DIY Projection Lens Triplet

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another lens maker

Another possible source:

http://www.knightoptical.co.uk/acatalog/LensesMeniscuslenses-CrownGlassPositiveType60diametermm.htm

I don't know if it is easier for you to buy from UK versus USA.

BTW: Most of the clear aperature & stop size stuff is more important in photography, where you are gathering diffuse light from all directions. Then you can discard light in the center of the image just to get very even exposure. (Longer exposure time compensates for less light.)

In DIY projectors we use a fresnel condensor system to direct most of the available light into the projection lens. Adding a stop to a duplet makes the center of the image dimmer, which is rarely desired.
 
guy you told us your center image is brighter than the corners (3 times brighter if i am not wrong), then do you think a F-stop won´t give you a more uniform brightens? (it would be better if we could somehow increase the outer brigtness, but i don´t know how could we do this).
 
stop aperature

<do you think a F-stop won´t give you a more uniform brightens?

When you use a lens for photography, diffuse light from a point on an object enters all parts of the lens. The rays that get through the lens are focussed on one point on the film. If the point of origin is close to the central axis of the lens, then the rays from the whole surface area of the lens will get to the film. If the point of origin is off to one side, then some rays striking the first surface will not get through the lens. You can close down the stop aperature, so that the center of the film gets just as much light as the edges. Then you just increase the exposure time to compensate for the smaller aperature.

In an LCD projector, we are using a fresnel condensor system to send most of the light to the center of the lens. This creates a "synthetic aperature", (same size as the projected arc image). If you use a stop aperature that is the same size as the arc image, or a stop aperature that is twice that size, it will make no difference at all: All the light fits within the smaller aperature.

Making it smaller than the arc image does have an effect: It gets rid of most of the light from the ends of the lamp arc. This may give you a sharper image, but it will also be much dimmer. (MH lamps are brightest at the ends of the arc.)

With the kind of very wide angle lens I am using, the corners are not dim because the field angle is loo low. Most of the differences in light levels are because of the fresnels and the optical design: I checked the effect of having a 220 mm fl condensor fresnel with a 15" LCD. The inverse square law says the light at the center will be 1.84 times brighter than the light at the corners. Changing the stop aperature size can do nothing about this. (But I think maybe a special non-parabolic reflector could help by putting more light at the edges and corners.
 
i think our setups with comon setups with near 40mm arc on the 60mm standar triplet, is more like difuse source than a unidirectional light. I mean for the triplet's viewpoint, it is like if it was difuse light, every pixel on the lcd will illuminate practically full triplet area. This is like if it was a CRT difuse source but i know that out of the fresnells working area, it won´t be diffuse (it will be much dimmer in the rest of directions). But i would say that for the triplet, it is exactly like if there was no fresnell and there was a very bright crt difusing light in all direction (but triplet would only capture part of it),

So my conclusion is, if it is like difuse source for the triplet's viewpoint, the f-stop will do a good work for us.
 
The inverse square law says the light at the center will be 1.84 times brighter than the light at the corners. Changing the stop aperature size can do nothing about this.

True, it wont get brighter, but it will make that part clearer. It chops the tails off your bell curve.

(But I think maybe a special non-parabolic reflector could help by putting more light at the edges and corners.

What special shape do you mean? Do an integral on the areas around an ellipse focus.
 
WOW

A 21.5 inch focal length is huge. And I can't see mounting two lenses about 10 inches apart. Six and a half inch diameter lenses are impossible to find, as well.

Have you looked for opaque projector lenses? I have one with an 18 inch focal length. It seems that a lens like that would work out better for you with the requirements that you have. Hopefully an opaque projector lens that can handle an 8.5 by 11 inch sheet of paper could project a 15 inch lcd.

Using a smaller diameter lens is bound to result in a loss of light, I wish I knew more about it. I'm out of my depth, here.

T.
 
How about a triplet?

