Normally, the focus ring is mechanically coupled to some of the lens elements, and turning the ring moves the glass.
Focus by wire is a lot like "drive by wire" where the gas pedal is not mechanically coupled to the engine but instead is an electronic switch that tells a computer how much gas to toss into the engine. In focus by wire, the focus ring is not mechanically coupled to the glass. It's just an input device like an old Atari paddle that tells a computer to go ahead and move the glass elements.
It doesn't necessarily stink, but it feels a little weird and you can't retract the front element unless the camera is turned on.
The manual focus ring is not physically coupled with the focusing mechanism. Rather, rotating the ring sends an electronic signal to the focusing mechanism. In practice, this means that the lens must be mounted on a camera body (and therefor receiving power) in order to change the focus. No big deal, really. And the manual focus ring has a very "loose" feel to it, more like a lack of resistance, compared to other lenses.
Focus by wire means a system where your turning of the focus ring actuates an electric motor that moves the glass elements of the lens to the proper position to achieve sharp focus. The alternative is a mechanical system where your turning of the focus ring moves the elements directly. Perhaps people say it stinks because it is not as fast or precise as mechanical focusing. That may be true for some Focus by Wire systems but the more advanced ones have 2 or 3 user-selectable speeds and can work very well. It's not a big deal for those of us who rely on Auto Focus most of the time.
The downside is usually because people associate 'focus by wire' with a slowness to focus. Case in point is the 85/1.2 L. Focus is very accurate but the image doesn't snap into focus say like an 85/1.8.
I never thought of the 85L as a sports lens and never tried to use it as such.
There does seem to be some negativity about focus by wire - I have to say, I don't get it, the manual focus ring on the 85L is soooooo smooth it's sexy.
the 85L isn't slow... it's precise... it's a lot more of a turn to move through the focus range than say... a 16-35L... think of it as a more fine-toothed gear...
the focus by wire works great for me... very smooth, and very precise.... once you get used to it!
