Will this lense work? My fresnels that I have now are 330mm focal, and I need a lense. I can get this one for $40. Is this usable? My monitor is a 15" 150mp. I am looking for a 100" screen.
"BUHL OPTICAL CO. long throw LCD projector lens. 15.5" F:4.2 lens for Eiki Video Projector 153-L155. Originally used on an EIKI LC330, it also fits their LC300/LC350 models, and probably others."
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An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
"BUHL OPTICAL CO. long throw LCD projector lens. 15.5" F:4.2 lens for Eiki Video Projector 153-L155. Originally used on an EIKI LC330, it also fits their LC300/LC350 models, and probably others."
way too long
No way would that 100 mm diameter lens cover a 15" LCD. Just the tube alone would severely restrict the Field Of View. Look for a lens that is 80 mm diameter or higher, and not much longer than it is in diameter.
18" fl opaque projector lenses work pretty well with a 15" LCD: They are mostly 5.5" diameter and 6" long.
No way would that 100 mm diameter lens cover a 15" LCD. Just the tube alone would severely restrict the Field Of View. Look for a lens that is 80 mm diameter or higher, and not much longer than it is in diameter.
18" fl opaque projector lenses work pretty well with a 15" LCD: They are mostly 5.5" diameter and 6" long.
Hey Guy,
I am using a 15.4" Sharp WUXGA and trying to figure out the best lens for my setup. I am torm between a opaque pj lens like diyprojectorcompany.com's or LL's new pro triplet. Would either have an advantage? Both of these lenses are long throw, but I would like a shorter throw typical to standard fresnel/triplet set-ups. The opaque and LL pro triplet both have like a 20ft throw right?
I am using a 15.4" Sharp WUXGA and trying to figure out the best lens for my setup. I am torm between a opaque pj lens like diyprojectorcompany.com's or LL's new pro triplet. Would either have an advantage? Both of these lenses are long throw, but I would like a shorter throw typical to standard fresnel/triplet set-ups. The opaque and LL pro triplet both have like a 20ft throw right?
throw distance
Any lens can work at any throw distance. If you use a 15.4" lcd, you get a 100" diagonal image from:
LENS................THROW_DISTANCE
300 mm..............88.5"
18".................135"
500 mm..............147"
22".................165"
I like a longer throw lens (ie. 22") because I can put my projector at the back of the room. If you want a shorter throw distance, then save your money and go with the standard 300 mm triplet.
Any lens can work at any throw distance. If you use a 15.4" lcd, you get a 100" diagonal image from:
LENS................THROW_DISTANCE
300 mm..............88.5"
18".................135"
500 mm..............147"
22".................165"
I like a longer throw lens (ie. 22") because I can put my projector at the back of the room. If you want a shorter throw distance, then save your money and go with the standard 300 mm triplet.
I guess my main concern is edge to edge and corner to corner focus. I can't get that with standard triplets so it seems my only viable options are the LL or DIYptojco type lenses right?
I should always try to match the FL of the fresnel to the triplet as close as possible correct? i.e. putting a 500mm triplet with a 330mm fresnel is bad news and would need funky tinkering to get it to work, right?
I should always try to match the FL of the fresnel to the triplet as close as possible correct? i.e. putting a 500mm triplet with a 330mm fresnel is bad news and would need funky tinkering to get it to work, right?
lenses & fresnels
I think you can get pretty good corner to corner focus with the standard 80 mm diameter 300 mm triplet. Any of these lenses will work fine with a 15.4" LCD. 17" LCDs are another story.
A field fresnel (second fresnel) that matches the LCD-to-lens distance is ideal. That distance is a product of the lens focal length and the throw distance:
1/fl = 1/throwDistance + 1/lcdToLensDistance
With a 300 mm fl lens, a 330 mm fl fresnel works well.
With an 18" fl lens, a 550 mm fl fresnel works well.
With a 500 mm lens, a 600 mm fl fresnel should work well.
With a 22" lens, a 650 mm fl fresnel should work well.
You can fiddle with the lamp-to-condensor-fresnel distance to adjust the distance of the arc image, in case your field fresnel is not the right length. I use a 550 mm fresnel with my 22" lens. Not ideal, but it works. I might get more light through my condensor fresnel if I used a 650 mm field fresnel, and put the lamp at the condensor fresnel's focal length.
I think you can get pretty good corner to corner focus with the standard 80 mm diameter 300 mm triplet. Any of these lenses will work fine with a 15.4" LCD. 17" LCDs are another story.
