more accurate than all but the most expensive, longest-throw helicoids. There are disadvantages as well. The lens will usually forget where it’s focused when the camera is turned off. Focus by wire doesn’t play well with muscle-memory still focusing and with many video scenarios.

But that’s not what I’m going to talk about today.

I don’t think that lenses with voice-coil focusing and no helicoid are likely to be usable on what in 10 or 15 years will be modern cameras, since they require hardware and support from the camera.

With view cameras, all that was necessary to use an ancient lens on a modern camera was to figure out a way to get the lens mounted to the right lens board. If all else failed, you could hand the lens and a blank lens board to any competent machine chop and they’d figure out a way to mount it.

With E-mount cameras, having custom adapters made is probably not cost-effective. Fortunately, it is unnecessary, as machine shops and actual high-volume manufacturing facilities the world over are churning them out in a dizzying variety of lens mount selection, precision, finish, and extra features (helicoids in the adapter, baffles, tilt/shift). Forgetting autofocus, all an adapter has to do is hold the lens in the right place, be light-tight, and avoid internal reflections. So, bring on the rangefinder lenses, the Nikon F-mount lenses (even ones that won’t work with modern Nikon bodies), Canon EF lenses, Pextax screw-mount lenses, pretty much any lens produced for a 35mm camera made in large quantity.

Now, let’s project ourselves into the future. Sony has stopped making E-mount cameras. Will you be able to use your current Sony E-mount voice-coil lenses on new cameras?

Don’t count on it.

An adapter for such a lens to some hypothetical future camera would have to go beyond the lens positioning and light-blocking that is all we demand of basic current adapters. It would have to allow the camera to focus the lens. That means either 1) that the adapter must translate focusing commands and data in both direction, convincing the camera that it is focusing a native lens and papering over the differences between actual lens that’s mounted and the lens the camera thinks it’s focusing, or 2) that the camera knows all about this legacy lens. What are the odds of either of those things? I dunno, but it’s not a slam dunk.

There’s one more potential fly in the ointment: the 18mm flange focal distance (FFD), the very thing that made it possible to mount just about any lens on a Sony E-mount camera, doesn’t leave much room for an adapter. Even if the cameras of the future have a smaller FFD, you know it’s not going to be zero.

I’d expect the same kinds of problems with optically-stabilized lenses. However, those lenses are useful even if the stabilization is turned off, and many, but unfortunately not all, optically stabilized lenses allow switching stabilization off at the lens itself.

Another example of a technology that may provide difficulty down the road is electronic diaphragm control. Canon has had this for years, and Nikon is using it on many, if not most, of their recent lenses.