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What's the rationale for "focus by wire"?


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<p>Many manufacturers produce so-called "focus by wire" lenses, particularly for mirrorless

systems (and indeed it might even be that <em>all</em> autofocus lenses for mirrorless cameras use the focus by

wire system).</p>

 

<p>Why is this?</p>

 

<p>I'm not asking about user opinions or usability here - I'm wondering what a manufacturer's design/engineering

rationale is for choosing a focus by wire design over lenses that have a physical/mechanical coupling between the

focus ring and the lens elements.</p>

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I would agree that focus by wire on mirrorless ILCs is to reduce the size and weight of those systems. I suspect all my Olympus M43 lenses are focus by wire, but I have had no MF issues with them. Even the cheaper lenses seem to have a well dampened focus ring.
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<p>I'm no engineer, but I guess it takes a lot more solid material to resist the clumsy force of a big guy like me, than the tiny AF motors' efforts.<br>

Creating real mechanics for such different operators is complicated. - Much easier to provide us with a sticky "by wire" ring and keeing the AFing mechanism as smooth as possible.</p>

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<p>No mechanical coupling is required between the focusing ring and helix. The construction is simpler and requires no change over (depends on the lens and camera). On my Sony, I find that fly-by-wire manual focusing is velocity controlled. A quick turn changes the focus quickly, while slow turning gives fine control. The lag time is still there, and extremely frustrating, unless you use manual control only for fine tuning.</p>

<p>Nikon AF-S lenses use a much better idea. You can switch to manual focus seamlessly. The ring doesn't rotate in AF, but there's no lag in manual focus. I think the ring moves the stator and AF moves the rotor (and helix).</p>

<p>It's fairly easy to switch between AF and manual focusing on my Sony A7, either on camera or on the lens. In manual mode, even temporary manual mode, turning the focusing ring activates the (optional) viewfinder magnification, just like an all-manual Zeiss Loxia lens. There's still lag, but a spoonful of sugar makes the medicine go down.</p>

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<p>I'm not a fan of focus-by-wire, but I've got to admit that it works pretty well using the 18-55 kit lens with my NEX 7. I guess I need to reserve judgement, though, because this is the only native E mount lens I own at this time. But the 18-55 is well damped and the turn of the focusing collar is sufficiently long such that fine focus is easy to achieve.</p>

<p>What I like the least about it is I have to go into the camera's menu settings to change between AF and MF . . . and DMF.</p>

 

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I have no micro thirds lenses that provide a mechanical coupling to the focus ring. Some of my larger ED Zuikos do and it is a nice feeling I must say because I am used to it from older days.... Like others I assume it is weight saver to make the lens as simple and light and small as possible to match the camera system size goals. It does the job in the lenses that I have bought so far including some costly fixed focal zooms..I do not love it nor hate it. I don't often use it and it has to be gotten used to...I am glad there are focusing aids to confirm focus when I go manual. What must pilots feel if they get no direct feedback from the rudder control...just idle wondering.
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Many mirrorless cameras use magnetic focussing where the lens elements are moved by electromagnets rather than being held by a metal or plastic frame that is then moved on a helical thread. There is therefore no mecahnical connecton possible between the rotating focus ring and the moving element.

 

This is also one reason why conventional SLRs focus slower in live view than a mirrorless camera. To focus accurrately a mirrorless camer will move the focus backwards and forwards by a tiny amount which is done easily and quickly with a magnetic system but a mechanical system suffers from a friction effect called stiction which makes moving it a tiny amount hard, you have to move it a larger amount backwards and forwards and then stop it in the right place.

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