Hi,
New around here, but have just got a question about the lens I need to use.
I am using a 15" lcd and I want the end projection to be 80" diagonal. So that is a magnification of 5.3x. So with a throw distance of 12 feet that would mean that
the focal length is 144/ (M+1)
144/6.3
=about 22.5 in inches
which equals 571mm
So I need a 571 mm fl projection lens. Is that correct? I have been looking around and all I have seen is a 450mm fl lens at diyprojector. I can't use that so any ideas on what I could use.
THanks for your help.
New around here, but have just got a question about the lens I need to use.
I am using a 15" lcd and I want the end projection to be 80" diagonal. So that is a magnification of 5.3x. So with a throw distance of 12 feet that would mean that
the focal length is 144/ (M+1)
144/6.3
=about 22.5 in inches
which equals 571mm
So I need a 571 mm fl projection lens. Is that correct? I have been looking around and all I have seen is a 450mm fl lens at diyprojector. I can't use that so any ideas on what I could use.
THanks for your help.
A cheap suggestion
See if you can find a 75 mm diameter +1.0 Diopter eyeglass lens and a 75 mm diameter +0.75 Diopter eyeglass lens. CR-39 plastic is okay, but anti-reflection coated glass would be better. You can ask an optician in a small eyeglasses shop to supply these. Tell him or her they are for a projection lens, so you need them as large diameter as possible, uncut, and not mounted in any frames. Cost me $30 US for uncoated plastic.
Then you find a 60 mm long piece of plastic pipe with inside diameter just over 75 mm. (I use ABS sewer pipe from a home improvement store.) You can CAREFULLY cut 3 mm wide rings from a large paper mailing tube to make heavy paper retaining rings, or you can cut them out of some more of the 75 mm ID pipe. Cut a section out of the retaining rings so they fit snuggly inside the pipe. Then use pairs of retaining rings to mount one of the lenses at each end of the tube, with the convex curves toward the outside.
You just made a 585 mm symmetric duplet. Should be close enough. You can experiment with distances shorter than 60 mm to get the focal length closer to 571 mm.
A much more expensive option: Buy a 600 mm process lens like a Rodenstock Apo Ronar 600mm:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=3874697252&ssPageName=STRK:MEWA:IT
See if you can find a 75 mm diameter +1.0 Diopter eyeglass lens and a 75 mm diameter +0.75 Diopter eyeglass lens. CR-39 plastic is okay, but anti-reflection coated glass would be better. You can ask an optician in a small eyeglasses shop to supply these. Tell him or her they are for a projection lens, so you need them as large diameter as possible, uncut, and not mounted in any frames. Cost me $30 US for uncoated plastic.
Then you find a 60 mm long piece of plastic pipe with inside diameter just over 75 mm. (I use ABS sewer pipe from a home improvement store.) You can CAREFULLY cut 3 mm wide rings from a large paper mailing tube to make heavy paper retaining rings, or you can cut them out of some more of the 75 mm ID pipe. Cut a section out of the retaining rings so they fit snuggly inside the pipe. Then use pairs of retaining rings to mount one of the lenses at each end of the tube, with the convex curves toward the outside.
You just made a 585 mm symmetric duplet. Should be close enough. You can experiment with distances shorter than 60 mm to get the focal length closer to 571 mm.
A much more expensive option: Buy a 600 mm process lens like a Rodenstock Apo Ronar 600mm:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=3874697252&ssPageName=STRK:MEWA:IT
Thanks guy - good idea, I'll use that instead of paying $$$ for a special lens.
I've found a good website here (http://www.knightoptical.co.uk/) which does crown glass postive meniscus lenses with an AR coating. Just out of interest would it be possible to use the two positive lenses and one negative lens to make a triplet.
I've found a good website here (http://www.knightoptical.co.uk/) which does crown glass postive meniscus lenses with an AR coating. Just out of interest would it be possible to use the two positive lenses and one negative lens to make a triplet.
unoptimized triplet
Yes, I have tried it. I used a +1000 mm fl -2000 mm fl +800 mm fl to give me an effective focal length around 600 mm. The positive lenses are acrylic and the negative lens in the middle is glass, and all are standard stock blank eyeglass lenses.
Using OSLO to model it, it looks like this triplet should have pretty good performance. But in real life, it is not very sharp. I think it has an internal reflection problem, since none of the lenses have anti-reflection coating.
Yes, I have tried it. I used a +1000 mm fl -2000 mm fl +800 mm fl to give me an effective focal length around 600 mm. The positive lenses are acrylic and the negative lens in the middle is glass, and all are standard stock blank eyeglass lenses.
Using OSLO to model it, it looks like this triplet should have pretty good performance. But in real life, it is not very sharp. I think it has an internal reflection problem, since none of the lenses have anti-reflection coating.
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