Film-Tech Forum ARCHIVE: FP20 drifting focus

This is topic FP20 drifting focus in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Jeff Else (Member # 4063) on 01-16-2010, 05:32 PM:

One of my FP20's likes to slowly drift out of focus so that i have to correct about every 10 minutes. Focus is sharp and uniform when correct, but always drifts. Why??

Posted by Ian Parfrey (Member # 5122) on 01-16-2010, 05:52 PM:

Jeff.

Could be a number of things such as -:

Lens loose in the mount
Too much heat at the gate with corresponding "popping" of the focus
Slop in the focus leadscrew
Worn skates and runners

There may be other possibilities which I'm sure other tech's will point out.

Good luck with it and let us know how you go and what you find.

Cheers
Ian

Posted by Jeff Else (Member # 4063) on 01-16-2010, 07:41 PM:

the lens is certainly in there tight, and the skate and runners are pretty new. the gate feels cool to the touch... i've got a 1200 cfm fan on two machines running 2k each (probably overkill i know but it was a free fan). is too much heat still a possibility somehow... if not, what can i do to fix the slop in the focus leadscrew?

Posted by Steve Guttag (Member # 268) on 01-16-2010, 08:15 PM:

Remove the lens and shake...verify it is not unscrewing itself.

Verify that your focus rack is operating smoothly and not binding mechanically or is gummed up. Remember, it is a spring that moves the lens one way so if it has a problem...there may be a time-delay between adjusting the focus and when the lens finally finishes its movement.

Steve

Posted by Jeff Else (Member # 4063) on 01-17-2010, 05:19 PM:

the spring is gunk free, is lightly lubed, and is moving freely. do you mean shake the screw without the lens mount in the holder? i did that and the screw does jiggle a bit.. perhaps it is just unscrewing itself? do i need a new screw?

Posted by Steve Guttag (Member # 268) on 01-17-2010, 10:00 PM:

No...shake the lens, itself.

Steve