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Camera/Projector lens question.



 
 
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  #1  
Old January 12th 05, 12:19 PM
brian
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Default Camera/Projector lens question.


"Eric Webster" wrote in message
...
Hello, I have a question about using a camera lens for a projection
system that I am building.

I am wondering if it would be possible to use a zoom camera lens with
specs ~= to 70mm-210mm f3.5-4.5 to project an image of a 17" lcd panel
onto a wall about 5'-6' away. The distance between the lcd and the film
side of the lens will be between 200mm to 300mm. The largest angle
between the central axis of the lens and the lcd will be about 39 degrees.

If I have the lens set to f=180 and the lens is 200mm away, then
according to 1/i+1/o=1/f the image should be in focus 1800mm(5.9') away.
Is this correct? Is there something about camera lenses that would
prevent them from working when the film(lcd panel) is that far away?

I have considered just using an overhead projector lens to do this but
it would be nice to have something that will let me change the
magnification.

Thanks for any help, Eric


f=180?????, I doubt if there is a lens that will stop down that far in the
first place, even my Sekonic L508 zoom master light meter only reads down to
f=128.5, ( just had to check it out). But even then, how will you be able to
project the image?, you would have to be able to shine a light through the
LCD panel and I think that would be near impossible especially with a tiny
aperture like f-180, you would have to use the light from an A bomb to shine
enough through that, to get an image, with film, it would have to be slide
film, or all you would get is a big negative image anyway.
Why not just go and buy a Projector, slide projectors can be had for a very
little money, and digitals are a tad more expensive at about 800 and up.
Anyway, why the hell do you need to magnify the image forma 17" LCD panel
anyway? if you want it bigger buy a bigger Panel, or simply connect your
computer up to your T.V and show the images on there.


Brian........................


  #2  
Old January 12th 05, 08:25 PM
Eric Webster
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Default

Mike Owens wrote:
Two things prevent this, light intensity and image inversion. First
off, there is no where near enought light coming from an LCD monitor to
make this possible. Even if you were in a completely dark room and
everything else worked perfectly as you intended (which it wouldn't),
the ambient light generated by the display would overpower the
projection. The problem is one of focused light. Projectors, whether
overhead, slide or the newer LCD projectors, all work on the same
principle. Take a light source and focus it on a projection lens.
Insert the image you want to project between the light source and
projection lens.

Since you don't have focused light, your "projector" won't work. But
even if, somehow, you did get it to work, the image projected would be
upside down and reversed. Your lens inverts and reverses the image. The
mirror/pentaprism in a SLR corrects this so you see the image properly.
The film/sensor, or screen in the case of your projector, sees the
uncorrected image.


Thanks for the informative reply!

I will be removing the lcd's original back light and replacing it with a
pair of fresnel lenses and perhaps a HID light source (I haven't decided
exactly how to do this yet). As for the image inversion I could simply
flip the lcd upside down so that the light is shining through it backwards.

p.s. This is a fun/spare time project. I'm not trying to create
commercial quality projector.
  #3  
Old January 12th 05, 10:40 PM
Paul Bielec
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"Eric Webster" wrote in message
...
Mike Owens wrote:
Two things prevent this, light intensity and image inversion. First
off, there is no where near enought light coming from an LCD monitor to
make this possible. Even if you were in a completely dark room and
everything else worked perfectly as you intended (which it wouldn't),
the ambient light generated by the display would overpower the
projection. The problem is one of focused light. Projectors, whether
overhead, slide or the newer LCD projectors, all work on the same
principle. Take a light source and focus it on a projection lens.
Insert the image you want to project between the light source and
projection lens.

Since you don't have focused light, your "projector" won't work. But
even if, somehow, you did get it to work, the image projected would be
upside down and reversed. Your lens inverts and reverses the image. The
mirror/pentaprism in a SLR corrects this so you see the image properly.
The film/sensor, or screen in the case of your projector, sees the
uncorrected image.


Thanks for the informative reply!

I will be removing the lcd's original back light and replacing it with a
pair of fresnel lenses and perhaps a HID light source (I haven't decided
exactly how to do this yet). As for the image inversion I could simply
flip the lcd upside down so that the light is shining through it

backwards.

p.s. This is a fun/spare time project. I'm not trying to create
commercial quality projector.


