8mm Forum: Projector Lens Distortion Reasons?

This is topic Projector Lens Distortion Reasons? in forum 16mm Forum at 8mm Forum.

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Posted by Stewart McSporran (Member # 128) on December 30, 2006, 10:12 AM:

Hi,

I've just received a B&H 1" f1.6 lens fitted into a sleeve for use in my Elmo 16-CL.

Unfortunately I'm getting rather severe pincushion distortion on the picture and I'd like to check my facts before going back to the supplier.

The diameter of the rear element of the lens is approx 11mm and has a very pronounced curvature, the diagonal of a 16mm frame is approx 12mm.

Given that the B&H lens is intended for 16mm projectors could the issue be that the lens isn't properly centred in the sleeve?

Stewart

Posted by John Whittle (Member # 22) on December 31, 2006, 01:58 PM:

I'm make sure that the lens was actually designed for 16mm projection. There are many people who mount 8mm lenses in sleeves to fit 16mm projectors and you get the results you described.

It doesn't seem that the element size you quote is large enough for the 16mm frame (it's .286 x .380 inches).

Any one inch lens is going to have more distortion than a 2 inch lens and also have less depth of field (hence very shallow focus range).

John

Posted by Stewart McSporran (Member # 128) on December 31, 2006, 04:19 PM:

It turns out that part of the issue was that the sleeve is too loose in the projector's lens holder. So when focusing it actually pivoted the lens sideways rather than straight back and forwards.

Wrapping masking tape around it and holding it when focusing helped.

I've arranged to return it.

Posted by Lee Mannering (Member # 728) on January 02, 2007, 02:56 PM:

Not much to help you my friend, but I did have some problems a few years ago with a cine projector lens. It was a Elmo F1.0 which someone had a go at cleaning.. BUT, they got the internal elements mixed up so the edges of proj image were out of focus. I stripped the lens down and got them back in right order and to this day it still works. More luck than anything.

I had an Elf 16mm projector which had one side of the screen soft. This was due to the swinging pressure pade needing adjusting. This could be done simply by projecting a large image with the shortest lens I had (1") and twiddling the small philips screw under the mount which pulled the swinging pressure pad true. The B&H may have something similar perhaps?

The problem is with short throw lenses I found over the years is that the whole optical side of things needs to be perfect or problems can crop up. If it does help you could send the lens to me and I will check it out. I do have a Bell & Howell machine somewhere here, and I think I have a new zoom lens in the box for the B&H.

Regards

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