Focus by wire

What does 'focus by wire' mean?

I don't understand what the advantage is. Doesn't this system just add another step to acquiring focus? Is it supposed to be more accurate?

Thank you. I think I've got it!

Jsayles86 wrote:

I don't understand what the advantage is.

Barrie Davis wrote:

There isn't an advantage.

There are several advantages, based on the Olympus FBW cameras I've used.

First advantage, you can change the rotation of the lens. If you are used to the backwards (Nikon) direction of focus, with FBW all it takes is changing a menu item.

Second advantage, speed sensitive focus. If you turn the focus wheel quickly it only takes 1/4 of a turn to go from near stop to infinity stop (or coarse focus).. If you turn the focus wheel slowly, it takes more than a full turn to go from stop to stop (or a fine focus).

I have one direct drive AF lens, and I really dislike it.

Yes, if you have a regular motor geared to the focus element, which is rigidly coupled to the focus ring, trying to focus manually when AF is on can strip the focus gears. So you are either in AF-only mode when you can't use the focus ring, or you are in MF-only mode when AF is completely off, and have to use a switch to go between the two. Such systems are easy to spot by the fact that focusing ring actually rotates when lens is auto-focusing.

Focus by wire means focusing ring is not mechanically coupled to anything, but there are electronic means of determining its rotation direction and speed. Usually, something very similar to the optical computer mouse principle. This info is then passed to the AF motor which moves the focusing element. The problem with this approach is that manual focusing uses up battery, and is as noisy as the motor.

More expensive lens like Canon ring USM use a motor design where focus element is driven by ultrasonic vibrations in the stator ring, and there is no mechanical gearing. Manual focusing on these is purely mechanical, very precise, uses no battery power and is completely silent.

Goodness, the more i read this forum the scarier the practice gets.

On lens that are not focus by wire there is the possibility of knowing where the plane of focus actually lies. In a project i just finished most of the work was macro and was more than difficult to be assured that the range, DOF, was in alignment with what i needed. So on these FBW lens, as far as i can tell from functional use it appears that optical physics is still in play, where you can use the 2/5 in front of plane of focus and 3/5 behind the plane of focus as a rule of thumb to anticipate what will happen as you stop down the lens. This raises many issues and problems from all sorts of applications, like street photography, while zone (prefocusing) focusing and using the hyperfocal distance to sandwich the distance range limits that you're working in so that you can anticipate what is in and out of acceptable optical sharpness ( where the circles of confusion are within the boundaries of acceptable sharpness). The down side of trying to use focus by wire in a precise manner lies the requirement of stopping the lens down well beyond the practical, guesstimated degree is that you introduce other optical imperfection issues that you don't want and shutter speeds that are slower than you might want or need and or by raising the ISO. Here is a copy of some recent correspondence with Zeiss on the matter, as i sit on the edge of buying a new camera ..........

