Focus problem on the RED ONE and S4 lenses

[Miguel Lindenberg]

Hello fellow AC's,

I'm working on a long feature film with the Red cam and COOKE s4 lenses.
We've been checking the cam back focus every once and then, without difficulties on doing fine adjusts on it. (Puting a 14, 18 or 25mm on and on the minimal distance, 25cm.)
Unfotunately, today something weird came up:
All the lenses, from 14 to 135mm, are good on their minimal distances.
But as we put then farther, 2 meters on from the chart, they all start having serious differences from their supposed ring distances. 10 to 25 cm differences on 2 meters and a lot more as we went farther from the chart like on 7 meters:
65mm - 8mts 75mm - 8mts 50mm - 8mts 40mm - 8,5mts 35mm - 12mts !!!

The biggest mistery is that the 100mm and 135mm are sharp on all distances, what excludes the back focus problem hypothesis.

The first thing that comes up to my mind is to get a second camera body (35mm that should be pre tested and reliable) and test the lenses on it. After that, be sure what lenses are reliable and with one them adjust the RED back focus,

Do any of you have had a similar problem working with the Red before or have clues on how should we proceed?
Facing the given results is really hard imagine an explanation to this problem.

Thanks in advance.

 

[Matthew Duclos]

I would test the camera and the lenses independently. Testing the camera with the lenses only works if you are absolutely certain the lenses are spot on at infinity. And same goes for the lenses. They should be checked on their own. If the camera flange adjustment is out, all of your lenses will read differently. You can use any number of tools to test the camera. The lenses should be checked on a collimator and projector.

 

[Jeff Kilgroe]

We've been checking the cam back focus every once and then, without difficulties on doing fine adjusts on it. (Puting a 14, 18 or 25mm on and on the minimal distance, 25cm.)

Do what Mr. Duclos suggests. Also, if you are going to check the camera back focus / flange depth with a lens. It's best to use the widest lens you have available (that you are sure is properly collimated) and with the focus set to infinity.