[Reinier]

Hello All,

 

Once and a while I heart about lenses which fail to focus, like the 17-85mm can have this problem. I have never had this problem, until now. I had on old Canon EF 28-90mm which worked fine, but the zoom ring was somewhat noisy. So, I thought I lubricate the zoom on the inside a little and also the small cogwheel from the autofocus as well.

 

It worked great. It is running smoothly...but since then the lens starts hunting for focus. Sometimes after three efforts it succeeds, but most of the time it just continue's hunting and won't focus at all.

 

The lubricant I used was suitable for plastic also.

 

Any ideas what causes the problem?

 

 

Kind regards,



Reinier

[Brightcolours]

My guess is that you lubricated an AF part that is not supposed to be lubricated, making the AF move too effortlessly for its own (designed) good. Now, it makes bigger steps than it thinks, overshooting focus all the time (and making the lens hunt).  

[Guest]

My guess is that you lubricated an AF part that is not supposed to be lubricated, making the AF move too effortlessly for its own (designed) good. Now, it makes bigger steps than it thinks, overshooting focus all the time (and making the lens hunt).  

yes, something along these lines, or the oil layer changes the gear ratio slightly, overshooting.

[miro]

Hello All,

 

Once and a while I heart about lenses which fail to focus, like the 17-85mm can have this problem.

I have never had this problem, until now. I had on old Canon EF 28-90mm which worked fine, but the zoom ring was somewhat noisy. So, I thought I lubricate the zoom on the inside a little and also the small cogwheel from the autofocus as well.

 

It worked great. It is running smoothly...but since then the lens starts hunting for focus. Sometimes after three efforts it succeeds, but most of the time it just continue's hunting and won't focus at all.

 

The lubricant I used was suitable for plastic also.

 

Any ideas what causes the problem?

 

 

Kind regards,



Reinier

 



 

Hi Reiner,

17-85 has aperture problem - this results in other type of issues.

Unfortunately I don't have the mechanical drawing of EF 28-90 nor experience with disassemble this lens.

I can share my general experiance. Most of this low costlenses are :

 1. glued and difficult to disassemble.

 2. AF gear is mixed - first stage is ruber belt followed with tooth - two stage tooth gear.

 3.The feedback opto coupler/encoder is located at dc motor shaft

 

If ef 28-90 uses belt type gear and put some grease inside you can expect such funny behavior.

 

PS:Not only low end lenses uses hybrid belt/tooth gears. E.g Tokina 100marco uses belt as first stage gear too.



 

 



 

My advice is to open and clean the lens. My experience is that plastic lenses doesn't use any lubricant while high end lenses use some kind of damping grease for zoom and AF ring. Good quality damping grease is very expensive100Euro/50g



 

In general by lubrication the photo rule "Less is more" is 100% valid. Tribologist said - it need the right amount with right viscosity, right consistency and type grease.




SOry that I can give you direct solution, but I hope that it is worth to try it.

Miro