Film-Tech Forum ARCHIVE: Modifying zooms?

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Posted by John Hawkinson (Member # 1135) on 06-26-2009, 06:33 PM:

For a quite a while, I'd been hunting for a video projector zoom for our projector that would fill the full width of the screen for letterboxed content (not that we show very much video, but it's nice to have). Previously, with the old lens, a Sharp AN-LV80EZ (147-206mm zoom), we could fit height for 4:3 content, but not the width.

Anyhow, I finally got my hands an AN-LZ55EZ today, which is a 102-143mm zoom. Full width is about 115mm, and that's great.

Unfortunately, at 143mm, the image is just slightly too tall, so it can't really handle 4:3 content. I'd really like it to go another 5 or 10 mm longer.

I haven't tried disassembling the lens, and I am kind of afraid to, but how reasonable is it to try to modify it to extend the zoom range? I assume that after some point the optical clarity will start to drop off, or there may be physical limitations. But is this just a matter of changing the position of a stop? Or something unpleasant-but-doable like adding more threads?

--jhawk

Posted by Mark Gulbrandsen (Member # 72) on 06-27-2009, 10:59 PM:

DO NOT UNDER ANY CIRCUMSTANCES EVER TAKE ANY ZOOM LENS APART UNLESS YOU HAVE A VERY GOOD IDEA OF IT'S INTERNAL MECHANICS BEFORE HAND AND KNOW THE ACCOMPANING ADJUSTMENTS TO SET IT BACK TO IT'S DESIGN SPECIFICATIONS. DITTO FOR ANAMORPHIC LENSES WHOSE ADJUSTMENTS ARE EVEN MORE CRITICAL!!

I used to service TV zoom lense back when I worked at Canon USA... we had all the correct factory tools, jigs, parts, shims, lens projectors, and all the gobbeldigook one needs to work on them. The Japaneese technicians were the best teachers!!!

The minimum you would need: Knowledge of how to dismantle the zoom and put it back together correctly so you don't damage anything, An appropriate collimeter as well as an appropriate lens projector and accompaning resolution aperatures. This is the only way of verifying that it will make specs. Many zoom lenses have to be adjusted at several different focal lengths while mounted on the projector in a darkroom.

Perhaps someone makes an adaptor that would fit in front of your lens that might allow it to work for you. Many of the ones made are very high quality and have very little light loss. You might check with Century Optics as a starting point.

Mark