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Attach a diopter to a projector?


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#1 Drew Lahat

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Posted 19 October 2010 - 12:55 PM

I got my hands on a small DLP projector, but my available placement for it is too far for its capabilities - the projected image is too large and dim. Could I use a diopter as a tele adapter, or do I need a proper teleside converter?

The projector's lens is built-in with no threading whatsoever, so I'm looking for a lightweight solution I could rig on the chassis' surface.
Its focal length is 25.7-28.3mm and I'm looking to reduce the projected image size by about 25%.

I've been doing some reading trying to understand the difference between a teleconverter, a high-power diopter, and a fixed-focus f/1 telephoto lens - brings back (vague) memories of high school physics...

Thanks,
Drew
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#2 Drew Lahat

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Posted 22 October 2010 - 12:56 PM

Bump?
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#3 K Borowski

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Posted 22 October 2010 - 01:55 PM

Well, sure you can use them, but it depends on the power you have, and whether they're positive or negative. I honestly forget now too whether positive or negative make a lens more close-focusing. Probably positive does this, as my severely-near sighted eyes require huge negative numbers, 8, & 7.


The way a dioptre works is that it is the reciprocal, in meters of the near-point it is correcting out from, with near-sighted eyes. So positives would probably have the opposite effect of halving, quartering, dividing in eigths, the near-point for focusing the lens.


Reading your question, you might need a teleconverter. Dioptres do effect size, but their primary purpose is adjusting FOCUS, if I understand the concept correctly. I'm not sure that 25% adjustment of near point corresponds to a 25% adjustment in image size, but if it does, that seems like a small adjustment, maybe less than +1 dioptre.

I would take this to mean, that, with a projoector lens, you'd use dioptres to close-focus the lens on a near wall.



There's probably a good wikipedia article on this subject. . .
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#4 Mark Dunn

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Posted 22 October 2010 - 02:09 PM

Dioptres (close-up lenses, we call them here) magnify. You need a reducing lens- as Karl says, a teleconverter, such as would correct his near sight. It effectively increases the focal length so you get a smaller image at a given distance, which is what you want. Try borrowing a weak spectacle lens for near sight of known strength. It's easy to tell which is which- you need one that makes things look smaller when you look through it. It probably won't be well enough corrected to use but if it works you could then look for a lens of matching strength- it'll be minus so many dioptres.
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