LCDs are polarised, that's why they are hard to see at certain angles.
So it might not be simple to pass light through the LCD and have it all
focused exactly where you need it to.


  #4  
Old January 13th 05, 02:56 PM
Mike Owens
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I didn't bother mentioning color drift due to viewing angle since the
normal backlight from a LCD wouldn't work. If one designed this like a
regular projection system, the colors would certainly change from the
center to the edge of the resulting projection due to the differing
angles the light passes through the LCD. How much color shift is
present would depend on the viewing angle of the LCD compared to the
distance from the projection lens. Newer LCDs with wider viewing angles
would definitely help.

The only way to prevent the color shift would be to have all the light
pass through the screen perpindicular to it. Of course, you'll need a
rather large parabolic reflector to focus the light properly on a 17"
panel, then another one to direct it towards the camera lens.

However, if you have an overhead projector handy, you might try placing
the disassembled LCD BELOW the glass that you normally place the
transparencies on. This glass is a large lens which takes the parallel
light generated by the reflector and focuses it (to some extent anyway)
on the projection head. Of course the heat of the lamp might destroy
the panel...
The whole thing would be an interesting science experiment.

  #5  
Old January 13th 05, 02:56 PM
Mike Owens
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Default

I didn't bother mentioning color drift due to viewing angle since the
normal backlight from a LCD wouldn't work. If one designed this like a
regular projection system, the colors would certainly change from the
center to the edge of the resulting projection due to the differing
angles the light passes through the LCD. How much color shift is
present would depend on the viewing angle of the LCD compared to the
distance from the projection lens. Newer LCDs with wider viewing angles
would definitely help.

The only way to prevent the color shift would be to have all the light
pass through the screen perpindicular to it. Of course, you'll need a
rather large parabolic reflector to focus the light properly on a 17"
panel, then another one to direct it towards the camera lens.

However, if you have an overhead projector handy, you might try placing
the disassembled LCD BELOW the glass that you normally place the
transparencies on. This glass is a large lens which takes the parallel
light generated by the reflector and focuses it (to some extent anyway)
on the projection head. Of course the heat of the lamp might destroy
the panel...
The whole thing would be an interesting science experiment.

  #6  
Old January 13th 05, 02:56 PM
external usenet poster
 
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Default

In message ,
Eric Webster wrote:

Hello, I have a question about using a camera lens for a projection
system that I am building.


Can the lens handle the heat?
--


John P Sheehy

  #7  
Old January 13th 05, 02:56 PM
external usenet poster
 
Posts: n/a
Default

In message ,
Eric Webster wrote:

Hello, I have a question about using a camera lens for a projection
system that I am building.


Can the lens handle the heat?
--


John P Sheehy

  #8  
Old January 13th 05, 05:14 PM
BC
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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

  #9  
Old January 13th 05, 05:49 PM
Peter Chant
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Eric Webster wrote:

I will be removing the lcd's original back light and replacing it with a
pair of fresnel lenses and perhaps a HID light source (I haven't decided
exactly how to do this yet). As for the image inversion I could simply
flip the lcd upside down so that the light is shining through it
backwards.


Try searching the archives of slashdot.org. Someone made a post there a few
years back about a come made projector made in a similar way. I think
their screen was somewhat smaller. I think that the main problems was the
heat playing havoc with the LCD.

Pete


--
http://www.petezilla.co.uk
  #10  
Old January 13th 05, 06:40 PM
Chris
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Default

In rec.photo.equipment.35mm BC wrote:
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


Just a little tangential information from a CRT projector owner:

Light output of my three CRTs is 225 ANSI lumens, so about 75 lumens per
CRT. And this is considered dim by the 2000 lumen DLP projectors out
there now (I like it, however).

My lenses are about 200mm (8" = 203.something mm) and have an f/1.1
aperture to allow as much of that light through as possible.

I've not heard of a 200mm lens for a 35mm system that has a max aperture
even close to f/1.1. Still, sounds like a fun project, good luck to the
OP.

Chris
 




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