A field fresnel (second fresnel) that matches the LCD-to-lens distance is ideal. That distance is a product of the lens focal length and the throw distance:
1/fl = 1/throwDistance + 1/lcdToLensDistance
With a 300 mm fl lens, a 330 mm fl fresnel works well.
With an 18" fl lens, a 550 mm fl fresnel works well.
With a 500 mm lens, a 600 mm fl fresnel should work well.
With a 22" lens, a 650 mm fl fresnel should work well.
You can fiddle with the lamp-to-condensor-fresnel distance to adjust the distance of the arc image, in case your field fresnel is not the right length. I use a 550 mm fresnel with my 22" lens. Not ideal, but it works. I might get more light through my condensor fresnel if I used a 650 mm field fresnel, and put the lamp at the condensor fresnel's focal length.
I've made 3 standard 15" PJ's and the corner focusing is less than OK (in my opinion of course), especially making it a touch worse with field fresnel keystone adjustment.
I saw test results of the new LL triplet on a 17" and the corner focus is as good as center focus.
The opaque triplets are designed typically for 11" square stage's so I am nervous about using it for a 15.4" LCD. Looking at it with my simple knowledge of optics I was guess the new LL worked well for even a 17" because it is a wider 100mm diameter. With that in mind i would think a 135mm opaque type lens would be therefore even better. I was told that the opaque type lense has a smaller aperature and wouldn't be as good as the new 100mm LL triplet. Is that B.S. or is there another design aspect for projection triplets that allow for different aperatures or "viewable stage areas" (maybe i am an optical moron and the two are not related).
BTW thanks for all your help and advice to me and many others!
I saw test results of the new LL triplet on a 17" and the corner focus is as good as center focus.
The opaque triplets are designed typically for 11" square stage's so I am nervous about using it for a 15.4" LCD. Looking at it with my simple knowledge of optics I was guess the new LL worked well for even a 17" because it is a wider 100mm diameter. With that in mind i would think a 135mm opaque type lens would be therefore even better. I was told that the opaque type lense has a smaller aperature and wouldn't be as good as the new 100mm LL triplet. Is that B.S. or is there another design aspect for projection triplets that allow for different aperatures or "viewable stage areas" (maybe i am an optical moron and the two are not related).
BTW thanks for all your help and advice to me and many others!
Field Of View
There are multiple factors working here. If a projection lens is long and skinny, then it will have a smaller FOV than a lens with the same focal length but a short and fat design. The light from the corners of a big LCD just can't get through a skinny mounting tube.
But even with the very same shape projection lenses, the designer tries to optimize the lens corrections for a particular size object. It would make little sense to design an opaque projector lens that had perfect flat focus corrections out to 17", if the opaque projector would only hold a 12" diagonal document. If we find that we can use that lens with a 17" LCD, then we just got lucky. The designer probably sacrificed the performance beyond 12" to optimize it within the 12" design target.
The FOV does not correlate directly with the lens diameter. The new LL lens will work perfectly with a 17" LCD because it was designed just for that.
On the other hand, maybe an opaque projector lens designed for an 11" by 11" square stage is not so bad for your purpose: That has a diagonal of 15.556"! So any such lens should work fine with a 15" or 15.4" LCD.
I have used a 600 mm process lens, an 18" opaque projector lens, and a 22" opaque projector lens with my 15" LCD. All gave me screen images where I could at least see individual red, green, and blue subpixels sharp in the center and I could still make them out in the corners. The process lens is so perfect that the screendoor is sharp and very black over the whole screen. (But that is a $2000+ lens!) The opaque projector lenses show screendoor that is a bit gray because they are not corrected as well. But I prefer them because they are large enough to put a lot more light on the screen.
I am sure you would be very happy with the image from the new custom-designed triplets. But you would also be happy with the results from an opaque projector lens. (About $75 on eBay.) Just make sure you get a matching field fresnel.
Try to avoid optical keystone correction if you want a sharp screen image.
There are multiple factors working here. If a projection lens is long and skinny, then it will have a smaller FOV than a lens with the same focal length but a short and fat design. The light from the corners of a big LCD just can't get through a skinny mounting tube.
But even with the very same shape projection lenses, the designer tries to optimize the lens corrections for a particular size object. It would make little sense to design an opaque projector lens that had perfect flat focus corrections out to 17", if the opaque projector would only hold a 12" diagonal document. If we find that we can use that lens with a 17" LCD, then we just got lucky. The designer probably sacrificed the performance beyond 12" to optimize it within the 12" design target.
