AVS Special Member
Join Date: Jun 2005
Location: Northern, VA - Washington, DC
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An add-on lens will never increase brightness, but will ALWAYS decrease brightness at the same image size. If the image is made larger by the lens, it will decrease in brightness by even more.
The reason projectors are brighter as you move them closer (keeping the image size the same) is because the aperture inside the lens opens up, allowing more light through the lens system. Every layer of glass beyond that reduces brightness and image quality. This is part of the photographic standard which has over 100 years of science behind it. ANY lens added to a series of lenses will reduce brightness, and the worst piece of glass in the system is the one which will impact image quality the most.
Hence, why photographers strive for the best lenses they can get their hands on and pay well more than we are paying for entire projection setups for just a single lens.
That said, I have used cheap wide-angle lenses when shooting video and have been happy with the results I've gotten, and I would expect with projection you can get a larger image from the same distance without problem. Still, any increase in size will reduce brightness appropriately and the addition of the lens will reduce brightness a bit as well.
Still, it looks like a fun setup, and if you think there is room to make it bigger, than do so.
BUT: Why buy the lens?
If you want the image size bigger, what happens if you move the projector back further? This may not be ideal for your final setup, but certainly for testing purposes you could move the projector back some can't you? (maybe you can't) But, if you can, do so, and see how large you can go. Move it back 20 or 30 feet or more, and see what the results are.
If you think the image gets too dim, then this is exactly what will happen when using an add-on lens. You could buy it and find out for yourself, but it will happen no matter what I promise you.
AV Integrated - Theater, whole house audio, and technology installation in the Washington DC metro area.