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ARRI BL3 ground glass adjustment ?


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#1 P Mitcheltree

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Posted 11 August 2011 - 01:48 PM

If possible, I would like to understand the mechanics of adjusting the ground glass distance on this BL3.
The camera has good focus on the focal plane but not accurate in the viewfinder.
The flange focal depth was measured and is good.
The 50mm Zeiss test lens is good, the ground glass image is not.
Both the lens and viewfinder system were checked on a collimator.

With the 50mm lens, cam body focal plane located 4 ft from test chart, lens has focus ring reading 4.5 ft when focused.
Same lens on a Arri 435 body reads 4ft spot on at 4 ft distance from chart.

How difficult is it to raise or lower the ground glass / block system ?
Do the adjuster screws B control this or is it single screw A ?
( I understand Arri may use 2 screws where Moviecam only uses one )

Screws B have fixed nuts below which do not turn.( 2nd photo )
As screws B are turned clockwise the heads retreat lower in the body, does this lower the ground glass support frame ?
Thanks for any input you can provide.

( we may be sending this body away for service but it would still be nice to understand how easy or difficult this adjustment is before we do that.)

( My email is hemshaw at gmail dot com )


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Edited by P Mitcheltree, 11 August 2011 - 01:50 PM.

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#2 Dom Jaeger

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Posted 11 August 2011 - 09:09 PM

To properly set the ground glass depth the camera needs to be mounted on a bench collimator, using a test lens that has itself been very finely collimated.

A 50mm lens has too much depth of focus (at the image plane), it will look sharp even if the back focus is out by several hundredths of a mm, which is more than the ground glass tolerance should be. I use a test lens of around 18 to 25 mm focal length, which has a shallower depth of focus, and so allows for a more accurate setting. Judging sharpness by eye through the viewfinder with an 18mm is very difficult, which is why a collimator is essential.

It's been a while since I worked on a BL3, but I think the 2 screws you've labelled 'B' are just plugs to cover the access holes. Underneath are two nuts that secure the slotted depth adjustment shafts. To adjust them, the nuts are undone half a turn, and the shafts minutely turned, equally to maintain flatness, then locked again with the nuts. It helps to have a special tool, basically a slotted driver within a socket driver, to hold the inner shaft and prevent it turning as you lock the nut. There is also a tool (and a complicated procedure) to measure flatness (necessary for even focus across the ground glass), but unless the shafts have been fiddled with, the ground glass should be flat from the factory, so equal adjustment of the two shafts will keep it that way.

The centre screw is an excentre to adjust horizontal framing, beneath the ground glass are two more screws to adjust vertical and twist.

Without wishing to sound like a party-pooping disciplinarian, as a rule whenever a camera screw has been sealed with paint (like the green stuff in your pictures) it is often a precisely set adjustment that shouldn't be fiddled with outside of a technician's bench. It's very easy to introduce more problems than you started with..
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#3 P Mitcheltree

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Posted 12 August 2011 - 07:08 AM

Thank you Dom for that detailed explanation !
As you said, it is easy to introduce more problems without the proper test gear.
The body will probably be sent out for repair. but as a technical person I appreciate understanding how the Arri procedure works.
Cheers.
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