The FOV does not correlate directly with the lens diameter. The new LL lens will work perfectly with a 17" LCD because it was designed just for that.
On the other hand, maybe an opaque projector lens designed for an 11" by 11" square stage is not so bad for your purpose: That has a diagonal of 15.556"! So any such lens should work fine with a 15" or 15.4" LCD.
I have used a 600 mm process lens, an 18" opaque projector lens, and a 22" opaque projector lens with my 15" LCD. All gave me screen images where I could at least see individual red, green, and blue subpixels sharp in the center and I could still make them out in the corners. The process lens is so perfect that the screendoor is sharp and very black over the whole screen. (But that is a $2000+ lens!) The opaque projector lenses show screendoor that is a bit gray because they are not corrected as well. But I prefer them because they are large enough to put a lot more light on the screen.
I am sure you would be very happy with the image from the new custom-designed triplets. But you would also be happy with the results from an opaque projector lens. (About $75 on eBay.) Just make sure you get a matching field fresnel.
Try to avoid optical keystone correction if you want a sharp screen image.
Thanks. I ended up using a OHP lense from the 3m 1800 series. It has a doublet entrance lens and singlet exit lens. It works perfect... Edge to edge perfect focus.
Check this thread:
http://www.diyaudio.com/forums/showthread.php?s=&postid=706538&highlight=#post706538
This lens has been discussed.
http://www.diyaudio.com/forums/showthread.php?s=&postid=706538&highlight=#post706538
This lens has been discussed.
longthrow DIY PJ
Hi Guy,
do you still use your longthrow DIY PJ?
How's the projection images? do you get bright, sharp and vivid color of the images?
I ask you this because my PJ is similar as your's, but until now I can't find cheap long FL lens.
Now I use combination of 22" FL lens and -160" (-0,25d) FL eyeglass lens to get 26,5" FL.
The result is not quite sharp, the image smears, the screen door look's light grey.
Can you inform me what lens do you use and where to buy it?
FIY, my lens to screen distance is 18 ft, I use 15" LCD, screen diagonal size is 100" (4:3).
Can I use field fresnel with 550mm fl from LumenLab? Do you use this fresnel?
Do you have website about your DIY PJ in detail?
Thankyou.
Gunawan.
Hi Guy,
do you still use your longthrow DIY PJ?
How's the projection images? do you get bright, sharp and vivid color of the images?
I ask you this because my PJ is similar as your's, but until now I can't find cheap long FL lens.
Now I use combination of 22" FL lens and -160" (-0,25d) FL eyeglass lens to get 26,5" FL.
The result is not quite sharp, the image smears, the screen door look's light grey.
Can you inform me what lens do you use and where to buy it?
FIY, my lens to screen distance is 18 ft, I use 15" LCD, screen diagonal size is 100" (4:3).
Can I use field fresnel with 550mm fl from LumenLab? Do you use this fresnel?
Do you have website about your DIY PJ in detail?
Thankyou.
Gunawan.
still using it
Hello Gunawan,
Yes, I am still using a long-throw lens. It is a 22" fl opaque projector lens that has lens elements about 4.75" in diameter. I use a 15" LCD, so from about 12.5 feet I get a 95" diagonal image. Since I removed the antiglare layer from the front of my LCD, I see very bright images. The lens is not perfect, so my screendoor is a bit gray instead of black. I do see the red, green, and blue subpixels from the center of the screen out to the edges but they are blurred a bit in the corners. I watch from about 10 feet away, so I can't see any defects in the image.
I use the 550 mm fl fresnel from 3dlens.com as the field fresnel and a 220 mm fl fresnel as the condensor fresnel. I have the lamp a bit closer than 220 mm from the condensor fresnel so I get the arc image focussed right at the center of my projection lens. That gets a lot of light to the screen.
Sorry, I don't have a website but I have posted a lot of pictures of different parts of my projector in various forum threads.
I bought my 22" lens from a man who had several surplus opaque projectors for sale. He no longer has them, but I see old opaque projectors and opaque projector lenses on eBay quite often. There is also a thread on this forum about 22" opaque projector lenses, and some people there have sources. These were commonly used by schools and businesses in the US about 10 to 40 years ago, so old machines sometimes reach the surplus market. If you do find one of these, ask the seller if he will ship you just the lens. The machines are very heavy. For your projector, I don't think one of these lenses would help much. I have never seen them with a fl longer than 22".