Dear Zeiss, I have been a photographer for 40 years. Most of that time was spent using Hasselblads with your name on the lens. Along with your name on the lens, there was also the healthy amount of demarcation for the plane of focus as well as the aperture etch markings for near/ far focus on both sides of that plane for use with zone focusing. I am just completing a project using an RX1 which will be on exhibit in Santa Monica, CA. all of April. Along with that camera i used a NEX 7 with a Sony macro. Having studied and taught photography in college I am aware of the practical 2:5 rule for focus as you stop down any lens. i.e less comes into focus in front of the plane of focus relative to the back side, proportionally. But you have left me into the area of only being able to guess and estimate since there are no marks on the lens barrel. I am about to embark upon purchasing a new camera system and am looking to find a camera (like the A7r) which would carry over this hyper-focal information etched on the lens so that i do not have to guess, but instead use the lens like an optical tape measure and make the determinations and know what i have done and that what I've aligned will correctly manifest in the capture. My question for you is why and on what grounds have you deemed it unessential not to have this feature and removed it from your modern lens series, particularly for this camera? And if you make a series of lenses for some other camera system that i am unaware of that include this feature would you be so kind to inform me. I am handicapped now from carrying cameras all my life. The RX1 has proven itself a fine tool in many respects, the lens being perhaps its best feature along with the mathematical algorithm for correction of its distortions in photoshop . It is unfortunate that Sony's support knowledge of the capabilities of this camera are pathetically absent. And i might add i think it quite odd for them to have announced/introduced a camera that there are yet to be lenses for, in particular ones that you make that would fit my needs... as in what i have mentioned above. On a technical note what i loved about the Hasselblad was your lenses being edge to edge ground for sharpness. Will these new lenses for the A7r carry that same optical sensibility. Would you be so kind as to respond to this request for details and information. and the response was....
Dear John, Thanks for your inquiry. The focusing ring of most modern AF lenses (e.g. the built-in Sonnar in the Sony RX-1) feature a "focus by wire" system, with no mechanical stops at infinity and at the minimum focusing distance (MOD). So it is impossible to engrave any distance markings or a depth-of-field scale on the focusing ring. The focusing ring can be turned freely when the camera is switched off. Such lenses are mainly intended to be used in AF mode only, manual focusing is just an additional feature with limited response (no hard stops, no DOF markings). If you´d like to use a lens with mechanical stops and a DOF scale like you are used from your Hasselblad system, you have to use a camera system together with manual focus lenses, e.g. our SLR lenses (ZE type with Canon EF mount, ZF.2 type with Nikon F-mount) or our rangefinder lenses (ZM type with Leica M-mount). Those lenses can be used directly on the corresponding camera systems or could be used on mirrorless cameras (e.g. Sony Alpha 7/R) together with a 3rd party adapter. All of our ZE, ZF.2 and ZM type lenses feature precisely engraved distance markings, hard stops at infinity and MOD, and a DOF scale. For further details, please do not hesitate to contact us.
Thanks to Bertram Honlinger, Camera Lens Division,Customer Care Center for Carl Zeiss AG for this response.
I hope that this has been helpful to your readers. OF course there are many autofocus systems which have anticipatory algorithms/solutions for capturing moving objects by means of reduced shutter lag, and things that faster phase detection and focus by fire can address.
It is odd that to encompass a larger range of capabilities with modern cameras that more specificity and purpose driven lens are required to get the job done. I am trying to side step the idea that to do more requires many more lenses (that perhaps even overlap in many ways ,like focal length) and costs more too. I am still contemplating the options.
peace, John

Jsayles86 wrote:

I don't understand what the advantage is. Doesn't this system just add another step to acquiring focus? Is it supposed to be more accurate?

There is not an advantage so far as the user is concerned. There are several advantages so far as the lens designer is concerned. Firstly, providing mechanical manual focus on an AF lens is quite complicated. Either a clutch mechanism is needed which disengages either the AF motor or the manual focus ring (depending on whether AF or MF is selected) or a differential gear mechanism is required which allows both the AF motor or the focus ring to move the focus scroll. This is more used in higher end AF lenses. Nowadays, mechanical arrangements are more expensive than electronic ones. For focus by wire, all that is needed is an encoder on the focus ring, which is read by the lens CPU, which in turn moves the focus motor itself - much simpler and cheaper to effect than the mechanical arrangements.

Second, focus by wire allows use of focus motor technologies which aren't so amenable to the mechanical focus mechanisms. One of these is the stepper motor, which can be coupled with an MF mechanism, but not so easily. The second is the voice coil motor. Whilst probably not impossible, it is very, very hard to devise a way of coupling this to a manual focus mechanism. The voice coil motor is particularly good for mirrorless lenses optimised for CDAF, it is low cost and, with a suitably designed lens, can find focus very quickly indeed.

scorrpio wrote:

More expensive lens like Canon ring USM use a motor design where focus element is driven by ultrasonic vibrations in the stator ring, and there is no mechanical gearing. Manual focusing on these is purely mechanical, very precise, uses no battery power and is completely silent.

This is not actually correct. The ring USM motors (and several micromotor designs, ultrasonic or otherwise) do use mechanical gearing, there is a differential 'gear' (though they tend to use smooth friction 'gears' rather than toothed ones) which allows both the focus motor and manual ring to be engaged at the same time. Essentially the focus scroll follows the difference between the two rings, manual and motor. Keep the focus ring still and move the motor ring, it goes with the motor. Keep the motor ring still and move the focus ring, it goes with the manual ring. Move them both and it does something in between.