Hi Fellow DIYers! I have a good news.
I contacted my local optics shop and they are willing to design one triplet for us.
Let me know of the specs that will fit the new 550mmFL Fresnel from 3dLens.
They asked me the following questions but I dont know the answer to these...
Are there any performance requirements for this lens, in terms of radius/EFL/BFL tolerance, diameter or thickness tolerance, wavefront quality, scratch-dig, etc?
How far away is your object and are imaging this to any sort of ...?
Rox and Guy, give your best shot!!!
Please provide me the design for this triplet.
Do you think if we reduce the diameter and increase the FOV it will be much cheaper to build? I just told him that I want a triplet and looking for an exact design. According to him they have equipment to do this triplet lens.
He has not give me a quote yet because he wants the full details.
I contacted my local optics shop and they are willing to design one triplet for us.
Let me know of the specs that will fit the new 550mmFL Fresnel from 3dLens.
They asked me the following questions but I dont know the answer to these...
Are there any performance requirements for this lens, in terms of radius/EFL/BFL tolerance, diameter or thickness tolerance, wavefront quality, scratch-dig, etc?
How far away is your object and are imaging this to any sort of ...?
Rox and Guy, give your best shot!!!
Please provide me the design for this triplet.
Do you think if we reduce the diameter and increase the FOV it will be much cheaper to build? I just told him that I want a triplet and looking for an exact design. According to him they have equipment to do this triplet lens.
He has not give me a quote yet because he wants the full details.
Buddy,
I can't offer any specs for you, but I'm sure Guy, Rox or the other resident gurus will help.
I do have some questions. What do you mean by local optics shop? I've never seen heard of an "optics shop." I have seen engineering optics suppliers on the web and their catalogs at the labs at school. They're pretty expensive for this sort of thing. Is this the sort of shop you're talking about?
Could the optics guy offer any sort of ball park figure or even order of magnitude?
I can't offer any specs for you, but I'm sure Guy, Rox or the other resident gurus will help.
I do have some questions. What do you mean by local optics shop? I've never seen heard of an "optics shop." I have seen engineering optics suppliers on the web and their catalogs at the labs at school. They're pretty expensive for this sort of thing. Is this the sort of shop you're talking about?
Could the optics guy offer any sort of ball park figure or even order of magnitude?
ideal long-throw triplet
I bet this will blow everybody away, in terms of cost. I know that it costs just about the same amount to make one lens as 100 lenses, because the tooling is most of the expense. The glass blanks are not all that much, if the designer pays attention to the cost. So make sure you get quotes for the design, production of one, and production of 100 lenses.
So tell them we want an air or oil-spaced Cooke triplet with an 80 mm clear aperature, and an Effective Focal Length of 450 mm. We will use it to project a 17" diagonal LCD to a screen about 14 feet away from the lens. This puts the LCD about 503 mm from the lens, so the required Field Of View is about 46.5 degrees (full angle).
All I know about performance, is that the LCD pixels are 0.293 mm squares, so they project to about 2.49 mm on the screen at that distance. We would like spots in each LCD pixel to send at least 95% of their energy to the corresponding screen pixel. And we want that level of performance over the entire FOV.
The outer lenses should be made from a glass with reasonable resistance to damage, and complete resistance to degradation by water. If the lens is air-spaced, then all six surfaces should be AR coated. If the lens is oil-spaced, then the two outer surfaces should be AR coated.
For anybody wondering, the rationale: You can't make triplets with the lenses glued together any wider than 80 mm total, or they come apart from thermal stress. Oil-filled lenses don't need AR coating on the surfaces in contact with the oil, and those surfaces also don't need to be finished as precisely. (So the cost may be lower.) A set of 550 and 220 mm fresnels will give you about a 550/220 * 25 mm arc = 62.5 mm arc image. That should fit through a lens with an 80 mm aperature, but with that wide an FOV, the outer lenses themselves will be much wider than 80 mm. Any lens that works for a 17" LCD will work fine for a 15" LCD. Any lens that works at a 14' throw, will work fine at shorter throws. Finally, we have to be reasonable about the spot size performance: We could spend $5000 each to get a perfect wide aperature process lens, but that would be wasted on LCDs with 0.293 mm features!
I bet this will blow everybody away, in terms of cost. I know that it costs just about the same amount to make one lens as 100 lenses, because the tooling is most of the expense. The glass blanks are not all that much, if the designer pays attention to the cost. So make sure you get quotes for the design, production of one, and production of 100 lenses.
