B4 to PL Mount - Setup and Proper Back Focus

[clem]

I purchased a B4 to PL Mount adapter from RED to use with a robotic B4 lens that we had laying around. I have tried 3 or 4 times now to back-focus this setup and still cannot get a sharp focus throughout the zoom range.

Here's what i have tried:
1. Setup a focus chart at 10 feet from the lens (also tried at 70ft, but it was too hard to tell if i was at the right spot on the lens without lens data)

2. Focus as tight as possible with the focus ring on the lens

3. Focus as tight as possible with the back-focus adjustment on the b4 lens

4. Focus as tight as possible with the back-focus adjustment on the RED ONE

I am able to get a tight, sharp focus at any focal length, but then when i zoom in or out, it goes out of focus, and if i zoom in too far, it's impossible to focus (even on a subject 70+feet away)


Is there something i'm missing, am i doing something wrong, or is this a limitation of the B4-PL mount adapter?

 

[Cail Young]

You should set the camera flange distance with a known good PL mount lens first. Then set the B4 lens backfocus as normal without adjusting the camera.

 

[Shane Kelly]

I'd agree with Cail, set the PL mount on the camera ( or return it to it's factory setting, if marked) then just do a backfocus on the B4 lens. Are you saying your B4 to PL adapter also has a backfocus adjustment?

 

[Rich Schaefer]

Yup, Cail is right.

First, Set you Red camera's PL Back-focus with a red-null, colimnator, set of trustworthy primes.

Secondly mount your 2/3 lens and back-focus. I like to back-focus 2/3 lenses like the old school TV sports guys. Here's how:
1- Open the aperture wide open! Zoom in on your Red LCD or EVF
2- Zoom the lens in to something far away 50feet plus, something with detail.
3- Zoom 70% out wide.
4- Adjust sharpness with back-focus adjustment only.
5- Repeat 2 through 4 at least once more
This usually works better than setting a chart at 10 or 8 or 6 feet and back-focusing. I have seen many a great AC struggle with video back-focus using the 10 foot method. The biggest problem is they assume the marks on the VIDEO lenses are a accurate

Best of luck!
Rich

 

[Jacek Zakowicz]

Yup, Cail is right.

The biggest problem is they assume the marks on the VIDEO lenses are a accurate

Best of luck!
Rich

OK Guys,
We've been through this so many times. First of all the marks don't need to be accurate- the lens just has to hold focus tele and wide to be correctly set.
Back focus procedure for zoom lenses:
I assume that your camera and adapter are setup correctly. If not do that first. There are many ways to do it but , at least the first time I would reccomend hiring someone with experience to help/teach/explain. It's well worthed.
1. Mount the lens on camera/adapter and zoom in. Set focus chart as close as you can-2-3 ft from camera.
2.With the lens wide open focus the image zoomed in with focus scale only.
3. Zoom out and refocus the image with the lens's rear collar only.
4. Zoom back in and repeat focusing with the scale on lens again.
5. Zoom out and see if you can refocus with the rear collar again.
Repeat above steps until no adjustments are necessary for either end.