Dear all,
I am planning to do a projectot based on a 7-inch wide screen LCD.
I have quite a lot of questions due to my misunderstanding.
does shorter focal length of the objective means bigger magnification?
then why don't people use a short facal length objective lens and fresnel to produce bigger image?
the function of the fresnel is to converge the light ray to the objective.
therefore its forcal length should be as same as the objective's one.
what is the shortest f/l of frensel in this world?
As i don't have enough space, i can build a big one.
I just have arround 6 feet to form the image.
Last question, can lenses in 35mm camera be used in projector?
thanks
Jonathan
Full Version: Small Lcd Projector
QUOTE (jonathanhui @ Jun 17 2006, 06:58 PM) 
Dear all,
I am planning to do a projectot based on a 7-inch wide screen LCD.
I have quite a lot of questions due to my misunderstanding.
does shorter focal length of the objective means bigger magnification? Yes it does
then why don't people use a short facal length objective lens and fresnel to produce bigger image? Because some people want to place their pj more far away from their projectionscreen. The size of the projected image doesn't need to be as big as it can be for the most of ppl
the function of the fresnel is to converge the light ray to the objective.
therefore its forcal length should be as same as the objective's one.
what is the shortest f/l of frensel in this world? It should be +- the same, not exactly. The bigger the tft you use the less small your focallength of your objective can be (I'd say 200mm as a minimum for 7", maybe you can try 150mm but I doubt it)
As i don't have enough space, i can build a big one.
I just have arround 6 feet to form the image.
Last question, can lenses in 35mm camera be used in projector? No they can not, not even with a 1,8" lcd they can't
thanks
Jonathan
I am planning to do a projectot based on a 7-inch wide screen LCD.
I have quite a lot of questions due to my misunderstanding.
does shorter focal length of the objective means bigger magnification? Yes it does
then why don't people use a short facal length objective lens and fresnel to produce bigger image? Because some people want to place their pj more far away from their projectionscreen. The size of the projected image doesn't need to be as big as it can be for the most of ppl
the function of the fresnel is to converge the light ray to the objective.
therefore its forcal length should be as same as the objective's one.
what is the shortest f/l of frensel in this world? It should be +- the same, not exactly. The bigger the tft you use the less small your focallength of your objective can be (I'd say 200mm as a minimum for 7", maybe you can try 150mm but I doubt it)
As i don't have enough space, i can build a big one.
I just have arround 6 feet to form the image.
Last question, can lenses in 35mm camera be used in projector? No they can not, not even with a 1,8" lcd they can't
thanks
Jonathan
A good cheap lens, is the 190mm focal copy lens at surplus shed. It is like $6 or something, and I get great image from my 8.4" lilliput. You will run two 220mm fresnels, or even better is two 200mm frensels.
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