Is your image sharp without the eyeglass lens? Maybe you could find a better -4000 mm lens extender than your eyeglass lens. Like an achromatic doublet.
With an 18 foot throw distance, it sound like you are running a theatre! Maybe you could just move your projector closer to the screen by hanging it upside down from the ceiling. 😀
Hello Gunawan,
Yes, I am still using a long-throw lens. It is a 22" fl opaque projector lens that has lens elements about 4.75" in diameter. I use a 15" LCD, so from about 12.5 feet I get a 95" diagonal image. Since I removed the antiglare layer from the front of my LCD, I see very bright images. The lens is not perfect, so my screendoor is a bit gray instead of black. I do see the red, green, and blue subpixels from the center of the screen out to the edges but they are blurred a bit in the corners. I watch from about 10 feet away, so I can't see any defects in the image.
I use the 550 mm fl fresnel from 3dlens.com as the field fresnel and a 220 mm fl fresnel as the condensor fresnel. I have the lamp a bit closer than 220 mm from the condensor fresnel so I get the arc image focussed right at the center of my projection lens. That gets a lot of light to the screen.
Sorry, I don't have a website but I have posted a lot of pictures of different parts of my projector in various forum threads.
I bought my 22" lens from a man who had several surplus opaque projectors for sale. He no longer has them, but I see old opaque projectors and opaque projector lenses on eBay quite often. There is also a thread on this forum about 22" opaque projector lenses, and some people there have sources. These were commonly used by schools and businesses in the US about 10 to 40 years ago, so old machines sometimes reach the surplus market. If you do find one of these, ask the seller if he will ship you just the lens. The machines are very heavy. For your projector, I don't think one of these lenses would help much. I have never seen them with a fl longer than 22".
Is your image sharp without the eyeglass lens? Maybe you could find a better -4000 mm lens extender than your eyeglass lens. Like an achromatic doublet.
With an 18 foot throw distance, it sound like you are running a theatre! Maybe you could just move your projector closer to the screen by hanging it upside down from the ceiling. 😀
Hi Guy,
thank's for your reply, wish I can put the PJ closer to the screen, so I can use normal triplets, but my wife disagree.
Actually I already watch DVD with this PJ since 2003, 3 years from now! what a time passing by... and I enjoyed it much...
The main problem is I can't get very sharp image, for example if I projecting a white dot 3cm diameter with black hairline bordering on black background, it has white smearing around 1cm wide outside the dot.
Any comment what causing this problem?
Is this because my -4000mm lens not good? do you know where I can find a better lens?
Thank's.
thank's for your reply, wish I can put the PJ closer to the screen, so I can use normal triplets, but my wife disagree.
Actually I already watch DVD with this PJ since 2003, 3 years from now! what a time passing by... and I enjoyed it much...
The main problem is I can't get very sharp image, for example if I projecting a white dot 3cm diameter with black hairline bordering on black background, it has white smearing around 1cm wide outside the dot.
Any comment what causing this problem?
Is this because my -4000mm lens not good? do you know where I can find a better lens?
Thank's.
smearing
I think this a problem with your -4000 mm lens. A simple spherical lens has inherent distortions and aberrations. There are a number of different solutions.
1. Instead of extending your 22" triplet, find a triplet or higher-element lens that has the focal length you need. That would have been designed to correct most of the distortions and aberrations. You might be able to find a large process lens with a 650 mm fl on eBay. They were very expensive lenses used in printing, but are mostly replaced now by digital processes so they sell for much less now. A process lens would give you an excellant image in perfect focus, but it is difficult to get all the light through the lens because they are not very large diameter.
2. Use a better extender lens with your 22" triplet. A combination of a positive lens and a negative lens of another material can be designed to reduce the chromatic aberration or the spherical aberration, but not both. Your problem is probably spherical aberration, since you do not mention red and blue colored fringes. The best prospect would be a wide angle adapter for a large camera lens. But i don't think anybody has made one with such a small change in the Field Of View.
3. You could try building a duplet from two plano-convex or positive meniscus lenses with focal lengths of around 1333 mm (0.75 Diopter). They would have to be anti-reflection coated. This would be an experiment. It might have worse distortion than what you have now, but this is a type of lens that was used in many overhead projectors. If you have access to eyeglass lenses, then you could also try this with aspheric lenses to see if they work better.
