Camera/Projector lens question.

BC wrote:
With only a single 35mm format lens you will only be able to project a
tiny part of the LCD screen. Mounting two lenses back-to-back will
allow you to project the entire 17" diagonal, but the image on your
screen will be extremely dim. I doubt you would even be able to see
the image at all except in a totally dark room. Basically one lens
forms an intermediate image of the LCD screen, and the second lens
re-images the intermediate image onto the wall. Since you want a 39
degree half angle at the LCD side you will need to use a wide angle
lens capable of covering 78 degrees or more on the LCD half of the
optical system. That means you'll need a focal length of 24mm or
shorter. The mating lens can have a longer focal length.
Unfortunately, since 35mm optics are not telecentric the coupled lens
pair will have mis-matched pupils. To correct this you would need to
put a field lens at the intermediate image in order to avoid severe
vignetting. A fresnel field lens will avoid introducing aberrations,
but unfortunately the rings will be very visible.

All things considered, I think you'll be extremely disappointed with
what you can achieve with 35mm optics. An overhead projector will work
alot better, but will still give an extremely dim image unless you can
drastically increase the panel illumination. An ordinary LCD panel
with fluorescent illumination will certainly result in an underwhelming
projected image.

Large format LCD projectors do exist, but they tend to use high
intensity arc lamps for illumination, and also use very large
projection lenses suitable for the format.
Brian
www.caldwellphotographic.com


Very great information! Thanks you!

My current thoughts are to replace the lcd's standard back light with a
metal halide lamp. They are very bright have 10k+ hour lifespans and
reasonable color. There are several problems. They emit light from a
small cylinder not a point and produce lots of heat.

I was hoping to take a big short cut by using camera optics to focus the
image. But I see that will be much more complicated then I'd thought.
(very naive understanding of camera lenses)

Thanks much!

(very naive understanding of camera lenses)

So what, the post was interesting. And it does sound like an interesting
project.