Film-Tech Forum ARCHIVE: Scope Lens orientation?

This is topic Scope Lens orientation? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Andrew Bangs (Member # 4938) on 09-04-2008, 12:47 PM:

I have just recently notice that our scope lenses were oriented with the oval shape in the virtical axis. As I am a bit of a nube... Is this correct? My first instinct is that they should be oriented horizontaly. Having said this the picture looks ok with it in the Vertical but I just want to be sure.

Sorry I think this is in the wrong thread and should be moved to film handlers.
I'll get the hang of this...

Posted by Jack Theakston (Member # 4484) on 09-04-2008, 12:54 PM:

Vertical oval is correct. Remember, when you're looking into the front, the light is being squeezed at the other end, not expanded.

Posted by Galen Murphy-Fahlgren (Member # 4535) on 09-04-2008, 02:54 PM:

You will know immediately if your scope lens is oriented wrong. I had one at my old theater that would creep over slowly, and if that theater had flat movies for a while, the first start with a scope movie could take you by surprise with the image as a narrow diagonal line across the entire front of the auditorium.

Posted by Monte L Fullmer (Member # 2797) on 09-04-2008, 06:40 PM:

..and the little numbers on the outside of the barrel means throw focal length from aperture to screen and the barrel is usually locked into place for that correct distance .. which only a tech should change that if needed..

Posted by John Wilson (Member # 269) on 09-04-2008, 08:44 PM:

And this is in Ground Level why? [Confused]

Posted by Monte L Fullmer (Member # 2797) on 09-04-2008, 08:56 PM:

cuz Andrew, being a noobie, forgot to change forums when he began this thread. As he mentioned above, he's learning.

Posted by Tim Reed (Member # 137) on 09-04-2008, 09:52 PM:

Remember, you're looking at a prism. The light is refracted outward (horizontally) when it hits those asymmetrical slopes on the sides.

I don't know where you can find one right now, but if you ever look at a cross-sectional drawing of an anamorphic lens, it will be obvious.

Posted by Randy Stankey (Member # 64) on 09-04-2008, 10:53 PM:

As Galen alluded, if your scope lens isn't turned the right way, the picture won't line up with the screen.

Therefore, if your picture fits the screen your lens is in the right way.

Posted by Andrew Bangs (Member # 4938) on 09-05-2008, 12:15 AM:

Cheers Guys [beer]

I appreciate it. I figured it was right but I didn't know if their was some Jedi Secret trick or not.

Monty Thanks for the additional info I was slightly curious about those numbers on the outer ring.

Ok since I Think I have your attention I have one more lens question. Actually I guess their are two.

First, I have noticed some pictures Keystone but only on one corner (usually upper left or lower left). What causes this? Is it the Lens orientation going out of whack or is the projector its self out of position.

Second, I have one screen where no matter what I do I cannot get the outer Edges in focus. The center of the screen is crystal clear but the sides of the screen are a blur. The last guy told me it had been like that forever and to just live with it but that is unacceptable to me. If I adjust the focus it just seems to shift from side to side so right now I have it split even. It is a pain and we get a comment on it once every couple of weeks. Thankfully it is on one of our smaller screens which gets the movies that are on the way out. But if it happens to one of our bigger theaters we will never hear the end of it.

Posted by John Wilson (Member # 269) on 09-05-2008, 12:45 AM:

quote: Andrew Bangs
If I adjust the focus it just seems to shift from side to side so right now I have it split even.

Your lens is not on the same plane as the film aperture meaning one side of the lens is closer to the projector than the other. There's no way you'll be able to get overall focus on that without adjusting. What sort of machine are you running?

Posted by Andrew Bangs (Member # 4938) on 09-05-2008, 12:55 AM:

They are Horizon Projectors. it takes two allen set screws to release tension on the lens plus I think their is a Spacer ring. I will check tomorrow if it is seated correctly. I am pretty sure This may also be the incorrect lens for this projector as they told me the old one was dropped so they grabbed a scope lens from our old theatre which used Strong Projectors.

Posted by Monte L Fullmer (Member # 2797) on 09-05-2008, 02:00 AM:

quote: Andrew Bangs
First, I have noticed some pictures Keystone but only on one corner (usually upper left or lower left). What causes this? Is it the Lens orientation going out of whack or is the projector its self out of position.

Is the machine center to screen, or off to the right some? If neither, wonder if you have an aperture plate that wasn't cut correctly to compensate keystoning. As long as the scope image is in true vertical-not twisted to either side, I'd call it bad aperture cutting.

quote: Andrew Bangs
This may also be the incorrect lens for this projector as they told me the old one was dropped so they grabbed a scope lens from our old theatre which used Strong Projectors.

Then, wonder if they'd even check the focal length of the prime lens between the one that the Horizon used, and the one that the Strong used.

Also, if that one that was on a Strong was dropped, 10 to 1 that the lenses inside that attachment got jolted some and has thrown the whole lens elements inside out of kilter. ... why you do take special attention and being careful with anamporphic attachments.

-Monte

Posted by Andrew Bangs (Member # 4938) on 09-05-2008, 02:21 AM:

The aperture on the Horizons Change Automatically with the rotation of the lens turret will Try to get you a pic But it is still possible that the aperture is miss alligned or cut wrong. My bet however is that the projector is not flat to the screen I just thought that would keystone the Whole Side not just the top or bottom corner. Perhaps their is a combination. Thanks for the Ideas of where to look.

Posted by Rick Raskin (Member # 1561) on 09-05-2008, 08:39 AM:

The ideal projection angle would be perpendicular to the center point of the screen with the projector set absolutely level (0 degree rake). Keystone is a function of projection angle and rake, and will vary accordingly. Properly cut aperture plates will [square the image and give the impression that] keystone [has been eliminated]. (Thanks Brad) In some theaters with excessive rake the screen was slanted back to help compensate.

[ 09-06-2008, 08:38 AM: Message edited by: Rick Raskin ]

Posted by Andrew Bangs (Member # 4938) on 09-05-2008, 08:30 PM:

I just found out today that the Screens in our theatre were suposed to be Tilted 7° but unfortunately that never happened so we will always have some keystone.

I did futs with The lens that was going blurry on the sides and after removing it and checking he spacer mount I heard a faint little pop. Either it's fixed or I broke the lens... Good news is it's Fixed. Thanks for tip on that one John.

Posted by Brad Miller (Member # 2) on 09-05-2008, 09:24 PM:

quote: Rick Raskin
Properly cut aperture plates will compensate for keystone.

Ok I know what you are trying to say, but to someone trying to learn that statement is incorrect.

What Rick is trying to say is that properly cut aperture plates can give the ILLUSION that the image isn't keystoning as bad as it really is...but the keystone remains.

Posted by Andrew Bangs (Member # 4938) on 09-06-2008, 01:37 PM:

not to be snyde but I did understand. I know the aperture just blocks light doesn't actually change it. The aperture is basically your first line of Masking. I may be a nube here and to film projection but I have been working with AV equipment for years as well as other lighting equipment. I used to be a theatrical lighting designer. All the way from Coffee Can overheads to the latest Intelligent moving head fixtures. Compared to those these Projectors are pretty simple and have allot more working space in them. But still it is kind of like learning a new dialect of a language you already know.

Having said that Thank you for looking out for us nubes. It is not often you get that in a forum. Usually you just get made fun of for not knowing what might be considered basic.

Cheers! [thumbsup]