Full Version: using commercial projector lens
So I'm new to this and have been reading a lot in the forum. I have an old lcd projector with a zoom f=50~75mm. I read the pinned note saying that the projection lens is not useful in a diy pj. I think I understand why such a short focal length would be problematic with regular sized lcd (I have a 17" that I got for free because the back lighting inverter board is bad), but I was thinking why couldn't a matched fl pair of fresnel lenses be used after the lcd. The light, a collimator, the lcd, a collector, and then another collimator, before entering the commercial lens? Thanks in advance.
Is there something wrong with the ideas or the question? Anyone with thoughts?
QUOTE (dallasb @ Feb 9 2009, 04:22 AM) 
Is there something wrong with the ideas or the question? Anyone with thoughts?
Focal length of your projector lens is too short by far. You need a focal length that is approximately the diagonal length of your LCD. So for a 17" LCD youīll need a lens with a focal length of about 17" '(or bigger)
Commercial projector lenses are designed to work with LCDs about or under 1".
donīt waste your time on it, it will never work.
I was trying to say I understand these rules of thumb, and why they are as such. The question I was trying to get at is would it be possible to add an extra fresnel that matches the focal length of the typical fresnel in a split design just before its focal point. In essence straightening out the beam of light while it has a very small diameter (like the one inch lcd's). Then shoot it through the glass fresnels and the projection lense of the projector. Is this possible or am I missing something? I've drawn a simple diagram.
Click to view attachment
Click to view attachment
you are missing for example that the focal length defines the distance between LCD ond projection lens. This means your LCD has to be placed 50-70mm away from the lens, otherwise you canīt focus on the panel.
you wonīt find a 17" fresnel with such a focal length.
btw. even if it did work the projected image size would be really, really big. (on a normal projection distance)
you wonīt find a 17" fresnel with such a focal length.
btw. even if it did work the projected image size would be really, really big. (on a normal projection distance)
If I'm reading your sugestion right...
The idea is to use a short FL projection lens, but using lenses to manipulate the image to 'appear' smaller to the projection lens.
Close, but no cigar. You simply can't manipulate the image that way. What you'll end up with is a completely unfocused blur of light.
You can't change the shape of light with a lens. The best that it can ever be is a focused enlargement (or shrunk) version of what you start with.
So in your diagram there, light leaves the lamp, goes through one fresnel lens, the LCD, and the other fresnel lens, then through another lens and on to the projection lens, is that right? It looks goon on that diagram, but consider instead this:
This is more or less your diagram, but with a representation of a black LCD with 3 small coloured points on it.
Click to view attachment
Becasue you won't actually have an IMAGE at the focus of the lens, but rather just a collection of light, what you'll get at the other end is... A blurred collection of light. You might be able to make out some of the basic shapes in the image, but that's about it, really. (Note that this is actually an optomistic representation, (In that it actually DOES focus SOMEWHERE) but you get the idea.
This is how it would turn out in the end.
The idea is to use a short FL projection lens, but using lenses to manipulate the image to 'appear' smaller to the projection lens.
Close, but no cigar. You simply can't manipulate the image that way. What you'll end up with is a completely unfocused blur of light.
You can't change the shape of light with a lens. The best that it can ever be is a focused enlargement (or shrunk) version of what you start with.
So in your diagram there, light leaves the lamp, goes through one fresnel lens, the LCD, and the other fresnel lens, then through another lens and on to the projection lens, is that right? It looks goon on that diagram, but consider instead this:
This is more or less your diagram, but with a representation of a black LCD with 3 small coloured points on it.
Click to view attachment
Becasue you won't actually have an IMAGE at the focus of the lens, but rather just a collection of light, what you'll get at the other end is... A blurred collection of light. You might be able to make out some of the basic shapes in the image, but that's about it, really. (Note that this is actually an optomistic representation, (In that it actually DOES focus SOMEWHERE) but you get the idea.
This is how it would turn out in the end.
Hi there,
i understand the need for recycling old lenses.
From 2 old lcd projectors i saved the lenses, and i found out that i could change the focal length,
by screwing off the first lens at the back.
it is a lens with zoom function. for thesting, i put focus and zoom in the middle position.
this changed the focal length from about 60 mm to about 180 mm from the first lens at the back
shining in it with a flashlight i could determine that the possible lcd size could be 90-120 mm diameter,
so that is like 5 inches?
with a 180 or 200 mm fresnel?
it still is quite small, 5 inches.
just by projecting an image on white papaer of a window in the room,
holding the lens in my hand (no building boxes etc)
i could determine that the window, 2500 mm away, 1220mm wide,
projected an image about 55 mm, 180 m from the lens
that means, with a 5 inch (127mm) screen, you get an image about 2,8 meter across 2.5 meter away
then, i presume, you could count the pixels.
a 5 inch screen has like 400? pixel resolution,
so pixels will be projected 7 mm big
(anyone, is that exeptable?)
the lens i screwed off is quite big, could be a nice collector lens for the lamp. bonus.
so, it is not optimal, but maybe do-able
i understand the need for recycling old lenses.
From 2 old lcd projectors i saved the lenses, and i found out that i could change the focal length,
by screwing off the first lens at the back.
it is a lens with zoom function. for thesting, i put focus and zoom in the middle position.
this changed the focal length from about 60 mm to about 180 mm from the first lens at the back
shining in it with a flashlight i could determine that the possible lcd size could be 90-120 mm diameter,
so that is like 5 inches?
with a 180 or 200 mm fresnel?
it still is quite small, 5 inches.
just by projecting an image on white papaer of a window in the room,
holding the lens in my hand (no building boxes etc)
i could determine that the window, 2500 mm away, 1220mm wide,
projected an image about 55 mm, 180 m from the lens
that means, with a 5 inch (127mm) screen, you get an image about 2,8 meter across 2.5 meter away
then, i presume, you could count the pixels.
a 5 inch screen has like 400? pixel resolution,
so pixels will be projected 7 mm big
(anyone, is that exeptable?)
the lens i screwed off is quite big, could be a nice collector lens for the lamp. bonus.
so, it is not optimal, but maybe do-able
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