Full Version: What would happen if you put another fresnel in
Just curious, as I dont even have my fresnel set yet, but what would happen if you put another condenser fresnel in, say, 4 inches behind the rear fresnel, as in 4 inches closer to the triplet?
this new fresnell would change the light path, the focusing point would be closer from the LCD (assuming a positive focal fresnell) so the light would be condensed somewhere between the lcd and the projection lens, if you donīt change the projection lens position then lot of light would be lost (the light would start to diverge from that focusing point and when it gets to the triplet lens, it would be too big to enter the triplet)
The fresnell lenses on the LL design are the "correct" ones, if you change them or introduce new ones, you need to design a new setup.
The fresnell lenses on the LL design are the "correct" ones, if you change them or introduce new ones, you need to design a new setup.
QUOTE (Rox @ Jul 19 2005, 06:25 PM)
this new fresnell would change the light path, the focusing point would be closer from the LCD (assuming a positive focal fresnell) so the light would be condensed somewhere between the lcd and the projection lens, if you donīt change the projection lens position then lot of light would be lost (the light would start to diverge from that focusing point and when it gets to the triplet lens, it would be too big to enter the triplet)
The fresnell lenses on the LL design are the "correct" ones, if you change them or introduce new ones, you need to design a new setup.
The fresnell lenses on the LL design are the "correct" ones, if you change them or introduce new ones, you need to design a new setup.
Right, so the focal point would then be further away (technically 330mm)? Basically what Im saying is since the light image when it reaches the second rear fresnel will be significantly smaller, as long as you move the triplet farther back to accomodate this, would the throw effectively be smaller in size? In theoretics this seems to be the case right?
Heres a REALLY QUICK and totally unexact drawing I made... hopefully someone gets this..
no, the focal would be closer (not farther) so smaller box.
if you need to extend the conbined focal, the you need a negative focal fresnell, you will hardly find one
.
anyway, why are you going this way?
if you need to extend the conbined focal, the you need a negative focal fresnell, you will hardly find one
.anyway, why are you going this way?
QUOTE (Rox @ Jul 19 2005, 06:53 PM)
no, the focal would be closer (not farther) so smaller box.
if you need to extend the conbined focal, the you need a negative focal fresnell, you will hardly find one.
anyway, why are you going this way?
if you need to extend the conbined focal, the you need a negative focal fresnell, you will hardly find one
.anyway, why are you going this way?
It was just a thought last night, because I need a smaller (longer) throw. I have a 24 foot deep room, intended to be a small theater, and I can't do a 20' wide screen, because I cant devote and entire wall to a screen. Its part of a commercial building and the room is multi purpose. It would serve as our dining room/conference room/small theater. It has 14' ceilings so dropping it is not really an option. The only option, IMO, is a zoom lens, and the zoom lens thread has produced no results thus far. Thus, I thought maybe my thought might have been true but I guess it isnt.
mmm, i think ther eis some confusion with the throw..
the throw is the distance from the projection to the lens. longer the trhow, shorter the lens to lcd distance. longer the throw, larger the projected image will be.
if you need smaller image, then place the projector closer from the wall.
your picture doesnīt look to work in my opinion, and you need to consider the distorsions introduced by the 2nd field fresnell at 4" from the main field fresnell (or at 5" from the lcd) as well.
if you cant place the proyector closer to the wall, then what you need is a longer focal projection lens, keep in contact with the news at lumenlab, maybe you will find one soon.
the throw is the distance from the projection to the lens. longer the trhow, shorter the lens to lcd distance. longer the throw, larger the projected image will be.
if you need smaller image, then place the projector closer from the wall.
your picture doesnīt look to work in my opinion, and you need to consider the distorsions introduced by the 2nd field fresnell at 4" from the main field fresnell (or at 5" from the lcd) as well.
if you cant place the proyector closer to the wall, then what you need is a longer focal projection lens, keep in contact with the news at lumenlab, maybe you will find one soon.
OK I believe I understand now. The reason it will be a shorter distance to the triplet from the 2nd rear fresnel would be the angle of light that reaches it?
Maybe if we used a collector in front of the 2nd fresnel it would straighten out the light pattern, then the 2nd rear fresnel would produce a smaller picture when it hits the triplet?
And no, as I said I cant really place the projector closer to my screen, because it will be in the viewing field of most of the people watching the screen. So I need to have a zoom effect, or a shrinking of the overall projected image.
Once again, I have not seen anyone try this with a second fresnel set at all on these forums, so when I had the thoughts last night, I wondered maybe it would work. Probably I was being shortsighted but I was curious just the same.