So tell them we want an air or oil-spaced Cooke triplet with an 80 mm clear aperature, and an Effective Focal Length of 450 mm. We will use it to project a 17" diagonal LCD to a screen about 14 feet away from the lens. This puts the LCD about 503 mm from the lens, so the required Field Of View is about 46.5 degrees (full angle).
All I know about performance, is that the LCD pixels are 0.293 mm squares, so they project to about 2.49 mm on the screen at that distance. We would like spots in each LCD pixel to send at least 95% of their energy to the corresponding screen pixel. And we want that level of performance over the entire FOV.
The outer lenses should be made from a glass with reasonable resistance to damage, and complete resistance to degradation by water. If the lens is air-spaced, then all six surfaces should be AR coated. If the lens is oil-spaced, then the two outer surfaces should be AR coated.
For anybody wondering, the rationale: You can't make triplets with the lenses glued together any wider than 80 mm total, or they come apart from thermal stress. Oil-filled lenses don't need AR coating on the surfaces in contact with the oil, and those surfaces also don't need to be finished as precisely. (So the cost may be lower.) A set of 550 and 220 mm fresnels will give you about a 550/220 * 25 mm arc = 62.5 mm arc image. That should fit through a lens with an 80 mm aperature, but with that wide an FOV, the outer lenses themselves will be much wider than 80 mm. Any lens that works for a 17" LCD will work fine for a 15" LCD. Any lens that works at a 14' throw, will work fine at shorter throws. Finally, we have to be reasonable about the spot size performance: We could spend $5000 each to get a perfect wide aperature process lens, but that would be wasted on LCDs with 0.293 mm features!
The local optics shop I was talking about is a shop that sells binoculars and telescope lens.
I talked to them regarding the triplet lens and they are willing to do it but they cant make one lens
just for me. According to them, Minimum order is 50-100 pcs.
They told me that I need a lens that is designed to be optimized for the 17” LCD.
Off-the-shelf triplet lenses would not work well at the four corners of the LCD.
They recommend me to consider a custom designed lens optimized specifically for the LCD (This will require a design and tooling fee). His estimate is that the best lens form is probably not a triplet given for the application.
Thanks Guy for the info. I gave those info to him and he'll consider it and minimize the cost as possible.
I dont have the quote for one but let us see at what price he will sell it. If not that expensive then I'll
buy 50 then distribute it to those who are interested. I'll probably get two.
I talked to them regarding the triplet lens and they are willing to do it but they cant make one lens
just for me. According to them, Minimum order is 50-100 pcs.
They told me that I need a lens that is designed to be optimized for the 17” LCD.
Off-the-shelf triplet lenses would not work well at the four corners of the LCD.
They recommend me to consider a custom designed lens optimized specifically for the LCD (This will require a design and tooling fee). His estimate is that the best lens form is probably not a triplet given for the application.
Thanks Guy for the info. I gave those info to him and he'll consider it and minimize the cost as possible.
I dont have the quote for one but let us see at what price he will sell it. If not that expensive then I'll
buy 50 then distribute it to those who are interested. I'll probably get two.
buddy123 said:
what do you mean by that?
guy, I agree on your lens specs, but the 80mm effective aperture I find it something small for the typical light source (27mm arc 400W ushio lam).
The 220/550 setup would project somewhere 67mm arc on the center of the ideal triplet... so if you raytrace the arc edge rays, it means that any triplet wider than 26mm will block the light.
I would say it is not posible to make a triplet lens thiner than 6cm so my conclusion is that it should be larger diameter than 80mm.I wonder if we can ask for a larger lens diameter...
edit: sorry, I didn´t understand that point you already xplaned on your post...
. My quick estimation is that 100mm aperture is the minimun so the light will not be blocked.Attachments
clear aperature versus diameter
I think that "clear aperature" takes that problem into account. An 80 mm aperature should pass all the light from a properly positioned 79 mm arc image. The diameter of the inner lens will be just a bit larger than 80 mm. The diameters of the outer lenses will have to be much larger, for the projection lens to have an 80 mm clear aperature.
The lens maker means that it might be cheaper to make this a tessar (a four element lens), because we want such a wide FOV. I think triplets are often made with up to 45 degrees of FOV, but we need a bit more for a 17" LCD. Those triplets may require expensive exotic glass. If he can make it with four lenses he may be able to use cheaper glass, get better performance in the worst cases, etc. As long as the surfaces are AR coated (or the lens is oil-filled), then a fourth lens does not absorb much more light.