I think this a problem with your -4000 mm lens. A simple spherical lens has inherent distortions and aberrations. There are a number of different solutions.
1. Instead of extending your 22" triplet, find a triplet or higher-element lens that has the focal length you need. That would have been designed to correct most of the distortions and aberrations. You might be able to find a large process lens with a 650 mm fl on eBay. They were very expensive lenses used in printing, but are mostly replaced now by digital processes so they sell for much less now. A process lens would give you an excellant image in perfect focus, but it is difficult to get all the light through the lens because they are not very large diameter.
2. Use a better extender lens with your 22" triplet. A combination of a positive lens and a negative lens of another material can be designed to reduce the chromatic aberration or the spherical aberration, but not both. Your problem is probably spherical aberration, since you do not mention red and blue colored fringes. The best prospect would be a wide angle adapter for a large camera lens. But i don't think anybody has made one with such a small change in the Field Of View.
3. You could try building a duplet from two plano-convex or positive meniscus lenses with focal lengths of around 1333 mm (0.75 Diopter). They would have to be anti-reflection coated. This would be an experiment. It might have worse distortion than what you have now, but this is a type of lens that was used in many overhead projectors. If you have access to eyeglass lenses, then you could also try this with aspheric lenses to see if they work better.
Hi Guy,
wish I can find 650mm fl, it will solve my problem.
there is very little red and blue fringes, it's white smearing, so it should be spherical aberration, right?
I already try with single eyeglass lens with fl 666mm (1,5D), the projection image sharp in the middle only, all side was blurry, is it because of the eyeglass lens shape (meniscus)?
What about duplet as your suggestion above? What is the best position: () or (( or )( ?
What is aspheric lens? does eyeglass has this type of lens?
Another problem, my field fresnel only has fl around 550mm, shorter than proj lens fl, is this the culprit?
wish I can find 650mm fl, it will solve my problem.
there is very little red and blue fringes, it's white smearing, so it should be spherical aberration, right?
I already try with single eyeglass lens with fl 666mm (1,5D), the projection image sharp in the middle only, all side was blurry, is it because of the eyeglass lens shape (meniscus)?
What about duplet as your suggestion above? What is the best position: () or (( or )( ?
What is aspheric lens? does eyeglass has this type of lens?
Another problem, my field fresnel only has fl around 550mm, shorter than proj lens fl, is this the culprit?
Yes, that is mostly spherical aberration. Aspheric lenses are designed to have less of that, by not using a spherical section for the curve. You can get aspheric eyeglass lenses, but they cost more than the standard spheric-section lenses.
The meniscus shape can have less spherical aberration than a double convex shape, if you pick the right radius for the lens material. But the inner surface of all eyeglass lenses has exactly the same curve radius, and that radius is not ideal.
The best orientation for a symmetric duplet is: ( )
You can try adding an aperature in the middle like this:
./....|....\
(...........)
.\....|..../
This will equalize the amount of light that gets through the lens from the middle and the corners of the LCD, but it will do that by limiting the brightness of the center. It is worth an experiment or two.
The short focal length field fresnel should only affect the amount of light you get to the lens. It will not affect the focus of your image, unless you are using a split fresnel design. Even in a split fresnel design, as long as it is only 10-20 mm from the LCD it should not affect the screen image focus.
I use one of those 550 mm fl fresnels with my 22" lens. I get the arc image to focus inside the projection lens by decreasing the distance between the lamp arc and the condensor fresnel. You should do the same.
The meniscus shape can have less spherical aberration than a double convex shape, if you pick the right radius for the lens material. But the inner surface of all eyeglass lenses has exactly the same curve radius, and that radius is not ideal.
The best orientation for a symmetric duplet is: ( )
You can try adding an aperature in the middle like this:
./....|....\
(...........)
.\....|..../
This will equalize the amount of light that gets through the lens from the middle and the corners of the LCD, but it will do that by limiting the brightness of the center. It is worth an experiment or two.
The short focal length field fresnel should only affect the amount of light you get to the lens. It will not affect the focus of your image, unless you are using a split fresnel design. Even in a split fresnel design, as long as it is only 10-20 mm from the LCD it should not affect the screen image focus.
