Torque values for zoom friction...?

[Gregg MacPherson]

Does anyone have torque values for zoom friction that they refer to when servicing..? I'm not a lens tech and I'm not planning to dissembowel a lens, I was just curious about the issue. With some engineering and design in my past, measuring these values would be no problem, but I need some background data to refer to. Like what are the target values that the service techs look for.

 

In normal engineering, there is a "break free friction", normally a bit higher than the friction while moving, where the movement begins from zero. Is it the same with zoom lenses..?

 

The lens that provoked my question was a 10.4-52mm Cooke, serviced a few years ago by Duclose, with very light focus friction, but notably higher zoom friction. So what are the target ideals..? And are they different, for lenses intended mostly for hand held vs on the tripod..?

 

I know that only Dom and a couple of other techs read the forum, so I will hope they read this. Much respect.

 

Gregg.

[Dom Jaeger]

Hi Gregg,

 

Manufacturers probably have torque tolerance values for focus and zoom barrels that they use as guides for quality control, I recall the tech at an Angenieux service training giving us torque values, but I suspect most experienced techs would simply use feel as a guide. Specific greases that manufacturers recommend for the various parts of a lens often determine the torque anyway, perhaps subject to lightening additives or how worn the threads or sliding surfaces are.

 

Usually the zoom barrel is stiffer than the focus, because it moves several element groups and can be subject to creeping if the lens is tilted, so a thicker lubricant can prevent that. Often the zoom is set to a focal length while the focus is continually shifted, so you want the zoom to be more stable.

 

Some assistants have preferences and ask for a lighter or heavier feel which can sometimes be done during prep for longer productions.

 

Different lenses can have different feels, Angenieux zooms are often lighter than Zeiss for example, and cam focus barrels like on an S4 or Master Prime are usually lighter than helical focus barrels like on an Ultra Prime.

[Gregg MacPherson]

Thanks Dom. Sounds like for you guys its done by feel, but maybe the factories sometimes test and compare the numbers. I'm still curious about this, but can't do much without some data.

Cheers.