Anybody interested can google for "metrogon" to see a four element lens that has a 90 degree FOV!
I think that "clear aperature" takes that problem into account. An 80 mm aperature should pass all the light from a properly positioned 79 mm arc image. The diameter of the inner lens will be just a bit larger than 80 mm. The diameters of the outer lenses will have to be much larger, for the projection lens to have an 80 mm clear aperature.
The lens maker means that it might be cheaper to make this a tessar (a four element lens), because we want such a wide FOV. I think triplets are often made with up to 45 degrees of FOV, but we need a bit more for a 17" LCD. Those triplets may require expensive exotic glass. If he can make it with four lenses he may be able to use cheaper glass, get better performance in the worst cases, etc. As long as the surfaces are AR coated (or the lens is oil-filled), then a fourth lens does not absorb much more light.
Anybody interested can google for "metrogon" to see a four element lens that has a 90 degree FOV!
I would say "clear lens aperture" is the diameter of the lens, after the housing attachement being considered.
the 135mm triplet has a 125mm clear lens aperture and it is measurable on the lens, or you can work out from F value;
F=3.6=efective focal/clear lens aperture=450/D
where D=125mm.
I don´t know if 4 element lens would be cheaper than a triplet, anyway is it posible to them to make a 100mm lens diameter triplet?
the 135mm triplet has a 125mm clear lens aperture and it is measurable on the lens, or you can work out from F value;
F=3.6=efective focal/clear lens aperture=450/D
where D=125mm.
I don´t know if 4 element lens would be cheaper than a triplet, anyway is it posible to them to make a 100mm lens diameter triplet?
Let's Finalize the design for the Triplet
Sorry for this late reply. So busy with my Job.
This morning I got a chance to stop by at the store and they told me that they are willing
to design a new triplet lens for me. Per my instructions, they will construct a custom lens
with these specs below:
-The lens will be constructed in 3 elements in 3 groups;
-Focal length is 450mm;
-Aperture of lens will be in diameter of 80mm;
-Angle of view will be 46.5 degree.
-Distance of projecting will be 3.5M to 3.6M
The sad note is, there is a design charges of USD3500.00 but if I will buy
30 pcs or more then they will waive the design charges. I guess it's not that
bad. The price of the new triplet will be USD195.00/piece.
I'm gonna buy 2 so i need 28 people more. You dont have to send me the money for now, just be manly enough
and assure me if you are willing to buy one so I have an idea if I will say Yes or No to the lens maker.
If you are near New Jersey then you can pickup your lens from my house so there is no shipping and handling.
Please! Please! Please! Don't say Yes if you are not willing to buy.
Please be considerate to me because I'll buy it from my hard earned money.
I don't think 30pcs lens is a good display or furniture in my house.
Guy, should I change the specs based on Rox advise and tell the lens maker we need a 100 diameter lens?
Let me know ASAP! Not sure if they will increase the price.
Rox and Guy, Could you provide a good drawing of this Lens with respect to distance and field of view like Rox drawing?
We have still a couple of days to finalize this Triplet Lens.
Sorry for this late reply. So busy with my Job.
This morning I got a chance to stop by at the store and they told me that they are willing
to design a new triplet lens for me. Per my instructions, they will construct a custom lens
with these specs below:
-The lens will be constructed in 3 elements in 3 groups;
-Focal length is 450mm;
-Aperture of lens will be in diameter of 80mm;
-Angle of view will be 46.5 degree.
-Distance of projecting will be 3.5M to 3.6M
The sad note is, there is a design charges of USD3500.00 but if I will buy
30 pcs or more then they will waive the design charges. I guess it's not that
bad. The price of the new triplet will be USD195.00/piece.
I'm gonna buy 2 so i need 28 people more. You dont have to send me the money for now, just be manly enough
and assure me if you are willing to buy one so I have an idea if I will say Yes or No to the lens maker.
If you are near New Jersey then you can pickup your lens from my house so there is no shipping and handling.
Please! Please! Please! Don't say Yes if you are not willing to buy.
Please be considerate to me because I'll buy it from my hard earned money.
I don't think 30pcs lens is a good display or furniture in my house.
Guy, should I change the specs based on Rox advise and tell the lens maker we need a 100 diameter lens?
