ALEXA back focus
Posted 30 March 2011 - 05:34 AM
this probably seems like a silly question but since i'm new tto the ALEXA i was wondering how one checks the back focus? can anyone advise me please?
Posted 30 March 2011 - 09:45 AM
In contrast to other user-adjustable systems, it seemed to hold pretty well, in my experience.
Posted 30 March 2011 - 01:03 PM
Posted 30 March 2011 - 01:55 PM
Posted 30 March 2011 - 04:58 PM
Posted 30 March 2011 - 06:04 PM
Put your lenses up one at a time and shoot two tests of each: focus by ground glass, focus by tape, on the sharpie line. When you look at those tests the detail in the newspaper shows very clearly if the actual focus went short or long. That's why you want to be way off square to the wall.
Repeat the test at other distances if you want.
Posted 30 March 2011 - 07:31 PM
A collimator or chart test (or diagonal newspaper test) are some of the methods available to check flange depth, but they all rely on the lens used being spot on. You can do multiple checks with different lenses, or check the test lens on a collimator first, but the simplest way is to use what Arri themselves use, a Denz Flange Depth Controller (FDC). It's basically a mini collimator housed in a lens, which gets fitted to the camera mount. The camera is connected to a monitor, and by adjusting the FDC barrel a simple bar graphic shows exactly when the flange depth is correct. A scale on the FDC barrel then tells you how many hundredths of a mm you need to add or remove from the mount shimming. Very simple and portable. I believe other companies now also make versions of the same idea.
Because of the robust build quality and design of Arri cameras generally, once the flange depth is set it rarely needs adjustment. But it's one of the checks a rental house does regularly.
Posted 31 March 2011 - 02:39 AM
FDC makes sense - i am familiar with the depth gauge so the FDC is teh "digital" version. got it. thanks!
Posted 03 April 2011 - 07:29 PM
Testing out all the lenses is crucial, because if it's just one lens that's off, then obviously it's that one lens. But if it's all the lenses, then certainly there's a backfocus issue.