Film-Tech Forum ARCHIVE: ?? About Adjustments on scope lenses

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Posted by Don Bruechert (Member # 1538) on 03-26-2003, 09:21 AM:

In one of our houses, which has a really short throw to the screen and a rather steep angle, we get a really trapeziodal picture when we project scope. I see there are several adjustments on the lens and I am wondering (without messing with them) if any of these adjustments can compensate for this trapezoidal effect. The adjustments appear to be locked off with a set screw so they can't be accidentally changed, so I am assuming they are not meant to be changed. Based on my camera experience I know that the projection angle will have a lot to do with the amount of trapezoidal effect, but in the camera world we have shift lenses which can adjust for it somewhat. Knowing nothing about projection lenses, I am not sure if there is a similar adjustment, or is it just focus and zoom??? Thanks.

Posted by Gordon McLeod (Member # 33) on 03-26-2003, 09:26 AM:

That distortion is usually reffered to as keystone distortion.
Non of the adjustments on your lens normally correct for that but one can get special lens mount collars to correct some of it but at a great loss of light

Posted by Christopher Seo (Member # 7) on 03-26-2003, 10:45 AM:

Don,

Are you referring to "swing and tilt" type camera lenses? Is there an exposure compensation necessary based on the severity of the angle off axis?

Gordon,

Would realignment of the lamphouse improve the light output after setting up one of the keystone correction lens collars?

Posted by Kris Brunton (Member # 432) on 03-26-2003, 11:40 AM:

The adjustment on the front of the anamorphic lens is for feild of depth adjustments.

Posted by Paul G. Thompson (Member # 655) on 03-26-2003, 12:58 PM:

According to a manufacturer's rep in ShoWest last year, a lense bushing/collar assy that has a correction adjustment for keystone worked fine for flat lens, but not for a scope lens.

I cannot recall what manufacturer had that bushing.

Posted by Don Bruechert (Member # 1538) on 03-26-2003, 01:08 PM:

Swing and tilt would refer to large format cameras. In the small format I believe the lens is referred to as a shift lens. I rented one once many moons ago to correct distortion caused by a wide angle lens in architectural photos, where the tops of buildings tend to lean in and stuff.... I thought this might just be a simple fix... I guess not [Smile]

Posted by John Pytlak (Member # 331) on 03-26-2003, 02:49 PM:

quote:
The adjustment on the front of the anamorphic lens is for feild of depth adjustments.

Actually, it adjusts the astigmatism of the lens -- bringing both horizontal and vertical detail into focus together.

See "Focusing Anamorphics" in the Film-Tech "Tips" section.

Schneider makes a "Keystone Corrector":

http://www.schneideroptics.com/projection/cinema_projection/keystone_corrector/

Posted by Paul G. Thompson (Member # 655) on 03-26-2003, 08:15 PM:

John, it was Schneider. When I saw that thing, I was ready to jump all over it because of what it was supposed to do. But the rep told me I was wasting my money if I wanted to use it for cinemascope.

Maybe they overcame that obstacle. Jeeze, that would be great if they did. [Smile]

Posted by Brad Miller (Member # 2) on 03-27-2003, 02:04 AM:

I remember Steve Guttag mentioning this Schneider lens mount to me. I never did follow up on it. I am very interested in seeing what it can correct for and if the benefits outweigh the disadvantages (if any).

Posted by Steve Guttag (Member # 268) on 03-27-2003, 06:35 AM:

The lens shifter or PC-Cine adapters as both Schneider and ISCO refer to them will do very well at keystone correcting...up to the point where you vignette the entrance pupil of the lens. For anamorphic applications, often the exiting portion of the lens will be the limiting factor.

There is no free in this world though...if you do correct for the keystone using a PC-Cine adapter, you will introduce linearity distortion which, in my opinion, is far less objectionable.

Since I do many 2-projector booths and most of them have 4-5 formats each, PC-Cine adapters are essential if you want all of the images to be centered on the screen for all formats and with minimal to zero keystone distortion.

Some projectors, like the Century have merely a lateral shifter built right in (only on the single lens machine). With a lens turret, you can often also achieve the benefits of what a dedicated PC-Cine adapter would have accomplished.

As for anamorphics with PC-Cine adapters...you can certainly use them it is just a whole lot more difficult. Since rotation is a part of setting up all of the PC-Cine adapters I have worked with (ISCO and Schneider)...adding the Anamorphic rotating into the mix is a big pain in the butt. Schneider's new PC-Cine adapter makes it much easier though it costs as much as a Schneider prime lens.

If you are doing special venue, you should definately look into PC-Cine adapters.

Steve