Film-Tech Forum ARCHIVE: Scope lens set up

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Posted by Richard May (Member # 2627) on 03-18-2007, 12:01 PM:

What is the proper way to set up a scope lens? When you are shooting with no keystone it seems pretty simple. When you have a keystone issue (even a slight one) it is more difficult. What do you look for when twisting the lens? Obviously you want the center of the picture to be level. What are the tricks?.............sorry, not the focus.......the actual shape of the pic....

[ 03-18-2007, 03:40 PM: Message edited by: Richard May ]

Posted by Floyd Justin Newton (Member # 1316) on 03-18-2007, 12:18 PM:

You'll find the alignment procedure is the 'Tips' section to the left. [Smile]

Posted by Gordon McLeod (Member # 33) on 03-18-2007, 12:19 PM:

first with the flat lens enure that the machine is projecting a square image of the target on the screen often I find people will twist the anamorph to compensate for leveling the machine
Then I focus the anamorph so that both vertical and horizontal target lines shift in focus at exactly the same rate

Posted by Richard May (Member # 2627) on 03-18-2007, 03:38 PM:

I'm not talking about focusing the scope lens. That I know how to do. I'm talking about the actual shape of the image in scope. I do know to first set up flat and make sure the machine is level. The problem arises when there is a keystone. Even if the machine and maskings are level, how do you know when you have the right twist in the scope lens since now the target film lines arent level because of the keystone?

Posted by Louis Bornwasser (Member # 3063) on 03-18-2007, 04:57 PM:

Like Gordon says: level the machine using the flat lens first. Then center using the flat lens. Scope will then be already level except for the "twist." Louis

Posted by Richard May (Member # 2627) on 03-18-2007, 06:15 PM:

I understand that. When you have a keystone though, how do you get the center of the pic level in scope other than holding a level up on the screen? The top and bottom lines on target film flare out.

Posted by Louis Bornwasser (Member # 3063) on 03-18-2007, 07:50 PM:

Gotcha! Drop a plumb bob on the screen center-line and disregard all else. At least the center is vertical.

If angle is very great, it is sometimes necessary to move the top of the screen back. One old roadshow house here in Louisville had the top 6 ft back of the bottom. Louis

Posted by Kenneth Wuepper (Member # 1174) on 03-19-2007, 06:35 AM:

Richard,

Be aware that tilting the screen may correct the keystone effect as viewed from the booth but it may cause other problems. I am thinkling of the effect on the reflection from the screen to the viewers. Even small gain screens, (Super-Glo) comes to mind, can have reduced light to the audience if the angle is wrong.

A compromise may be necessary between keystone and light.

KEN

Posted by Frank Angel (Member # 248) on 03-19-2007, 07:45 AM:

The keystoning issue exists whether it's an anamorphic (scope)lens or spherical (flat) lens. It is produced by the angle of the projector in relation to the screen when it is not projecting from a position that is at the center of the screen but above it (or below it in the case of many Drive-ins. It is a fixed error and the keystoning is a fixed result of that error. Neither the flat or scope lens can be adjusted in any way to correct it.

Projector aperture plates and the screen side masking tabs are used to "correct" the keystoning. The apeture is cut at an opposite angle of the keystone splay so as to "hide" that portion of the picture that is angling outward past the screen's perpendicular edge. Although the plate forces the edge of the image to meet the perpendicular edge of the screen, the distortion remains in the image and any lines in a target film will still splay outward toward the bottom of the image. Nothing except tilting the screen back will correct this as Louis mentioned. That solution comes with its own negatives as Kenneth said.

The best you can do is make sure the plates are filed correctly and the picture is in focus, then let the lines fall where they want.

Posted by Richard May (Member # 2627) on 03-19-2007, 07:52 AM:

Great idea Louis. Thanks. That was the answer I was looking for. Luckily there is no downward tilt keystone since the projector is the same height as the center of the screen. There is only a slight sideways keystone but just enough to skew the lines a bit.

Posted by Frank Angel (Member # 248) on 03-27-2007, 02:00 AM:

quote: Richard May
Luckily there is no downward tilt keystone since the projector is the same height as the center of the screen. There is only a slight sideways keystone

Richard, the projector is at screen vertical center but not centered from left to right? Was it originally a single screen split down the middle? Or a change-over booth but now only a single projector is being used? Even so, I can't imagine that small angle in a change-over booth would cause keystoning of any significance.

Posted by Monte L Fullmer (Member # 2797) on 03-27-2007, 02:08 PM:

..you doing all of your scope centering with the aperture out of the projector?

If you're not centered, you might have to cut new aperture plates using undersized apertures...

Posted by Richard May (Member # 2627) on 03-28-2007, 07:44 AM:

Frank, You are right. This is a quad that was built originally as a quad with changeovers. Yes it is vertically centered. And yes it is only slightly off center to one side by about 2 feet. Like I said, it is only skewing the horizontal lines on target film ever so slightly. I'm just trying to get it perfect. Thanks again. [Smile]

Posted by Steve Guttag (Member # 268) on 03-28-2007, 08:50 AM:

If you are using the current 35PA target...look at the four circles in the corners (with cross hairs in the middle of them). They are there to aid in getting the scope lens perfectly squared up since all four will only be circles when the lens is twisted correctly.

In the document RP40...reference target "E" and Annex A.4.

Steve

Posted by Frank Angel (Member # 248) on 03-30-2007, 10:57 AM:

Richard, Hi5 to you [beer]

We need a lot more perfectionists and a lot less, "can't-be-bothered-ists."

Posted by Richard May (Member # 2627) on 03-30-2007, 01:15 PM:

LMAO Frank...I like that....Thanks.

Posted by Richard May (Member # 2627) on 04-05-2007, 09:15 AM:

Ok, I dropped a plumb bob like Louis suggested. I leveled the console using the flat lens. I lined it up with the center line on the target film. This showed how out of level the top and bottom maskings are. Once the flat picture was level, I put in the scope lens and twisted it until the center line on the target film was perfectly inline with the plumb bob again. I was only off a little bit doing it by eye. Great idea Louis. Thanks.
Steve, I also played around with the circles in the corners but found it a bit easier using the line. Thanks again.

Posted by Louis Bornwasser (Member # 3063) on 04-05-2007, 11:13 AM:

Glad to hear of your success !!!

The plumb bob system means that the center is correct even if the rest isn't. Since most action is centered this means that the distortions are in areas that are less critical. Louis