Incorect Flange Focal Distance

[Dmitriy Tartishnikov]

Hello,
We have one stable problem in our red one. Now we are shooting and it's still on.
Using on PL mount, scale on lens doesn't fit with sharp picture on red evf, monitor etc, so pulling focus with scale is useless, because flange focal distance is incorrect. With Cooke S4/i lenses is +/- ok, on Arri Ultra Primes its liek +/- 2cm incorrect, but whats most problematic, Zeiss High Speed MK2, there's sometimes incorrect in meters. We twice tested lenses on calimator, and they are good. So there a question, can we somehow adjust the mount(like on any film camera, for ex. 435, arricam).
I wrote about this some time ago, but i checked lenses my self on calimator, and even on other cameras(435exteeme, moviecam), and they are correct.
Did someone have this problem?
Thanks

 

[martinnoweck]

- what mount are you using?

- did you adjust the backfocus of the red mount / pl mount of the camera?

(you can't adjust the backfocus with shims like a filmcamera)

do adjust / check the backfocus of the camera:
- choose your widest and high quality lens
- check this lens on a collimator or projector
- put the lens on the red camera
- put a siemens star 1m distance to the sensor plane
- use the 1x1 magnify mode and focus by eye
- check the distance on the lens - you should read 1m or the backfocus of the camera / mount is off

regards,
martin

 

[chuck colburn]

If you can check the lens on a collimator and it is on at infinity then just put the lens on the camera body and use the collimator to adjust the back focus control on the camera body at infinity focus. Quick and easy and by far the most accuret way to do it.

 

[Dan Hudgins]

f/ stop

f/ stop

...but i checked lenses my self on calimator,...

Did you check the scales on the lenses with them all set to the same f/ stop?

There is a reason I am asking.

 

[chuck colburn]

Yes some lenses of older or more simplistic design suffer from focus shift with change of stop. Most often if you set a lens with this inherent problem up at infinity and wide open, when you shut it down it shifts past the film plane. (into the red)

 

[Dmitriy Tartishnikov]

Did you check the scales on the lenses with them all set to the same f/ stop?

Of course, there was 2.8 on all of them.
Red answered me by this question to try adjust the mount(by the way, i'm talking about pl mount) manually, unscrewing screws on camera, and adjust it on scale board. But strange is that on different lenses, its different situation, cooke are cool, but High speed totally out of focus...

Ill try what red support suggest to do, but im afraid of one thing, that if I change it to High speed, will it be cool with cookes still.

 

[Steve Gal]

You must make sure those lenses are seating properly into the Red's PL Mount. There are sometimes PL adapters on lenses and they have a 45 degree angles and the Red PL mount does not have proper chamfer 45 degree cut and doesn't let the lens seat right. Not saying this is your problem but just make sure and check.

 

[Tim Sutherland]

It sounds like some of your lenses are not collimated correctly. If the scales come up right on a lens that is correct, then your flange depth is correct, but if all the lenses aren't spot on, then you'll have this problem.

Another thing is you should be setting your backfocus (flange depth) on the camera when using your widest lens - at least 18mm.

You could also try stopping by a rental house with a lens projecter and have them take a look at your lenses first, that would be a good start.

 

[chuck colburn]

Lens projectors don't show you proper flange focale depth. They show you how the lens looks across the image plane. A collimator with a back focus micrometer shows you proper collimation. A lens projector can show you flange focale depth error but are generally not used for such test.