# Need help with lens for long throw projection set up

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#### Daniel Neuman

Hello All,
I sent basically the same message to the forum at diyprojectorcompany.com (thats where the theory is coming from) but I am trying to cast as wide a net as possible.

This is my first post to this group. I have looked at a lot of messages and read thru the theory section on the website and have a few questions on what is the best lens to use for the following setup.

Source image-3by5" - 5.8" diag
Projected image = 6by10' or 72by120" - 139.9" diag
Distance to screen is 20' or 240"
This is a magnification of 24x

Rearranging the equations in the theory to solve for focal length with the above numbers I get a focal length of 9.5" or 242.5mm. Does this sound right to y'all?

With this setup what would be the best focal length fresnal lens to use and why?

I've read the theory section but one thing that confuses me is where do you place the projection lens in relation to the image? Do you place the image at the focal point of the projection lens?

Thanks,

#### Guy Grotke

simple equation

1/fl = 1/LCD to lens + 1/lens to screen

lens to screen = 240"
lens fl = 9.5"

so LCD to lens = 9.9"

LCD to lens is always more than the focal length of the lens. If you put it at exactly 9.5", then the image would focus at infinity!

I would get a pair of 220 mm fl fresnels, and then build a split design:

Lamp arc / 220 mm space / fresnel with grooves facing LCD / 20 mm space / LCD / 20 mm space / fresnel with grooves facing LCD / 220 mm space / triplet.

Fire it up, adjust triplet position to focus at your 20' throw distance, then adjust lamp arc to fresnel distance until you get a nice even image.

#### Daniel Neuman

Thanks again for the reply Guy,

This is the kind of info I need. I do understand the thins lens equation but it does not seem practical because its for focus at infinty. But I have heard that infinity is 10X the focal length? Is this an approx. that people use?
Is there not any way to narrow down the range of LCD to projection lens distance?

I know this is probably basic projection 101 but please help me understand the purpose of the fresnals. Its my under standing that if you put the light source at the focus of the first fresnel (in your suggested split design) it will produce a collimated beam of light accross its entire area. This collimated beam then passes thru the image-lighting it evenly then thru the second fresnel lens which does the opposite of the first and starts to focus it down to a point-before its focused to a point the projection lens intercepts the light and focuses the light out to the projection screen.
The fresnels also must be placed a certain distance away from the image yes? Like 2cm on each side? If my image is small enough can I (should I) use a glass lens instead?

I am worried that the fresnels will be to close to the ultra hot light source. What if I used the split fresnels with the light side having a much larger focal length to push the light farther away? Then the downstream fresnel (sorry I don't have the correct term. y'all use) could have a shorter focal length to keep the projection lens closer?

#### gunvald

I've been looking into this stuff recently also. Here's what I know from looking at the physics book for about a hundred pages;

You've got the right idea.

Fresnels are cheater lenses, used instead of large, heavy, expensive convex lenses. If you can get a lens of the right size for not too much cash, that's the way to do it. Fresnels make more sense for the sizes of DIY projectors.

If you want to push back the light, get a fresnel with a longer focal length, just like you thought.

Alternately, I've read in other forums of people using low-E glass (low energy emission) to keep the heat off the fresnel and lcd panel. I don't know if this blurs the image though (scatters the light from your point source). Sounds like a great idea if it works.

#### Guy Grotke

yes & no...

>Is there not any way to narrow down the range of LCD to projection lens distance?
A: You need adjustability since your throw distance may change. Use the thin lens approximation to plan and then some real experiments with your actual throw distance to find the place to mount your lens in the center of its adjustment range.

>fresnel work like this...
A: Yes, you've got it.

>fresnels 2cm [from LCD]?
A: Any closer and you will see fresnel rings in the screen image. They can be much farther away on the lamp side if focal lengths, etc. require it. Better to keep them on the lamp side of the LCD, if possible.

>can I (should I) use a glass lens instead?
A: Expensive, & heavy, but glass will work fine.

>What if I used the split fresnels with the light side having a much larger focal length to push the light farther away?
A: Okay, but as you move away from the lamp, the light intensity drops by distance^2. Another strategy to gather more light is to put a precondensor lens close to the lamp, adjusted to capture a wide cone of light and direct it all (in a narrower cone) to the condensor fresnel.

>...downstream fresnel [the field fresnel] could have a shorter focal length to keep the projection lens closer?
A: The LCD to projection lens distance is fixed by the lens fl and the throw distance. The field fresnel will not affect that much. The ideal fresnel arrangement is to put them together 20mm before the LCD, with the lamp arc at the focal distance of the condensor fresnel, and the projection lens at the focal distance of the field fresnel. This is not always possible! So then you get to fiddle with the lamp arc to condensor fresnel distance to get those light rays focussed into the projection lens.

>heat control?
A: Add IR filter glass (DIYprojectorCompany.com) or a piece of Rosco Thermashield BEFORE the condensor fresnel. Use a fan to pull air from around your lamp and push it outside the enclosure. Your fresnels and LCD will stay nice and cool then.

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