I was recently reading the comments — I know, I know, I shouldn’t read comments — on Canon Rumors post about Lens Rentals Tear down of Canon’s new EF 70–300 f/4.5–5.6 IS USM II and a poster was lamenting about how Canon is redefining full time manual to mean something other than the focus ring being mechanically connected to the focusing mechanism.

I see arguments like this come up over with some regularity when people talk about focus by wire, or electronic focus, lenses. Presumably, based on the tone of most if not all of these kinds of posts, it’s meant to be a complaint or criticism of electronic focusing lenses. And I just don’t see the point, I guess you could call it, that they’re making.

First of all, let me address one specific point that one poster made, as it’s somewhat tangential to the discussion as a whole.

Though Canon has re-defined Full-Time Manual focusing in MY opinion it means that you can turn the focusing ring even if the lens is not attached to the camera.

Now I do fully recognize that this poster says that they’re talking about their opinion here, and I’m not saying that their opinion is wrong. However, for those that are students of history, there is something worth noting here. The first Canon lenses with full-time manual focusing were in fact electronic focusing designs, not mechanical ones. Canon’s own EF Lens Work III has this to say about full-time manual focusing:

Full-time mechanical manual focusing
This function allows the photographer to manually focus the lens as soon as one-shot AF control is completed without switching the focus mode switch to manual focus. Full-time manual focusing originally employed an electronic focusing method for the EF 85mm f/1.2L USM and other early EF lenses, but today uses a mechanical system in almost all USM lenses equipped with a manual focusing ring and a distance scale, such as the EF 24–85mm f/3.5–4.5 USM, the EF 16–35mm f/2.8L USM, and the EF 300mm f/2.8L IS USM.

Mechanically interconnected manual and autofocus drive systems weren’t the original configuration, electronic focusing was — which also explains why electronic focusing options existed in Canon’s EOS bodies long before Canon started shipping modern STM and Nano-USM lenses.

In any event, let me get back on track here.

The argument that you should be able to focus your lens when you don’t have it attached to a camera, and when the camera is off is one I don’t find especially compelling anymore. We’ve long since left the era of an add-on metering prism on an otherwise mechanical camera.

All EOS bodies, since the inception of the system have required power to operate. Not just AF, but to open the shutter, advance the film, and actuate the aperture stop. With the move to digital systems, even the “film” now requires power to operate.

Without power, a modern SLR — and this is even more true of a mirrorless camera — is a paperweight at best.

That said, I do recognize that there are some arguments for being able to look through the viewfinder and focus the camera without taking a picture. Some wildlife photographers will use their big lenses like spotting scopes when they’re not shooting, and argue that they should be able to focus the lens without having to power up the camera; after all power in the field isn’t always accessible and convenient.

The other argument stems from the need to collapse the focusing mechanism of an overall extending lens, like the EF 50mm f/1.8 STM, for protection during storage. And actually this is a point I’ve never actually seen brought up specifically with respect to focus by wire lenses when criticizing them. This is also a point that’s moot for an internal or rear focusing lens anyway too.

With all that said, let me make an argument for focus by wire.

As with all things in engineering, and this is very much my view of the world, designs are about trade-offs and what capabilities they enable. Engineering takes this a bit further, and asks about the economics of the design, but for the sake of this discussion that’s not especially relevant.

One of the biggest design changes that came with autofocus lenses was a direct change to the relationship between the ring’s rotation and the focus position of the lens. Manual focus designs, because they relied on the human mind and body to drive them were inherently optimized to human scale operation.

Take for example, an inexpensive manual focus prime, like Canon’s FD 50mm f/1.8. Even with a comparably long minimum focusing distance of 0.6 meters, the lens’s focus ring rotated though almost 180°.

Shift forward to autofocus. Computers and AF sensors, even primitive ones, are much more capable of quickly measuring focus than the human eye is; even aided by a vernier system like a split prism. Moreover, as technology improved, AF processors were able to make 10s if not 100s of measurements per second. Even low-end cameras in the late 90’s and early 2000’s could do things, like track fast moving birds in flight, that required extensive practice, skill, and a large amount of luck, to do when manually focusing.

Introducing computers to the focus system shifted the demands placed on the AF mechanisms in the lens. Instead of needing to be able to allow humans to be able to place focus precisely, they needed to enable an AF system to drive the lens faster.

The overwhelming response to make these kinds of adjustments, bearing in mind that again the AF processor and computer controlled drive systems, can make much finer adjustments to the lens faster than a human can, was to shorten the focus throw and steepen the angle of the focusing helical.

Going from a MF 50mm like the one noted above to a AF one, like Canon’s EF 50mm f/1.8 or EF 50mm f/1.8 II, shortened the throw of the focus ring to somewhere around 90°. Fine, mostly, for a computer, but not so great for a person trying to focus manually.

This brings me to electronic focusing.

Electronic focusing completely decouples the manual focus input from the focus drive in the lens. No longer is there a physical mechanical connection that has to be respected. The internal focusing mechanism can be optimized for fast AF performance; or in the case of Nano-USM not even use a traditional rotating helical drive at all. Lens designers are freed from having to consider human scale issues when designing the internals of the lens’s focus drive system.

At the same time, the manual inputs can be optimize for human operation. We see this rather extensively in Canon’s newest focus by wire lenses, like the EF 50mm f/1.8 STM. On the EF 50mm f/1.8 STM, the focus ring is back to a nearly 180° throw when doing manual focus.

Going beyond optimizing the internals and human controls for their respective optimisms, decoupling the mechanism form the controls allows a lot more flexibility. Canon’s Nano-USM motor is an example of that. Unlike virtually every previous focus mechanism which uses a helical to convert a rotational motion into a linear one, the Nano-USM motor is linear from the start.

There are also advantages on the user interface side of things; some currently implemented and some that could be implemented in the future.

One example of this is the ability to simply ignore the focus ring completely while in AF mode. This becomes useful in a number of ways. For example, while shooting night landscapes or astrophotography, I want to lock focus at infinity. With a traditional full-time manual focus lens, to prevent accidental focus shifts, I would tape the focus ring in position. Since I use 2-button focusing, the shutter release doesn’t trigger the AF system, and I don’t have to set the lens to manual to prevent the shutter release from actuating it.

With an electronic focus lens, I can simply configure the camera to ignore the focus ring in AF mode. On a 5D mark IV, for example, this is found in the AF 3 menu under “Lens electronic MF”. Moreover, given Canon’s custom user functions, I can set this and all the associated settings I use for night landscapes, to one of the C modes which makes the camera easy to reconfigure so it’s on when I need it and off otherwise.

Ultimately in the end how does all this play out?

Focus by wire does eliminate the ability to focus a lens when it’s not on the camera and when the camera isn’t on. This certainly can compromise the ability to use a DLSR as a spotting scope without consuming power — a point that’s entirely moot on a mirrorless camera anyway. On the other hand, it affords the ability to optimize the both the internal mechanism for the demands of fast efficient autofocus, while allowing the user interface to be optimized for the imprecisions of human operation.

My experience with electronic focusing lenses, is that the benefits of having a manual focus ring that’s better tuned of the limitations of a human operator, vastly exceeds any detriment that comes from not being able to focus the lens manually when the camera is off.