cooke 18-100 T3 question about focus

[Sergey Afanasiev]

Hi guys! If you worked with the cooke 18-100 T3 lens maybe you can help me. I got this lens and found a problem with the focus. For example: Focus well adjusted. But every time when I'm changing the zoom I am losing the focus. Is something wrong with my lens? Or this is OK for coook 18-100?

 

[ericyoung]

A cinema zoom lens should be parfocal - ie hold it's focus throughout its zoom range. If you zoom right in to 100mm, focus and then zoom out to 18mm, is it still in focus at 18mm?

 

[Jay Kim]

Check your backfocus on the camera if you are using Red cameras.
My Cooke 18-100 Classic holds focus when I change the focal length.

 

[Ryan Patrick O'Hara]

It should hold focus. If not the camera flange, it is the lens's back focus and that will need to be serviced.

 

[Jeff Kilgroe]

Camera back focus (FFD) needs to be checked. Lens may need to be collimated so its mount is in the correct position. Could be as simple as adding or removing a shim or two on the lens' mount. A lens technician should be able to help you out and give the lens a tune-up if your camera mount checks out OK.

 

[Sergey Afanasiev]

If I will chanche from 100 to 50 or 35 or 18 I will lose the focus... Back focus on the camera is ok... It looks like I have to call to tech guy.... I am using red epic M camera.

 

[Sergey Afanasiev]

Thank you for the information!

 

[Bob Rudis]

Male sure the screw on mount is tight…

 

[Stephen Williams]

The Lens need to be collimated, a zoom will be perfectly focus at 3 points when correctly set up as focus works on an 's ' curve, however is should look perfect throughout the range, Once you know the lens is good you should check that the mount is totally clean, if the focus drifts the mount is wrong & needs adjusting.