| germanchiwawa |
I have a 17" lcd in 4:3, and i was wondering what lenses from surplusshed could be used in this setup. My fresnel will have focal length of 330mm and i was planning on buying this achromatic lens for using at the projector lens: 42.7mm Achromat Lens with focus length of 333mm.
Will this be sufficient to get the image on the screen without distortions?
How can i make the projector lens into a long-throw lens, so that the projected image can be smaller. Do i just add a positive meniscus to the projector lens setup? |
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| Guy Grotke |
Simple achromatic lenses are not very good at wide angles. You may need a triplet to get a 17" LCD on the screen. lumenlabs.com has a $29 320 mm fl triplet.
To increase the focal length (and thus decrease the size of the projected image), you can add a negative lens. The equation is:
1/fl + 1/fl = 1/combined fl
PCV, DCV, or negative meniscus lens have negative focal lengths. So, you can use a -500 mm fl lens to adjust a 320 mm fl triplet to have an EFL of about 889 mm:
1/320 - 1/500 = 1/889
Or use a -800 mm fl lens to adjust a 320 mm to 533 mm:
1/320 - 1/800 = 1/533
You can use the same equation to see what a positive meniscus lens would do. |
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| dnsey |
| Whilst combining lenses in this manner will certainly adjust the focal length, it will also destoy much of the geometry and colour correction that constitutes a 'good' lens. OK for experimenting, but not ideal for a final project. |
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| Guy Grotke |
Many great photos have been taken using single meniscus lens extenders. And germanchiwawa was asking about Surplus Shed lenses he could buy (ie. for $4 to $6 US!), instead of ordering a custom triplet made. I was just giving him the equation to demonstrate how positive and negative lenses work.
But I agree: With experimenter's surplus lenses thrown together, you won't get results as good as a real compound lens design. The color aberration will more visible, and the more "adjustment" you do this way, the more geometric distortion you will get.
Specifically, if you need a wide angle lens (to give you a large image over a short throw distance), then you will be unhappy with anything but a real triplet or a metrogon. These designs will be sharp over an entire 45 degree field. Projector designs that use longer throw distances have lower entrance angles, so you can get away with simpler lenses. |
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| germanchiwawa |
Thanks, ill get that triplet for my projection lens, the reason i was trying to find a way to change the focal length is because with a 17" lcd, the image will be WAY too huge for my wall, even at
somewhat short distances. |
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| Guy Grotke |
| You could look in the lumenlabs.com store. They have a 450-500 mm fl lens for $200. You would also need a longer focal length upper fresnel to use with that. |
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