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Setting up a 0.8:1 short throw on-axis lens [message #311813]

Sat, 12 April 2008 13:51

Mike Tomei, CTS
Messages: 29
Registered: March 2007
Location: Cambridge, MA

Should Get Out More

Hello everyone,

Just wanted to run this scenario by all of you and see if my thinking was correct:
I was freelancing for a hotel AV department the other night. They had rented an Eiki LC-X5 and a 0.8:1 short throw on-axial lens.

They wanted to do front projection, so they had the projector on 2.5' tall table in front of a 10' wide fast fold. The projector was the same height as the bottom of the screen. I expressed concern, since I knew that the on-axis short throw lens they were using wants to be positioned on the same horizontal and vertical plane as the center point of the screen. I suggested rear projection but they said it wasn't an option. So I went ahead and showed them what front projection from this low position would look like. I extended the projector's front feet fully, had to raise the lens shift a bit, keystoned the heck out of the image and showed them that the top corners of the image were pin cushioning.

I'm not a lens expert, so I wanted to make sure that I didn't miss something obvious here. Was my thinking correct in assuming that this on-axis lens really needed to be on the same plane as the center point of the screen?

Thanks!

Mike Tomei

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Re: Setting up a 0.8:1 short throw on-axis lens [message #311817 is a reply to message #311813 ]

Sat, 12 April 2008 13:58

E. Lee Dickinson
Messages: 962
Registered: February 2007
Location: Richmond, VA

Has No Life

Mike, you did what you could - exhausted the lens shift and keying adjustments. Most short throw lenses are, as you said, center-field.

E. Lee Dickinson
Advanced Visual Production Inc.
sound - lighting - video - design
www.avpric.com

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Re: Setting up a 0.8:1 short throw on-axis lens [message #312387 is a reply to message #311817 ]

Mon, 14 April 2008 15:34

Shane Gavin
Messages: 104
Registered: November 2006

Has No Life

You were right. The lenses are ground differently for large projectors vs. tabte top projectors.

Short throw lenses seem to work best if you have the projector somewhere in the middle 1/3 of the screen, but dead center is obviously best.

I like to zero out all lens shift and keystone settings and use physical geometry of the projector to get the image right.

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