Let me know ASAP! Not sure if they will increase the price.
Rox and Guy, Could you provide a good drawing of this Lens with respect to distance and field of view like Rox drawing?
We have still a couple of days to finalize this Triplet Lens.
hold off for a while
I admire your decisiveness, but I recommend you keep your $6000 in your pocket for a while: Lumenlab has an almost identical project underway right now. You can look at the information at the bottom of this web page:
http://www.diybuildergroup.com/
I think it would be a better idea to wait and see if there is anything wrong with LL's new triplet before you procede. I am sure your lens company would charge more for a 100 mm diameter lens, so your price would be about the same as LL will be charging. If we (the DIY community) do find something lacking in the new LL triplet, then you could go ahead with a new design to solve those problems.
But there may not be any problems. The new LL triplet + the 550 mm fl fresnel may be just what we need for a great long-throw projector. Then the only missing "ideal parts" will be a 10000+ hour 400 Watt MH in a small package with a color temp of 5000-6000 K, a cheap dichroic spherical reflector to fit it, and a cheap hot mirror big enough to fit the light cone.
It would be pretty silly to have two similar "ideal" projection lenses, and no money to invest in creating the "ideal" hot mirror!
I admire your decisiveness, but I recommend you keep your $6000 in your pocket for a while: Lumenlab has an almost identical project underway right now. You can look at the information at the bottom of this web page:
http://www.diybuildergroup.com/
I think it would be a better idea to wait and see if there is anything wrong with LL's new triplet before you procede. I am sure your lens company would charge more for a 100 mm diameter lens, so your price would be about the same as LL will be charging. If we (the DIY community) do find something lacking in the new LL triplet, then you could go ahead with a new design to solve those problems.
But there may not be any problems. The new LL triplet + the 550 mm fl fresnel may be just what we need for a great long-throw projector. Then the only missing "ideal parts" will be a 10000+ hour 400 Watt MH in a small package with a color temp of 5000-6000 K, a cheap dichroic spherical reflector to fit it, and a cheap hot mirror big enough to fit the light cone.
It would be pretty silly to have two similar "ideal" projection lenses, and no money to invest in creating the "ideal" hot mirror!
Guy, thanks for the Info. I wasn't aware that lumelab is looking for another supplier of their
pro lens. That's a good news and It's nice to hear that. Lets wait for that lens then.
Wow, the price is a bit expensive, more than $250. Well, they probably have to pay for the design fee and other misc. cost. Do you have any idea when they are going to have it?
Anyways, I talked to the owner of optics shop this late afternoon. He made a call to the lens maker
for a change in specs. The good news is, they can change the specs of the triplet and make it as a
100mm diameter for the same price but they are asking for 50-100 pcs to compensate for the design cost. According to them, the materials are cheap but the design and testing will be a bit
time consuming and it has a price. Design, Testing and mass production phase is about 1 month
according to the lens maker.
Well, for now, let's just wait for the lumenlab triplet. I'm just excited to strip my 17" LCD
and try the new fresnel and new triplet.
Thanks again Guy!
pro lens. That's a good news and It's nice to hear that. Lets wait for that lens then.
Wow, the price is a bit expensive, more than $250. Well, they probably have to pay for the design fee and other misc. cost. Do you have any idea when they are going to have it?
Anyways, I talked to the owner of optics shop this late afternoon. He made a call to the lens maker
for a change in specs. The good news is, they can change the specs of the triplet and make it as a
100mm diameter for the same price but they are asking for 50-100 pcs to compensate for the design cost. According to them, the materials are cheap but the design and testing will be a bit
time consuming and it has a price. Design, Testing and mass production phase is about 1 month
according to the lens maker.
Well, for now, let's just wait for the lumenlab triplet. I'm just excited to strip my 17" LCD
and try the new fresnel and new triplet.
Thanks again Guy!
Actually the lens in the optics shop is very much cheaper. $195/piece but we need 30 pcs for 80mm diameter
and no design fee.
For 100mm diameter, same price as $195/piece but they need a minimum of 50 pieces/ order and no design fee.
If the lumenlab's triplet is not good enough then we can talk to the optics shop again.
Thanks for your nice words CCMCornell.
and no design fee.
For 100mm diameter, same price as $195/piece but they need a minimum of 50 pieces/ order and no design fee.
If the lumenlab's triplet is not good enough then we can talk to the optics shop again.
