# How to make the projected image smaller by adding one ore more lenses in front of the built in projector lense?

I have a projector that creates a large image, even if the distance to the screen is short. The device is very small, approximateley 10x10 cm if you look from above. The height is only 3 cm.

I could remove and replace the builtin "lense pack" to make the size of the projected image smaller, but I'd prefer to keep the device unchanged.

What type(s) of lense(s) in front of the unchanged device would be reqired to make the image dramatically smaller?

I have measured the following with the original device.

If the distance d from the focal point to the screen is $300\,cm$, the width $w_1$ of the image on the screen is $\approx200\,cm$. And if d is as far as $900\,cm$ then $w_1= 600\,cm$.

What can I do to make the image smaller, so it's new width $w_2$ would be approx. $100\,cm$ at a distance $d= 900\,cm$?

Lenses, mirrors or prisms would be OK, but I somehow guess 1 or 2 lenses would do the trick. What type of lenses would this be, how do I choose and calculate the type and the parameters of the lense? Can you teach me how to choose and understand the formula? The quality of the image and the resolution should still be good afterwards.

Of course I can have a $1\,m$ image if I move the projector close enough, but that is not the solution I am looking for.

I tried also to find this out by myself already. I visited two camera stores and talked with the owners. I also visited optician retail stores. But anybody there could answer this question. I have some basic knowledge of physics and also tried to solve this by reading books about optics and geometrical optics, but without any success. I also tried with some lenses I could get my hands on but I could not solve it. The problem I also had to deal with was that as soon as I moved the lenses a little further away from the device, they were too small to catch the whole beam.

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## 2 Answers

A single lens with the object located at a distance greater than 2*f produces a smaller image (upside down).

http://www.phys.hawaii.edu/~teb/java/ntnujava/Lens/lens_e.html

A doublet can also reduce the size: http://ressources.univ-lemans.fr/AccesLibre/UM/Pedago/physique/02/optigeo/doublet2.html

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Upside down is fine. The projector can switch the image accordingly. – SHernandez Sep 10 '12 at 13:47
Thank you for the links. I will take some time and try and understand what they can teach me. – SHernandez Sep 10 '12 at 13:49
Just move the object on the left with the mouse and you see the image on the right. You just have to place your screen where the image is. – Shaktyai Sep 10 '12 at 13:50

You might find this prohibitively expensive. Two reasons:

• a simple lens will not be good enough, because it will introduce chromatic aberration: divergence of rays of different colors. This is because the glass's index of refraction is slightly different for different wavelengths (this is called "dispersion"). You will need to use an achromatic doublet or triplet, which is two or three lenses made of different kinds of glass specially tailored to cancel out each other's dispersion.

• as you found out, the lens has to be big enough to catch the entire beam, and so will require a lot of glass. You can mitigate this problem by placing the lens closer to the projector, but then you will need a shorter focal length; in addition to making the chromatic aberration worse, it will also introduce spherical aberration, which is distortion of the image in simple lenses of small focal lengths. To prevent this, you will need a aspheric lens, which is ground in a complicated shape and also expensive.

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One suggestion which might make this doable for a low cost: look into using a camera lens from an old (pre digital) SLR camera. These can be found very cheaply on ebay and craigslist. If you have a friend with a nice camera see if you can borrow a zoom lens, you may be able to find a configuration which does what you want. – user2963 Sep 10 '12 at 16:12
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