Focus by wire

Started Jun 8, 2012 | Discussions
Jsayles86 Junior Member • Posts: 49
Focus by wire

What does 'focus by wire' mean?

dlevitt Senior Member • Posts: 1,164
Re: Focus by wire

Jsayles86 wrote:

What does 'focus by wire' mean?

It means that there is not a direct mechanical connection between the focus control and whatever lens elements move to focus the image on the sensor. Instead the control tells the camera's internal controller and motors to move the lens elements.

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OP Jsayles86 Junior Member • Posts: 49
Re: Focus by wire

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?

Philip Kendall
Philip Kendall Contributing Member • Posts: 789
Re: Focus by wire

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?

If there's a direct mechanical linkage between the focus ring and the actual internals, turning the focus ring at the same time as the AF system is attempting to drive the focus means you're crunching the AF motor, which is not going to be a good thing in the long term. Disconnecting the two means you can't do that.

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Barrie Davis
Barrie Davis Forum Pro • Posts: 21,460
Re: Focus by wire

Jsayles86 wrote:

I don't understand what the advantage is.

There isn't an advantage. It's just the most sensible way to add a manual focus facility to standard electric Auto Focus.

Instead of the camera evaluating the image and moving the lens elements electrically to the position of maximum sharpness...

... with "focus by wire"...

... the photographer evaluates the image, and works a pair of switches to push/pull the lens electrically to his best estimate of the position of maximum sharpness.

With true manual focusing the electric motors moving the lens elements are disengaged entirely, and a helical screw focusing motion is turned with the fingers.

Under certain circumstances, say, subjects with little clear texture, the photographer can do a better job focusing than the AF can... that is why the facility is provided.
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OP Jsayles86 Junior Member • Posts: 49
Re: Focus by wire

Thank you. I think I've got it!

Mike_PEAT Forum Pro • Posts: 13,344
There are several advantages to focus by wire (fbw)...
1

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.

scorrpio
scorrpio Veteran Member • Posts: 3,595
Re: Focus by wire

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.

Thorbard Contributing Member • Posts: 691
Re: Focus by wire

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.

Most importantly, manual focus can be done in a USM lens without moving out of autofocus mode and without risk of damaging anything.

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john reiff williams New Member • Posts: 8
Re: There are several advantages to focus by wire (fbw)...
1

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

Karroly Forum Member • Posts: 57
Re: Focus by wire

It is perfectly possible to make a fully mechanical focus ring that allows manual focusing when autofocus is engaged without breaking the gears. A good example is the Pentax-DA 18-55mm F3.5-5.6 AL kit lens. Do not ask me why other lens manufacturers do not use this trick. Patents maybe...

Another advantage of focus-by-wire is design simplicity, particularly with lenses featuring internal focusing. The focusing lens group is "burried" inside the lens - unlike lenses which have a rotating front lens group - so it may be more complicated, and thus expensive, to design a mechanical link between the internal focusing lens group and the external focusing ring.

Among drawbacks of focus-by-wire not listed in this thread are the following ones :

- DSLR users often frame, zoom and focus with the camera off to check before eventually turning the camera on to take a picture. This saves battery power. They can no longer do that with a focus-by-wire lens.

- If the lens has a rotating front element (no internal focusing) and the lens was left on, or close to, its minimum focusing distance when the camera is turned off, there is no way to retract the lens manually. You must turn the camera on again to set the lens to the infinity position and then turn the camera off.

- There may also be an issue if you want to preset the focusing distance, let's say on the hyperfocal distance,  then turn the camera off to turn it on later and quickly take a shot. The camera may have reset the lens to the infinity position, unless it has a option to not do so.

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bobn2
bobn2 Forum Pro • Posts: 71,226
Re: Focus by wire
1

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.

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bobn2
bobn2 Forum Pro • Posts: 71,226
Re: Focus by wire
1

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.

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