Thanks for your nice words CCMCornell.
While on the topic of lenses here.... $250 is getting kinda pricey for a lens. OUCH! But... My question is for Guy (or anyone who knows the answer):
How do you find the FOV of a lens? What are the spec you need to know to find it out? I have seen many lenses for sale and they have the right FL, but if the FOV is bad... Forget it. The FOV of this new lens sounds good but at $250, that's too much for me. Any help would be greatly appreciated. Thanks!
How do you find the FOV of a lens? What are the spec you need to know to find it out? I have seen many lenses for sale and they have the right FL, but if the FOV is bad... Forget it. The FOV of this new lens sounds good but at $250, that's too much for me. Any help would be greatly appreciated. Thanks!
how do you find the FOV?
Pretty simple: Every compound lens is designed for a particular application. The designer starts with the required focal length and performance over a particular Field Of View. Then he selects individual lens elements and spacing that meet those requirements.
For example, a telescope objective might require a focal length of 450 mm, and excellant performance over a FOV of just 2 degrees. (Only light that gets through the eyepiece matters.) So you can find a beautiful telescope objective that will be horrible as an LCD projection lens. The designer traded off the performance at 20 degrees for better performance in the central 2 degrees. Why not?
Or you can find an overhead projector lens that was designed to have a FOV that covers a 10.5" by 10.5" stage when the image is focussed at a 6 to 12 feet throw distance. That is a diagonal distance of 14.85", so it will work okay for a 15" LCD.
About the only old lenses you can buy that were designed for an FOV big enough for a 17" or larger LCD, are process lenses. These were designed for making lithography masters from artwork. The litho masters are big enough to fit a large magazine page or poster. The same lenses can be used for large-format photography. I have one that can fill an 11" by 14" film plate!
In summary: Look at the original application of the lens. If it was designed for an application that only had a small FOV, then it will not work very well with a much larger LCD. And NO, you won't find one that "just happens" to work well at the larger FOV.
Pretty simple: Every compound lens is designed for a particular application. The designer starts with the required focal length and performance over a particular Field Of View. Then he selects individual lens elements and spacing that meet those requirements.
For example, a telescope objective might require a focal length of 450 mm, and excellant performance over a FOV of just 2 degrees. (Only light that gets through the eyepiece matters.) So you can find a beautiful telescope objective that will be horrible as an LCD projection lens. The designer traded off the performance at 20 degrees for better performance in the central 2 degrees. Why not?
Or you can find an overhead projector lens that was designed to have a FOV that covers a 10.5" by 10.5" stage when the image is focussed at a 6 to 12 feet throw distance. That is a diagonal distance of 14.85", so it will work okay for a 15" LCD.
About the only old lenses you can buy that were designed for an FOV big enough for a 17" or larger LCD, are process lenses. These were designed for making lithography masters from artwork. The litho masters are big enough to fit a large magazine page or poster. The same lenses can be used for large-format photography. I have one that can fill an 11" by 14" film plate!
In summary: Look at the original application of the lens. If it was designed for an application that only had a small FOV, then it will not work very well with a much larger LCD. And NO, you won't find one that "just happens" to work well at the larger FOV.
$250 for a lens
Well, it's just like they say at the hot-rod shop:
"Speed costs money... How fast do you want to go?"
You can build a great projector with a 15" LCD and an OHP-type triplet. It just won't work for a long-throw application. Or you can use one of those triplets to make a long-throw projector, if you use a smaller LCD. You just won't get as good a picture because of the limited resolution and contrast of smaller LCDs.
But this is DIY! I encourage everybody to build a projector that fits their needs and budget. If you need to, you can scrounge an old OHP and an LCD monitor with a broken backlight, pull a power supply and fans out of some junk PCs, and build a "good enough" projector for $0! Don't scoff: It has been done and the results posted on this forum.
Well, it's just like they say at the hot-rod shop:
"Speed costs money... How fast do you want to go?"
You can build a great projector with a 15" LCD and an OHP-type triplet. It just won't work for a long-throw application. Or you can use one of those triplets to make a long-throw projector, if you use a smaller LCD. You just won't get as good a picture because of the limited resolution and contrast of smaller LCDs.
But this is DIY! I encourage everybody to build a projector that fits their needs and budget. If you need to, you can scrounge an old OHP and an LCD monitor with a broken backlight, pull a power supply and fans out of some junk PCs, and build a "good enough" projector for $0! Don't scoff: It has been done and the results posted on this forum.
pretty likely
If the projector you mentioned is a commercial product, then the lens will not have much more FOV than it needed for the 3.1" LCD.