Also, why would the image distort? Since I'm not keystoning, shouldnt the image remain relatively square?
Maybe if we used a collector in front of the 2nd fresnel it would straighten out the light pattern, then the 2nd rear fresnel would produce a smaller picture when it hits the triplet?
And no, as I said I cant really place the projector closer to my screen, because it will be in the viewing field of most of the people watching the screen. So I need to have a zoom effect, or a shrinking of the overall projected image.
Once again, I have not seen anyone try this with a second fresnel set at all on these forums, so when I had the thoughts last night, I wondered maybe it would work. Probably I was being shortsighted but I was curious just the same.
Also, why would the image distort? Since I'm not keystoning, shouldnt the image remain relatively square?
the rear fresnell is called to the one between lcd and the bulb, i thought you we talking about the field fresnell (330 focal one).
the fresnell lens at 5" from the lcd will magnify the lcd looking it as if it was 19" TFT, but this magnification is done by a poor fresnell lens, this kind of lenses are good to bend light from the bulb but if you use it on the IMAGE path, (let's understand using it as part of the image projection lens system) then there will be cromatic, spherical, coma, ... distortions introduced to the image. think about a singlet lens instead of a fresnell lens.
what you need is definitelly a longer focal projection lens. So you can place the proyector at the back but your image size will be size you need. Actually there isnīt much choice on long throw lenses...
the fresnell lens at 5" from the lcd will magnify the lcd looking it as if it was 19" TFT, but this magnification is done by a poor fresnell lens, this kind of lenses are good to bend light from the bulb but if you use it on the IMAGE path, (let's understand using it as part of the image projection lens system) then there will be cromatic, spherical, coma, ... distortions introduced to the image. think about a singlet lens instead of a fresnell lens.
what you need is definitelly a longer focal projection lens. So you can place the proyector at the back but your image size will be size you need. Actually there isnīt much choice on long throw lenses...
alternatively, you can adjust your projector placement.
What I was planning on doing is build a stand-up projector that is set off to the side and adjust the keystoning with a mirror. It would allow you to place the projector closer to the wall you're aiming for and it would be less intrusive at the same time.
Another possibility is building your projector to fit inside a table (some examples lurk about).
The only way to adjust the throw of the box is to adjust the non-fresnel lenses. You may be able to use the pro lens to do this, I'm not sure.
What I was planning on doing is build a stand-up projector that is set off to the side and adjust the keystoning with a mirror. It would allow you to place the projector closer to the wall you're aiming for and it would be less intrusive at the same time.
Another possibility is building your projector to fit inside a table (some examples lurk about).
The only way to adjust the throw of the box is to adjust the non-fresnel lenses. You may be able to use the pro lens to do this, I'm not sure.
Well see thats a problem too. After reading the countless pages on the keystoning thread, those people have gotten no final results either. Its a semi-helpless feeling right now. I want a projector but I dont have a 11x11 room.
My alternative would be to project width wise, which may end up being better, but will still cause problems. We'll see. One thing that really isnt an option is paying $250 for a new lens. That defeats the whole purpose of the affordable DIY projector.
My alternative would be to project width wise, which may end up being better, but will still cause problems. We'll see. One thing that really isnt an option is paying $250 for a new lens. That defeats the whole purpose of the affordable DIY projector.
well you will certainly have to be at least a little innovative here to get the results you want, its DIY after all. If a stand up is what you are interested in, joe's design is here:
http://www.lumenlab.com/forums/index.php?showtopic=2129
You'll have to come up with a way to tilt the mirror horizontally to adjust the keystoning. It is also possible to tilt the front fresnel, but I wouldn't since it can do funky things with your light path.
I would also not build the box exactly like that, more like this:
http://www.lumenlab.com/forums/index.php?showtopic=2710
so that you have room to build a mirror-tilting mechanism to adjust both horizontal and vertical keystoning at the same time.
http://www.lumenlab.com/forums/index.php?showtopic=2129
You'll have to come up with a way to tilt the mirror horizontally to adjust the keystoning. It is also possible to tilt the front fresnel, but I wouldn't since it can do funky things with your light path.
I would also not build the box exactly like that, more like this:
http://www.lumenlab.com/forums/index.php?showtopic=2710
so that you have room to build a mirror-tilting mechanism to adjust both horizontal and vertical keystoning at the same time.
mirror tiltind does not correct keystone TheAxeMaster, it has never been demostrated (on paper), do you mind being the first?
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