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?
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?
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.
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.
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.
Not ideal, but
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.
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.
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.
somewhat short distances.
other options for long-throw lens
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.
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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