Auto and Manual Focus on SLR camera lens

[YS2003]

Most of the lens now come with auto focus (such as Canon's USM). I am curious to know if auto focus mechanism wear out if you use too much. Does manual focus increase the longevity of the lens?
Also, is it recommended to switch to MF (manual focus) before adjusting focus? I was told as long as I don't hinder the lens's auto focusing, focus can be adjusted in AF mode.

Edit: I just noticed there was a typo on the title. It should be manual without f.

 

[beavo451]

Most of the lens now come with auto focus (such as Canon's USM). I am curious to know if auto focus mechanism wear out if you use too much. Does manual focus increase the longevity of the lens?
Also, is it recommended to switch to MF (manual focus) before adjusting focus? I was told as long as I don't hinder the lens's auto focusing, focus can be adjusted in AF mode.

Well with anything mechanical, things do wear out, though I doubt the USM motor will wear out anytime soon. There are two main types of AF drives. One is the screw type where a motor in the camera body turns a screw that moves the lens elements back and forth to adjust focus. The second is a motor in the lens itself (Canon USM, Nikon AF-S). It communicates with the camera via electronic contacts. With USM and AF-S, you can turn the focus ring while the camera is focusing and it will immediately revert to manual focus until you restart the focus process. Do NOT try this with a regular (screw-type) AF lens. It will sound very horrible and probably damage the camera's motor.

I wouldn't worry about the AF wearing out.

 

[ChrisA]

The answer is "It depends". I'm a Nikon user but this applies to Canon too.

The new AF lenses are designed to have an "instant" manual focus override where you simply turn the focus ring and the AF stops and lets you overide it manually. The lens is designed this way and will not be damaged. Other AF lenses that are not markd "AFS" will be damaged unless you disengage the AF motor by use of a switch either on the Lens or on the camera body. Canon uses different terms but same deal

How long will the lens last? The modern AF lenses that use "ultrasonic" motors from Nikon or Canon will likely last for decades. But they are no match for the build quality of older 1960's, 70's or 80's vintage manual focus lenses that were made of solid machined brass and glass. When AF came out they had to lighten the lenses because a small focus motor could not quickly turn a big hunk of solid brass and also there was to much drag in the old style focus helix. The older manual focus Nikon lenses still work fine in the new AF bodies and if you like MF you can buy these MF lenses now for cheap. But even the new lenses that use some plastic parts are well made, some better then others but I'd expect the lenses to outlast the newer camera bodies.