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Old 4th July 2002, 04:43 AM   #121
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hmmm, what defines a meniscus lens apart from a normal one?
I'm thinking of using a 2.5inch lcd and a metal halide bulb and a fresnel lens to collimate the light, but i'm unsure what kind of lens setup to use on the projection side. I'd like to get a picture about 150" or slightly less diagonal on my screen...
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Old 4th July 2002, 08:09 AM   #122
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Default lenses

Xblocker-

You are right about not needing a lens that is the size of your panel. However, I have to disagree about needing a doublet (or triplet) with a focal length that is minimally the size of the panel being used. If you have a fresnel before your panel, then the light coming through the panel is converging and you can use whatever focal length doublet or triplet you want - as long as it is large enough (in diameter) to catch the converging cone of light. The practical reason that the focal lengths of typical doublets on OHP's are at least a foot, is to give the proper magnification to get a big image. Using the following equation

(focal length x screen width) / panel width = throw distance in inches

which is the same as

M = F2/F1, where F2 is throw distance, and F1 is focal length of the lense, and M is the magnification

Using these equations you can determine the focal length you need to get the size on the screen you want in a certain throw distance. This is how the
OHP's work. Now it is not by accident that the focal lengths turn out to be greater than the panel width, take for example:

throw distance = 10 ft = 120 inches
6 ft screen width = 72 inches
10 inch panel width

so now plug into our equation

FL= (TD*panel width)/screen width = 120*10/72 = 16.7 inches

So long focal lengths come directly out of this equation, but you could use shorter focal lengths,
you would just get a bigger magnification.

Anyway, this is how I understand things, but if you know something I don't, I would love to hear it! So, let me know what I am missing.

BTW I totally agree with you about needing a double or triplet, without them you will get chromatic and spherical aberrations, plus even higher order effects. That being said they could turn out to be small effects, which might be why Undream's picture looks fine. I have a buddy that knows how to use Z-max (a ray tracing program), and I have been trying to get him to model this, he said he would, but he is kind of busy building real astronomical instruments. Anyway hopefully I will get some kind of model going here sometime.

Thanks

J
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Old 4th July 2002, 08:36 PM   #123
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jco9w,
watching the interdependence between fl, object size and imagesize, you're right about the formulas. But now let's assume we have a projection lens with a fl=2" to project a 10" panel. You would have a field angle of 2*cos(2/5)=133degrees! Now tell me, which projection objective is capable of doing that? This must be a very special wide angle objective. Not to talk about a single lens! Another critical point is lens thickness and aperture. The thicker a lens, at a given FL and aperture, the more disturbing elements concerning imaging come into game. Excellent obectives have an F-ratio of 1.0, which means aperture diameter = focal lenght, this also is only possible with corrected lenses.
We cannot only discuss magnification ratio without looking at quality. There have to be always compromises between math calculations and real physical-optical components.
Also, the bigger (steeper?) the transmitting angle of light through LCD the poorer the results, but thats another topic.
In Photography all big format cameras have longer FLs than mini formats...Why?
well, all in all i cannot see, why this shouldn't be the case for DIY-projectors. Make yourself a test with 10" panel and a slide projector objective and tell me what you can see!!

(grr, what a torture in english!)
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Old 4th July 2002, 08:46 PM   #124
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Default good point

xblocker-

You make good points. My goal in the previous email was not to say that practically you weren't correct (in fact I showed that you do need a long
focal length), but to make sure people understand the physics. Agreed, you need a giant diameter lens if it has a short focal length, and they are hard to come by and can cause other problems, however, you could (in theory) still make it work.

Point being, a rule of thumb is a rule of thumb and physics is physics. Your rule of thumb works well with the projectors we are talking about, but
I didn't want people to come away that it is the law.

Anyway there is no need to argue, I agree with you about the focal lengths, I just thought that I would put the formulas out there so people got the whole story.

J
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Old 4th July 2002, 09:19 PM   #125
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hmm, I just calculated with my setup that I'd need a lens with a Fl
of 3.6" for a 2 inch lcd and a 100 inch screen at a throw distance of 180"... Is the feasible??? Or will it work... And how to I work things out if I'm gonna need more than one lens to eliminate aborrhations?
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Old 4th July 2002, 10:56 PM   #126
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jco9w,
i agree, i only made some points, because i often see a big lack of information about optical systems here.

Duo,
with a 2" LCD you're nearly in the dimension of the slide projectors. I would try any objective of an old slide projector or modify such a projector. Look at the small LCD projector thread. There you can find a lot of hints. You can also try 2 mensicus lenses, each with about 7" FL to get a total of 3,5".It should look like that: ( ) . Eye glasses have meniscus shape. I experimented a lot with used or missworked eye glasses, which i got for nothing from an optometrist.

Good luck!
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Old 4th July 2002, 11:31 PM   #127
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haha, my glasses and contace have an fl of about 3, but I'm nearsighted so the lenses are concave not convex as far as I know... Where is a good place to get lenses of these types if I can't find them at the optometrist... Also, did you have any luck with the lenses from glasses?
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Old 5th July 2002, 01:12 AM   #128
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Duo,
Eye glasses with focal lenght of 3? What measure? And, are you talking bout FL or dioptrics? Dioptrics are defined as 1Dpt=1/f (f in meter,Germany).
I don't understand why opticians shouldn't give away defective lenses. I've already built simple telescopes and projection objectives out of it. So what!?
You also can try 2 planconvex lenses. In every case it's better to have 2 thinner lenses than 1 thick!

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Old 5th July 2002, 01:23 AM   #129
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hmm, good point, my glasses could mean that number in a different way. Anyhow, I know very little about lenses: whats a planconvex lens???
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Old 5th July 2002, 06:08 PM   #130
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Default about "low e-glass"

Hay guys just a note about low E-glass this is not a great thing. If anyone has ever seen low e glass it looks like tinted glass. The loss in light transmittance is any ware from about 25% to 35% and there are more problems, the glass really has a "shelf life" due to oxidation problems. Not to mention that glass itself is pretty opaque is anyone has ever seen a case if glass… you cannot see thru it! So too much glass can be a problem! Just a thought.
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