Film-Tech Forum ARCHIVE: Focus Issues -

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Posted by Adrian Hauser (Member # 4723) on 12-11-2008, 05:16 AM:

Hi Film Techers,

Before I start I will say I have already done the alignment test as per the
anamorphic procedure in the tips section.

Im having problems adjusting my focus.
It seems I can get the centre in focus but the sides are out. It is more noticable
on the anamorphic but also minutely present on the Flat lens as well.
This says to me it is a projector problem not "necessarily" a Lens problem.
Probably both, yes, but Im keen to find out what checks can be done
with the projector and lamp to eliminate what it may not be.

Projector Cinemattica 12 . Lenses Isco .
Open gate screen Luminance 14fl. 5600 Kelvin.
Non Variable output lamp.

Ive read on other topics that Lamps and radiant heat can also effect flat field focus.
What can I do to test these.

I have played with the gate tension rails and this has had no effect. It is very hard to tell if the film is bowing within the gate itself.

Id prefer constructive help rather than people bagging my cinemattica [Smile]

Many thanks in anticipation,

Adrian Hauser

Posted by Jim Cassedy (Member # 4115) on 12-11-2008, 10:34 AM:

I'm not familiar with a Cinematacca Projector so I'm not sure this suggestion will be of any help but I had a similar problem with a Century projector last year.

I could get the center of a scope image in in sharp focus, but the edges were soft, or I could focus to get the edges sharp and center was slightly out.

In my case, it turned out not to be the guide rails on the film gate, but the side guides on the film trap, which somehow had gotten slightly out of alignment by several thousandths of an inch. This caused a minute 'cupping' of the film which was invisible to the eye when looking at the projector running, but very apparant on screen, especially in scope.

I did not have a guage to re-set the width properly but I was finally able to do it through trial and error and I've had no focus problems since.

Posted by Monte L Fullmer (Member # 2797) on 12-11-2008, 12:54 PM:

(is this a Cinemeccania machine?)

Sounds like you've got a flat gate and trap machine there with bad runners that's causing slight film bowing in the middle due to heat from the bulb.

Posted by Benjamin John Doman (Member # 4365) on 12-11-2008, 01:02 PM:

I saw something similar many years ago on a DP75 with Peerless Magnarc conversion. We could focus one side or the other, but not at the same time. Worse on ‘scope. It had been a problem since the theatre had been twined. It turned out that the mirror mounting was not square in the lamphouse.

Posted by Adrian Hauser (Member # 4723) on 12-11-2008, 04:33 PM:

Thanks For the Replies,

Regarding the gate trap. Are the runners you are describing the little spring loaded
guides down the inside edge of the gate?

Are the side guides and runners the same thing?

Adrian

Posted by Monte L Fullmer (Member # 2797) on 12-11-2008, 06:57 PM:

Sorry on that - runners are the rails that the flat part of the film is pressed against when the gate closes. If there is any wear in this area, the film will not lay flat between the two surfaces and can flex causing focus issues.

Posted by Gordon McLeod (Member # 33) on 12-11-2008, 07:16 PM:

I am assuming that you are referring to a cinemcanica vic12 which is also the portacine

my suspision is that you have several alignment issues
optically the lens mount axis must align with the lamphouse axis and perpendicular to the film plane
The vic 12/portable was less than exact in all of those issues

Posted by Adrian Hauser (Member # 4723) on 12-11-2008, 10:23 PM:

Thanks Gordon and Benjamin.

Yes it is the Vic 12.

given your statement below how would I double check the alignment of these.
When you say lamp house axis are you describing the Lamp itself, the mirror, or the entire housing?

Im keen to make this the best Vic 12 there is..... because its the only one we got and
were not about to invest in another.

>my suspision is that you have several alignment issues
>optically the lens mount axis must align with the lamphouse axis and >perpendicular to the film plane
>The vic 12/portable was less than exact in all of those issues.

Adrian Hauser

Posted by Jim Cassedy (Member # 4115) on 12-13-2008, 09:17 AM:

quote: Adrian Hauser
Regarding the gate trap. Are the runners you are describing the little spring loaded guides down the inside edge of the gate?

I'm not sure what they look like on your machine, but here is the actual
film trap I had the same problem with that I described in my post to you.

The rails marked "1" and "2" had somehow gotten out of
alignment & were causing a slight 'cupping' of the film.

I did not have a proper gauage to re-set them but I was
able to do so using a piece of scrap film and a bit of trial & error.

Have had no focus problems on this machine since.

Posted by Tim Reed (Member # 137) on 12-13-2008, 12:59 PM:

Those are the studio guides. The "alignment" for them is very easy; shove them apart as far as they will go. They're totally unnecessary.

Film lateral alignment is taken care of, surprisingly enough, by the lateral guides.

Posted by Louis Bornwasser (Member # 3063) on 12-13-2008, 02:14 PM:

Tim is right! Every unit shipped by ORC was wrong. Had huge "disagreement" with Jerry Kampbell who was their director of manufacturing. (Simplex XL, curved gate; you can see the edge of the film from the lateral guide roller to the sprocket.) Louis

Posted by Justin Gorka (Member # 3727) on 12-13-2008, 05:06 PM:

May seem a bit obvious....burnt lens?

Posted by Rob J. Buskop (Member # 3338) on 12-17-2008, 09:58 AM:

This is a fairly common problem.
If there is an area that simply will not focus there's one most
probable answer to your problem:
The lamp and mirror unit has to be perfectly realigned with the
optical axis of the projector.
To do this properly you need special tooling to get it really right.
Alignment shortcomings are the most common cause for your problem.
so I'd like to suggest getting a properly equipped technician around and have everything realigned.
Also it's worthwhile to check the concentricity of the cathode-
anode gap with the fitting end of the lamp.
It is not uncommon to find that the arc is out of centre as well.

Posted by Jeff Else (Member # 4063) on 12-18-2008, 02:10 AM:

i have a newbie question out of curiosity... how is it that lamphouse alignment can affect picture focus? If the film plane is perfectly perpendicular to the lens, why should the image not be in focus (albeit poorly and unevenly lit)?

Posted by Gordon McLeod (Member # 33) on 12-18-2008, 09:42 AM:

The optical system is just not the lens but the optics of the illumination system as well
In fact a projection system is a optical bench for all intents and purposes

Posted by Steve Guttag (Member # 268) on 12-19-2008, 08:22 AM:

Jeff....the simple answer to your question is that what you are REALLY focusing is LIGHT, not an image or the film. The film creates a shadow in the light that we see as an image.

This is the same reason you see a fringe around the image when an aperture plate is inserted...you are seeing the light hit the aperture plate first, then the film. The amount of fringe can be affected not just be the distance between the film and aperture but also the reflector design (how straight the light rays are when they hit the aperture) and the lens (how is its inlet pupil designed IN RELATIONSHIP TO THE REFLECTOR design.

If you had a poor lamp alignment you will be getting multiple light paths to the film plane and ultimately the lens...this will translate to multiple images on the screen or a soft/blurry picture.

Steve

Posted by Jeff Else (Member # 4063) on 12-22-2008, 10:38 PM:

ahh, thanks!