8mm Forum: Optical keystone in front of 8mm projector

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Posted by Matthieu van der Sluis (Member # 6040) on August 24, 2017, 11:36 AM:

Has there ever be an optical keystone to be placed in front of your 8mm projector, so you can project from an angle and correct this optically to a perfect square picture again?

You find this in many videoprojectors now, but as a digital version and, if you use it too much that lowers the resolution to almost half.

Posted by Brian Fretwell (Member # 4302) on August 24, 2017, 01:23 PM:

The Eumig 926 stereo had an enlarged front element that could be moved up and down to do this, but people think it was not a very good lens (I believe). It was dropped from the next model.

Posted by Matthieu van der Sluis (Member # 6040) on August 24, 2017, 03:10 PM:

That sounds more like an optical Lens Shift, which is nice also, and prefered in most videoprojectors.
A keystone is more for correcting the picture from a for example tilted a little bit downwards projector.

Posted by Brian Fretwell (Member # 4302) on August 24, 2017, 03:54 PM:

I took it to be "keystone correction" which is what both systems do. The Lens shift was the only one I remember being available on 8mm.

On some 70mm they did make "rectified" prints with keystone in the opposite direction printed in which was corrected when shown. There are examples in the on-line "American Widescreen Museum" site.

Posted by Matthieu van der Sluis (Member # 6040) on August 25, 2017, 01:45 AM:

With prints like that you'll end up with the same quallity degree as with the digital keystone in projectors although,... the resolution on 35mm film is so much higher, I probably would not mind about that.

Actually,... optical Lens Shift could be working too, but I don't think it would shift as much compared to what the JVC or Epson projectors do.

Posted by Maurice Leakey (Member # 916) on August 25, 2017, 02:35 AM:

Keystone correction in 35mm cinemas was corrected by filing out the masking plates.

Posted by Winbert Hutahaean (Member # 58) on August 25, 2017, 02:37 AM:

This is just a wild idea.

If there is no correcting keystone lens, do you think a little play with the screen will do?

I never try it my self, but should the picture is keystoning what will happen if we move the upper side/bottom side of the screen? [Wink]

Posted by Maurice Leakey (Member # 916) on August 25, 2017, 03:49 AM:

Winbert's suggestion is quite correct.
Tilt the screen at 90 degrees in line of projector's light beam.

Posted by Matthieu van der Sluis (Member # 6040) on August 25, 2017, 04:43 PM:

That solves it indeed, but not having to do this, thats why the keystone is needed.
I want to place the projector above a kabinet, so it can be placed further from my stationary screen I use also for video projection.
Only the projector is too high than, so I like to tilt the projector downwards to fil the screen, but that picture needs to be corrected this way.
Some videoprojectors have great Lens Shift for this situation, so they can be place straight forwards and with the Lens Shift the picture can be lowered to fit the screen with a straight picture.
for it needs a large shift.

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