Film-Tech Forum ARCHIVE: Best Lens Specification for Short Throw ?

This is topic Best Lens Specification for Short Throw ? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Kerry Fleming (Member # 1657) on 03-29-2003, 07:36 PM:

I recently aquired a Century model S A projector and have encountered some lens problems.

I have a limited throw distance of apx. 20 feet available and want to obtain the biggest picture possible on a 10 foot screen.

I have tried a Sankor Deluxe MC (flat) lens (F 2.0 f=50mm 1.97 in.) and can not get focused. Lens bottoms out!

The base lens for my scope adapter is a Super Snaplite BX290 (f=1.7). I can focus that lens when used alone. But when I add a B&L scope adpater I can not obatin a completely clear focus. And the picture is much smaller than I would like it to be.

I welcome any suggestions or advice you have to offer for obtaining the largest focused picture possible given the constraints of this problem.

Thank you for your replies.

Posted by Pat Moore (Member # 380) on 03-29-2003, 07:51 PM:

Hi Kerry;
That 50mm lens should be about right for that screen (assuming 4.5' x 10'). I haven't played that much with Sankors other than some tests against ISCO's. What do you mean, "bottoms out"? You can't get it back close enough to the film?
Your scope lens should be around 75mm, but I doubt of the B&L Anamorphic will focus on that short a throw. That might be tough with the newer anamorphics, though I'll say I haven't had to try much at 20ft. Couple other people here may have tried 20ft throws though...
Good luck, and welcome!

Pat

Posted by Brad Miller (Member # 2) on 03-29-2003, 08:51 PM:

ISCO and Schneider both make lenses that are suitable for your application. I've not had a problem with either focusing down to 20 feet. In fact, I've used both of them down to about 15 feet with no real issues.

One recommendation though, sell the Sankor and buy ISCO or Schneider lenses. The difference in quality is really worth it. If you don't have a large budget, check around for used lenses.

Regarding your "bottoms out" comment, I am assuming this is a single lens projector. Have you tried mounting the lens in it's holder loose, such that once you insert it into the projector you can reach in with a finger and slide the lens forward of backwards a little? You may have already tried that, but I just don't see where you wouldn't be able to focus a 50mm lens at that throw.

Posted by Gordon McLeod (Member # 33) on 03-29-2003, 09:06 PM:

For the century Sankor offers a extension ring that moves the back element farther back in a extended 62.5mm tube
Centurys are one of the few projectors that lens can sometimes not physically get far enough back in due to the shroud over the tube

Posted by Phil Hill (Member # 371) on 03-29-2003, 09:09 PM:

Brad: I agree... ya can't beat a Schneider! I've done many lens tests over the years; I ***Personally*** think Schneider is the best.

Although short-throws do present many problems.... if you keep the magnification to with-in reason, the image will be great!

>>> Phil

Posted by Steve Guttag (Member # 268) on 03-30-2003, 01:11 AM:

Phil,

It depends on the criteria...ISCO and Schneider both have their strong and weak points....both are substantially better than anything else out there too.

As to 20-foot throws...just did one with ISCO Ultra Star Plus series lenses....on a "rear-screen" no less...resolution was exceptional! I believe our 1.85 lens was in the under-40mm range (larger picture than on the one being discussed). I was quite pleased with the results.

Steve

Posted by Paul G. Thompson (Member # 655) on 03-30-2003, 02:33 AM:

To re-enforce Steve's comment:

Remember....with projection lens issues, you get what you pay for.

Posted by Phil Hill (Member # 371) on 03-30-2003, 05:10 PM:

I agree both have their good and bad points. I have found that in the shorter focal lengths like 24-35mm, the Isco has more edge chromatic aberration than the Schneider. I find the ISCO to be "sharper" in the center as compared to the Schneider in that focal range. But that seems to work against you in that it tends to show the magnified film grain a "little" too much and detracts from the image. Just my personal preference.

And Paul, you usually get what you pay for in most things you buy....um except monster wire....but that's a different topic [evil]

>>> Phil

Posted by Steve Guttag (Member # 268) on 03-31-2003, 12:55 AM:

Chromatic abberations are also often due to poor lamphouse alignment and/or reflector design. I've had no problems with the ISCO line on short focal lengths (nor Schneider).

The annoying thing about the newer generation of ISCO and Schneider lenses is that they tend to look around the aperture plate! The "fringe" area is huge.

Steve