| Fresnels, focal length and physics... - Click HERE for Original Thread |
| Figero |
Evening all,
Ok, first things first - I've done alot of reading on this forum and read a fair few websites about this sort of project - thankyou all for the info you have provided. I have just a few questions to ask:
Fresnel lenses - These seem to have specifications such as:
Size: 180mm x 255mm
Thickness: 0.5mm
Weight: 25 grams
Material: optical PVC
Groove pitch: 0.35mm
Focal length: 325mm
(from http://www.bigscreenprojection.com/contents.htm)
What should I be looking for to get maximum quality:
Lowest groove pitch?
longer focal length?
Is optical PVC a good material?
Also what is all this focal length stuff about, what does 32.5cm focal length actually mean, is it the distance my triplet lens needs to be from the fresnel?
Finally is one of these fresnels enough? I have read about people splitting their fresnel to put the LCD between the two layers. I'm guessing this is because the lens was for an OHP. Is a fresnel lens a double layered lens by definition or are most of the ones on sale actually just single lenses (so it would be necessary to buy two of them and put the back to back).
Any ideas? |
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| wout |
just buy the "OHP Fresnel", those are high quality. All fresnel's have two layers.
i think :) |
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| Figero |
Thanks, I've ordered some of my parts now;
15" Compaq TFT (model 500)
Fresnel from some dodgy guy at ebay
250W MH light
I still need another lens to project my image, I've read a bit about the triplet lens, are these vital to a good quality projector or will I be ok with a normal Magnifying glass? |
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| faithblinded |
Definitely stay away from magnifying glasses.
A good triplet will be essential to having a nice picture. It's not the absolute best, but a good triplet is about your best value if you're looking to build an inexpensive PJ that actually produces a nice picture.
I would be skeptical of the quality of the fresnel you'll receive from one of those big screen rip off artists on eBay.
You might do well to buy yourself a nice OHP and get the fresnel and triplet at the same time that way. If not, I'd reccomend sandwiching the fresnel in beteen to pieces of glass to keep it flat. |
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| cruser |
| Figero that 180mmX225mm fresnel is nowhere near big enough for a 15" monitor |
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| Figero |
very good point, thankyou!
what size fresnel is right for a 15" monitor, 30cm by 20cm? I think maybe I will just get a projector (new or second hand) and use the fresnel and lens from that, I'm sure that would be big enough - and would have one of those triplet lenses that everyone is going on about. |
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| ace3000_1 |
| quote: | | Definitely stay away from magnifying glasses. |
Actually they work, ive used them before, they will only out focus the edges if the lens is too small, they are only good for a small lcd and they do give a nice sharp and bright picture.
Trev |
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