Before the rush

Before the rush
by evan-pak

Submit your Photo
Hall of Fame

Please participate in Meta
and help us grow.

Photography Stack Exchange is a question and answer site for professional, enthusiast and amateur photographers. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I’m curious about the smooth movement of the zoom or focus rings on a lens. How is the smooth movement done? Is there some kind of liquid with high viscosity? That doesn’t sound very probable. On the other hand, I have no idea how to get the smooth movement by purely mechanical means. Please enlighten me before I go disassembling my lenses. :)

share|improve this question
up vote 21 down vote accepted

The twisting motions you apply to focus and zoom rings are converted to forward and backward movement by helical threads and tracks cut into the barrels inside the lens. This photo shows an example of the threads that do the focusing duties in a partially-disassembled Nikkor prime:

Disassembled Nikkor prime

Note the tracks cut into the inner barrel and the metal rails in the outer one which force the inner one to slide forward and back. Zooms that don't use a push-pull mechanism do something similar with less-dense threads so you don't have to rotate the ring many times to get through their entire range. (This technical illustrator's site has some excellent examples as well.)

High-quality lenses have precisely-cut metal threads that fit together with very tight tolerances. A very light coating of viscous grease provides smooth travel and the drag that we tend to associate with "quality." Less-expensive lenses forgo the metal and instead rely on molded plastics. This isn't always a bad thing; many auto-focus lenses had to sacrifice the feel and keep weight down as a way of reducing the load on focus motors to make them work faster.

If you're interested in seeing how all of this works without taking your lenses apart, order a service manual or ask a repair shop if they'll let you browse through one. Most show a complete tear-down of the lens and the illustrations will give you some idea how all of the parts work.

share|improve this answer

Part of it is due to the grease on the moving parts, yes. On my Canon 70-200mm f/2.8L, I can feel the notches in the gearing. I think plastic parts have more "give" and also wear down slightly to give a smoother feel.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.