If it was a DIY job, then who knows? The builder could have used a 480 mm process lens, which would also work fine for a 17" LCD. One way to tell is to look at the ratio of length to width of the lens. A long skinny lens will not have a very wide FOV.
The focal length and the FOV have a very limited relationship: The lens works well out to a particular angle from the central axis. This is called the field angle. The focal length affects how far the LCD will be from the lens. If it is far from the lens (long fl), then a small field angle will still give you a large FOV. If the LCD is close to the lens (short fl), then you will get a small FOV even if the field angle is large. The best example of this is a CRT projection lens. The focal length is so short that the LCD has to be right up against the lens. So the FOV is no wider than the lens itself.
If the projector you mentioned is a commercial product, then the lens will not have much more FOV than it needed for the 3.1" LCD.
If it was a DIY job, then who knows? The builder could have used a 480 mm process lens, which would also work fine for a 17" LCD. One way to tell is to look at the ratio of length to width of the lens. A long skinny lens will not have a very wide FOV.
The focal length and the FOV have a very limited relationship: The lens works well out to a particular angle from the central axis. This is called the field angle. The focal length affects how far the LCD will be from the lens. If it is far from the lens (long fl), then a small field angle will still give you a large FOV. If the LCD is close to the lens (short fl), then you will get a small FOV even if the field angle is large. The best example of this is a CRT projection lens. The focal length is so short that the LCD has to be right up against the lens. So the FOV is no wider than the lens itself.
I'm just wondering that we already have the 330mm FL Fresnel.
The optics shop can make any kind of triplet lens.
In terms of quality, why do we have to make a triplet to match the
new fresnel lens which is 550mm FL? Is there any other benefits aside
from long throw? Is the 330mm FL Fresnel is enough for the 17" with
a custom made triplet?
The optics shop can make any kind of triplet lens.
In terms of quality, why do we have to make a triplet to match the
new fresnel lens which is 550mm FL? Is there any other benefits aside
from long throw? Is the 330mm FL Fresnel is enough for the 17" with
a custom made triplet?
very short throw projector!
The 330 mm fl fresnel is just about perfect for a projector with a 300 mm fl triplet. The problem is that a projector using a 300 mm triplet and a 17" LCD will give you a 100" diagonal image with a throw distance of just 81 inches. (Much like an overhead projector: because they also use 300 mm projection lenses.) To view a 100" image, you would want to sit at least 100" back from the screen. 150" would be even better. So the %#$*& projector has to sit right in the middle of your viewing area!
People deal with this by sitting on either side of the projector (ie. just 81" from the screen, in this case), or they put the projector up near the ceiling and try to do keystone correction. Neither is a very good solution.
This is why people want to build long-throw projectors: Even with a 17" LCD, a 450 mm fl triplet will give you a 100" screen image from a 122" throw distance. So they can put the projector at the back of the room, just like a movie theatre.
Note that this is all dependant on the size of the LCD. If you use an LCD that is half the size, then you get an image that is also half the size. Or an image of the same size, but at twice the throw distance. So the 300 mm triplet & 330 mm fresnel are much better suited for projectors that use a smaller LCD.
The 330 mm fl fresnel is just about perfect for a projector with a 300 mm fl triplet. The problem is that a projector using a 300 mm triplet and a 17" LCD will give you a 100" diagonal image with a throw distance of just 81 inches. (Much like an overhead projector: because they also use 300 mm projection lenses.) To view a 100" image, you would want to sit at least 100" back from the screen. 150" would be even better. So the %#$*& projector has to sit right in the middle of your viewing area!
People deal with this by sitting on either side of the projector (ie. just 81" from the screen, in this case), or they put the projector up near the ceiling and try to do keystone correction. Neither is a very good solution.
This is why people want to build long-throw projectors: Even with a 17" LCD, a 450 mm fl triplet will give you a 100" screen image from a 122" throw distance. So they can put the projector at the back of the room, just like a movie theatre.
Note that this is all dependant on the size of the LCD. If you use an LCD that is half the size, then you get an image that is also half the size. Or an image of the same size, but at twice the throw distance. So the 300 mm triplet & 330 mm fresnel are much better suited for projectors that use a smaller LCD.
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