Film-Tech Forum ARCHIVE: How many degrees off screen centre is a potential focussing problem

This is topic How many degrees off screen centre is a potential focussing problem in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Adrian Hauser (Member # 4723) on 04-30-2008, 06:04 AM:

Hi,
Sorry to bring this up again but my last post went off topic slightly so Im
posting again with a particular question.

Our projector gate is about 5 degrees of axis with the centre of our projecton screen.
IE : it is positionedat the rear of the theatre to the left of centre by about a meter.
I am having focussing problems with my anamorphic lens and am led to believe that anamorphics have less "on screen" depth of field.

The room is quite small. Ill put the room dimentions below. please let me know if you think its worth moving the projector to fix this problem or if you think that with these dimentions the problem lies else where.

Lens anamorphic ISCO cinemascope Ultrastar F80mm
Screen width = 16FT
Screen to projector = 29FT
Projector off axis/centre by 5.5 degrees.

Many thanks
Adrian

Posted by Brian Guckian (Member # 1678) on 04-30-2008, 06:51 AM:

Looks like you're just outside tolerance re. projector placement (5 degrees is a rule of thumb for maximum horizontal offset), but it shouldn't be enough to cause serious focussing problems. Your 'Scope lens focal length also seems correct insofar as you are not using excessively short lenses / over-sized screen for your auditorium.

Have you eliminated potential misalignment of the anamorphic adapter in relation to the backing lens and are you satisfied it is correctly focussed? Is the lens itself mechanically OK (potential misaligned elements?)

Likewise turret / lens holder and lamphouse / mirror alignment.

How does the focussing problem manifest itself on screen? Is it left-right / top-bottom / diagonal? Or centre / edges?

Posted by Adrian Hauser (Member # 4723) on 04-30-2008, 09:06 AM:

Hi Brian ,
Thanks for the reply,
The focussing problems are a left to right phenomena. It feels
like when I rack the focus it almost racks sharps across the screen from on
edge to another.
I have set up using the alignment procedure suggested on the info page here for anamorphics. I have also set up the lamp correctly. The Lens itself is fixed , not on a turret that folds out. I feel Ive done everything right. The only thing I hvent checked is if the film i buckling in the gate. Is there a way to visually check this without burning ones eyes out?

Adrian.

Posted by Gordon McLeod (Member # 33) on 04-30-2008, 10:33 AM:

You may have to stop the lens down to get a greater depth of focus

Posted by Louis Bornwasser (Member # 3063) on 04-30-2008, 02:05 PM:

What make/model of projector? While it can be a lens problem, this sounds more to me like a gate (mis)alignment problem. Very common in Century/Westrex. Louis

Posted by Adrian Hauser (Member # 4723) on 04-30-2008, 05:17 PM:

Hmmm,
the Projector is a VIC 12. Good condition.
A:
Stopping down the lens, where to filter the lens to best achieve this?
Perhaps I can test this by placing some ND filters in the prime position to se if field of focus gets better.

B:
How does one test gate misalignment?

Thanks

adrian

Posted by Brian Guckian (Member # 1678) on 04-30-2008, 05:23 PM:

I don't know the Vic 12 but if it has any sprung lateral guide rollers like the later models, they may be stiff or even seized, pinching the film in the gate. I've seen this quite a lot on Vic 5s.

Posted by Adrian Hauser (Member # 4723) on 04-30-2008, 05:40 PM:

Thanks Brian,
How can i check to see if this is happening visually?

adrian

Posted by Brian Guckian (Member # 1678) on 04-30-2008, 06:03 PM:

If you lace up some film and turn the mech over by hand you should be able to carefully observe the film as it goes into the gate and see if it is sitting completely flat. Check for resistance as you turn over the mech. Check for free lateral movement in any sprung rollers fitted.

Another possibility is that the lens holder itself is misaligned and is not square to the gate.

Posted by Richard Fowler (Member # 893) on 04-30-2008, 09:45 PM:

ND filters will not do the trick; you need a "waterhouse" stop ring or iris within or the rear of the back up lens to increase your depth of focus. 80mm lens if mounted to the anamorphic as an "integrated unit" may limit some adjustment. You could mount the anamorphic part on a bracket on the front of the V-12 head and use a back up lens with a 62.5mm barrel; mount it in a 70.6mm eccentric lens adapter to give you some optic shift that the single lens mount of the V-12 is unable to give. The V-12 is based on the Portacine and the film trap is a typical Cinemeccanica flat gate with top guide rollers.

Posted by David Kilderry (Member # 142) on 06-18-2008, 05:59 AM:

Adrian, if the Vic 12 is getting up in hours, I would check the gate back plate for wear, uneven or otherwise.

I would move the projector closer to 0 degrees rather than stop down the light, especially if you are on or under 16fl.

If you can find a similar (does not have to be the same) focal length anamorphic set-up, try that too. It will tell you if the lens itself is a problem. You may also try spinning it 180 degrees.

As pointed out above, 5 degrees is right on the limit, your resolution is high due to the screen size, so focus will be more critical.

David

Posted by Stan Gunn (Member # 554) on 06-20-2008, 06:11 AM:

Hi Adrian send me your email address and I will send you my phone

number so as to maybe help you directly. [beer]

Regards.

Stan Gunn