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ALEXA back focus


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#1 christian mann

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Posted 30 March 2011 - 05:34 AM

hi there,
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?
thanks!
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#2 Seamus Mulligan-Ferry

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Posted 30 March 2011 - 09:45 AM

It's set like a traditional film camera, by using shims behind the PL mount. This means it should be done during/before the checkout by a qualified technician with the proper tools (i.e. a collimator).

In contrast to other user-adjustable systems, it seemed to hold pretty well, in my experience.
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#3 christian mann

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Posted 30 March 2011 - 01:03 PM

okay, that's what i had in mind...some 1AC confused me when saying he checks the back focus during gearcheck....so it's the rental house's responsibility in a way.....thanks!
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#4 John Sprung

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Posted 30 March 2011 - 01:55 PM

True, but it's the first's responsibility to check that they did it correctly. Shoot a diagonal newspaper test if you're worried.






-- J.S.
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#5 christian mann

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Posted 30 March 2011 - 04:58 PM

diagonal newspaper test? not familiar with that...what is that about? like the "harp-test" to check the groundglass is in correct position?
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#6 John Sprung

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Posted 30 March 2011 - 06:04 PM

OK, get a full sheet from a newspaper, with lots of detail on it and a fair variety of type sizes. Classified ads work well. Tape it to a wall roughly 30 degrees off level, then use a yardstick and sharpie to make a vertical line down the middle of it. Place the focal plane of the camera 10 feet from the sharpie line, but instead of square on to the wall, you want to be quite far over, like about 45 degrees or so. The only critical thing is getting the distance from the sharpie line to the focal plane exact.

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.




-- J.S.
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#7 Dom Jaeger

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Posted 30 March 2011 - 07:31 PM

Unlike a film camera, which can be mechanically checked very precisely with a depth gauge, digital cameras need to be checked optically.

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.
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#8 christian mann

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Posted 31 March 2011 - 02:39 AM

okay, so the newspaper test is just like the harp-test we do this side (cape town). just a different name.
FDC makes sense - i am familiar with the depth gauge so the FDC is teh "digital" version. got it. thanks!
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#9 Mitch Gross

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Posted 31 March 2011 - 10:29 AM

A lot less expensive way is to use the the NULL Lens & Target that we sell at AbelCine. Put it on and if the image comes up sharp then you're all set. It is extremely accurate and costs a fraction of the FDC.

http://www.abelcine....mator-Kit/#tabs
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#10 Jonathan Bowerbank

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Posted 03 April 2011 - 07:29 PM

The basic prep practice (described in any AC manual) of always taping out your lenses to make sure they're sharp (starting with each lens' close focus, then working outwards) will always tell you if there's something wrong, whether the lenses are out or the back focus is out. At which point you alert the rental house's tech and they should troubleshoot it for you.

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.
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