Why don consumer DV cameras have “real” lenses with focus marks?
“Real” lenses use helical grooves to rack focus; the resistance you feel when you focus such a lens is the natural friction of the rotating barrels sliding through the lightly-greased grooves. That smooth friction, alas, plays havoc with autofocus systems which all consumer cameras must have, so goes the conventional wisdom; strong and battery-draining motors are needed to spin such barrels, and they can’t obtain the fast focus response that’s so useful in optimizing autofocus algorithms. Thus autofocus lenses use lighter, more easily positioned internal focusing elements (which are also advantageous from an optical standpoint) with lighter, faster, more thrifty focus servos. The “focus ring” you manhandle isn’t actually connected to the focusing mechanism. It’s a free-spinning ring with an optical or electromagnetic sensor attached: when you spin the ring, a series of pulses is sent to the focus controller. The faster the pulse train, the faster the controller changes focus.
