DIY Projector using a LF Process lens

[Super Dave]

Hello!

I have a little different question for this board. I am currently collecting parts for a DIY projector. The problem I have is finding a projection lens that meets these requirements:

1. A viewing circle large enough to "see" a 17" (diag) LCD screen

2. A focal length between 480mm and 550mm

3. A lens diameter large enough to "see" a light arc 27mm long going through first a 220mm FL fresnel and then a 550mm FL fresnel (so the arc would be about 68mm by the time it gets to the projection lens)

4. Relatively inexpensive to find on either ebay or a surplus, etc...

My set up goes: Bulb -> 220mm (maybe 330mm) fresnel -> 550mm fresnel -> LCD -> Projection lens

My basic question is: Does a process (large format) lens exist with a viewing circle of 440mm+, a focal length of 480mm, and a lens diameter of 80mm+?

Thanks for any info you can provide!

[Juergen Sattler]

I don't know about cheap, but here is a list of lenses for 11x14 cameras andthey should all meet your requirement for image circle. Hopefully someone else will be able to help you with your other quations. http://www.largeformatphotography.info/lenses/LF11x14in.html
Good Luck

[Richard Ide]

Hi Dave

You don't say what you are building exactly but it doesn't sound like you need an image forming path.
One thing to keep in mind is that process lenses and their relatively small maximum aperatures are great for exposing film or enlarging. If you are projecting an image such as overhead transparencies, slides etc. for viewing purposes; then the larger the aperature, the better. A lens which is used close to same size has a large image circle, so perhaps you can use a shorter focal length lens which would give smaller dimensions to what you are building. Hope I haven't assumed too much.

Richard

[tor kviljo]

You certainly do not use a slow (small aperture) process lens for a projector: enormous waste of light & problem with heat build-up once you have collected strong enough light source for a acceptable bright image... However, there is plenty of LF projection lenses around from two sources: old but high quality (Elgeet, Leitz) lenses for up to 4"x5" lantern projectors (Typically f 3 to f 5: that is 10 cm free aperture and a fl. of 300 - 500mm ) , and heaps of cheaper but extremely fast acrylic projection lenses for projection television screens (close to the use you plan for it, I belive). Both types have at times been available from Surplusshed (www.surplusshed.com if I recall right) and sometimes from Edmund scientific. Cost: a handful $$ each. The big pro lantern projectors were earlier found in any auditorium - most possibly now on the dump or (hopefully) in the cellar. If You could do with a shorter lens, I have a nice Elgeet 300mm f 3 (coated 3 element all-glass projection lens: this is one of the sharp ones) in the attic somewhere. Good luck with your project!

[Super Dave]

Thanks so much for the info! Some one was asking earlier about my setup and why these requirements. Here is a more detailed look:

I am building a projector using parts from unexpected places. My "reticle" is an LCD screen from a Dell 17" flat-screen monitor. The housing is made of wood. In my living room (where the projector will go), the throw (distance to screen) is going to be between 96" and 120" (depending on my final design). The screen size is going to be between 72" diag and 85" diag (due to limited space on my wall). So I have a long throw (distance to screen) and a "small" screen size. This is why I need the longer focal length. I have a 17" and not smaller LCD screen because I wanted to have 720p HD (HD is 1280x720; 17" LCDs have a native resolution of 1280x1024; 15" LCDs are only 1024x768). This is why I need a lens with a big viewing circle (at least 17" or 440mm). The 220mm fresnel (back fresnel) is for focusing the bulb's rays so they become parallel. The 550mm fresnel (field fresnel) is for condensing the rays into the projection lens. The projector is going to be lit using a metal halide bulb with an arc gap (the space where the light comes from) around 25mm - 27mm. The magnification result of the fresnel lenses (magnification = field / back) is 2.5. So by the time the projection lens "sees" the arc, it will be 62.5mm - 67.5mm (arc length * magnification). This is the reason for the large lens diameter (or large aperture).

Here is a picture to help with the description. There are four designs in the runnings (three shown).

1. Unsplit: The two fresnels are back to back. Sharper image on screen. No keystone adjustment. Horizontal setup.
2. Split: One fresnel before the LCD, one after. Not quite as sharp an image. Keystone adjustment. Horizontal setup.
3. Folded (mirror) Unsplit: Same as unsplit but folded light. Smaller box. Vertical setup.
4. Folded (mirror) Split: Same as split but folded light. Smaller box. Vertical setup.

Parts list:

1. Metal Halide Bulb (Yellow)
2. Spherical Reflector (Green)
3. 220mm FL Fresnel Lens (Dark Blue)
4. 550mm FL Fresnel Lens (Light Blue)
5. 17" 1280x1024 LCD Screen (Orange)
6. Projection Lens (Red)
7. Front Surface Mirror (Grey)



These are not drawn to scale. The length of the box is shorter by about 1.5ft (possibly more) with a folded design. The folded split design is just a split design with a mirror.

I hope this helps get an idea of what I'm trying to do. You can google "DIY projector" for more info too. Again, I appreciate all the help!

[Donald Qualls]

Suggestion for testing before you spend a huge amount of time and effort and possibly a non-trivial amount of money (for the lens and screen) is whether your chosen LCD can survive operating in the high light flux and potentially rather hot environment of a projector housing. One of the big challenges in making LCD projection TVs and computer displays has been finding a way to keep the active LCD chemistry from bleaching in a very strong light or at operating temperatures -- necessary because the strong light is required to make a large screen bright enough for comfortable viewing, especially in a non-darkened room, and projection lights are almost invariably quite hot running.

It'd be a shame to get the whole thing built and find out your display goes blank white on you after it's been on for ten or fifteen minutes -- and since laptop and desktop computer displays aren't made for projection, there's no reason they'd necessarily include the more robust (and presumably more costly, if the price of the displays is any indicator) bleaching resistant liquid crystal material.

[Super Dave]

Donald, you are absolutely right. In fact there are a few components that I did not include to keep the diagram simple. There is IR and UV glass for filtering out a lot of that nasty heat and bleaching light. The rest of the heat is taken out via a couple of 90 CFM fans (intake and exhaust). These are blowing directly over the fresnels and the LCD display and out past the bulb and ballast.

Very real concerns and they need to addressed as you said. If you don't control the heat, you'll end up with bowing fresnels and faded GCDs (gas crystal display hehe).