I have a triplet setup from an OHP that a friend passed along to me. Each lens is separate and seems to have a magni/minification effect when the middle element is moved fore or aft. Could this sort of setup be useful?

On a slightly different slant...

In real life I'm a PhD student doing brain imaging research related to vision processing. In order to project computer images into the MRI for subject to view, which is around 30 feet away from the projector (LCD projectors don't like high magnetic fields...go figure!) we use a standard LCD projector with what appears to be a 50mm lens a few inches in front of and facing the projector focusing lens, followed by a a zoom lens a few inches after that (facing forward). Follow that? Well this setup allows us to project out to 30' easily and scale the image down to 8-10" if we want. Or scale up if need be....the zoom lens works great for resizing. It's a great system for long throw and picture quality is excellent. Now is there any reason to believe that we couldn't just nix the focussing lens on my projector and use the 50mm and zoom lens to focus? Or do you think that initial lens on the projector itself is key? Just thought that with this sort of setup I might be able to avoid the hunt for a 700mm+ fresnel and possibly keep the PJ box smaller than would be necessary with that long FL fresnel.

Any thoughts?

Dee
 
I think your idea has merit. I’ve been thinking about something simular. I’ve noticed there are tele-photo extenders available that increase the effective focal length of a tele-photo lens by 1.5 or 2.0 times. If we could find one which had a large enough aperture we could posibaly have an objective lens focal of 480mm or 640mm without requiring us to physically place the lens at a large distances from the LCD. You could simply try using a pcv lens in the same manner but I have my reservations about how well it would preform compared to a proper colour corrected extender.

DJ
 
OHP triplet

Dee,

That's probably the best setup for reasonable money. I get good results with mine - at the verge of what xga can do. Guy's setup is better but he found a rare lens for cheap. I wonder where he got the 650mm fresnel?

You might be able to extend to a very long throw with a good image if you use the lenses out of an old binocular set.
 
extending focal length

What you need is a weak achromatic doublet with a negative focal length. For example, a -1000 mm fl doublet would extend a 330 mm fl triplet to have an EFL of 493 mm. The best supply of such lenses would be telephoto lens extenders. Those come in large enough diameters that you should be able to find one to fit your triplet.

You could try going a lot cheaper by just using an eyeglass blank, but that might introduce some chromatic aberration. A -1.0 Diopter eyeglass blank has a focal length of -1000 mm. One limitation: These are never larger than 80 mm diameter. Most are more like 65 mm.

>I wonder where he got the 650mm fresnel?

I am using one of lumenlab's "790 mm" field fresnels (which I measure as 770 mm fl), by adding some space between the fresnels and the LCD.
 
The dreaded long throw question.

Hi all.

I recently bought a projector that will remain un-named as it has got a lot of bad reviews (Abus HD-12 Pro) which I actually like, except for the racket of the fan. and its inability to decrease image size except by moving the projector back and forth.

Yes, I should've bought a more expensive one, but my redundancy package wouldn't go that far..

All I want to do is move the thing back about a metre (into a convenient cupboard with a window - which I have yet to cut out, but keep the projected size the same. Long throw lenses are over £1000 which is a bit harsh as the projector cost less than £200. It uses a 5 inch LCD screen and all I can tell you about the lens is that it has a 68mm diameter at the front - I haven't found any info on focal lengths etc. on the web to date.

I have the projector 3.18m from the wall which gives a projection size of 100 inches (2.54 metres) diagonal. I want to move it back to 4.16 metres and keep the 100 inch projection size, the size would be 130 inches (3.3 metres) diagonal at that distance.

Not knowing a thing about lenses, is there anything in the Optics Shed that I could fit on the front or the back of the existing lens that won't put my mortgage payments in jeopardy? Could I alter the existing lens with a hacksaw, hammer and Duct tape? Would a chat with a friendly optician (if I find one) help?

Any (polite suggestions would be appreciated.

All the best,
Fitch.
 
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