I use one of those 550 mm fl fresnels with my 22" lens. I get the arc image to focus inside the projection lens by decreasing the distance between the lamp arc and the condensor fresnel. You should do the same.
symmetrical duplet
Hi Guy,
thank's for your advice, last night I made a test with white dot again, I got less smearing effect if I projected it right at the middle of the screen and became wider if I moved it toward the side of the screen. Is this spherical abberation problem?
With aspherical lens, do I just need to buy 1 lens with 1,5d (650mm FL) to get the best result?
Do you know what to say aspherical lens in "optic shop" language? or just the same as we call it?
I think, I will try the symmetrical duplet using 2 raw eyeglasses, it's interesting and cheap to buy the lenses.
For this configuration, it's no need to use aspherical lenses, right?
About aperture, if my lenses has 6cm diameter, how big is the aperture diameter to get the best result?
What happen if the arc image doesn't focus inside the projection lenses? In my case it fall 2-3cm before the proj. lenses.
I can't focus it to the lens because my field fresnel only has 450mm fl, if I push the arc image into the proj. lens, I get a hotspot/uneven brightnes on the screen.
Is this the condition that create smearing effect?
see you.
Hi Guy,
thank's for your advice, last night I made a test with white dot again, I got less smearing effect if I projected it right at the middle of the screen and became wider if I moved it toward the side of the screen. Is this spherical abberation problem?
With aspherical lens, do I just need to buy 1 lens with 1,5d (650mm FL) to get the best result?
Do you know what to say aspherical lens in "optic shop" language? or just the same as we call it?
I think, I will try the symmetrical duplet using 2 raw eyeglasses, it's interesting and cheap to buy the lenses.
For this configuration, it's no need to use aspherical lenses, right?
About aperture, if my lenses has 6cm diameter, how big is the aperture diameter to get the best result?
What happen if the arc image doesn't focus inside the projection lenses? In my case it fall 2-3cm before the proj. lenses.
I can't focus it to the lens because my field fresnel only has 450mm fl, if I push the arc image into the proj. lens, I get a hotspot/uneven brightnes on the screen.
Is this the condition that create smearing effect?
see you.
lots of questions!
If you are projecting those white dots from your LCD onto a flat screen, then you are seeing the effect of not having flat-field correction. To test for spherical aberration, you need a single point-source light that sends rays to the entire lens surface. If the image dot gets sharp when you block the rays around the edges, and fuzzy when you use the whole lens, that demonstrates spherical aberration.
Standard lenses are ground with a spherical section, just because that is easy to do. But that shape makes an imperfect lens. Parallel rays passing through the lens near the edge do not focus in the same point as rays passing through the center.
Lenses can be ground with an aspheric shape, to get rid of almost all the spherical aberration. I think you can just ask for eyeglass lenses in the usual manner (ie. so many Diopters) but in an aspheric shape. These have to be molded or ground with special computer-controlled machines, so they will be harder to find and much more expensive than spheric lenses. But that still won't fix the flat-field correction.
I would try the duplet with normal lenses first. If you can find 0.75 D aspheric lenses for not too much money, then you could try making a duplet with them. But I think maybe the aspheric lenses would cost too much.
I don't recommend you use an aperature. If you adjust the fresnels right, they give you the same effect with no loss of light. An aperature would be more important if you were using the lens for photography, where you can sacrifice light for even exposure.
If you can't get the arc image into the lens, then some of your light will not get through the lens. Maybe the distance you need is just too far for the fresnels you have. I think lumenlab.com has some longer fl fresnels. I have one of their 790 mm fl fresnels, but I think their new ones are 650 mm fl. But even if you don't have the arc image in the best location, that should not affect the focus of your screen image. Just the evenness.
If you are projecting those white dots from your LCD onto a flat screen, then you are seeing the effect of not having flat-field correction. To test for spherical aberration, you need a single point-source light that sends rays to the entire lens surface. If the image dot gets sharp when you block the rays around the edges, and fuzzy when you use the whole lens, that demonstrates spherical aberration.
Standard lenses are ground with a spherical section, just because that is easy to do. But that shape makes an imperfect lens. Parallel rays passing through the lens near the edge do not focus in the same point as rays passing through the center.
Lenses can be ground with an aspheric shape, to get rid of almost all the spherical aberration. I think you can just ask for eyeglass lenses in the usual manner (ie. so many Diopters) but in an aspheric shape. These have to be molded or ground with special computer-controlled machines, so they will be harder to find and much more expensive than spheric lenses. But that still won't fix the flat-field correction.
I would try the duplet with normal lenses first. If you can find 0.75 D aspheric lenses for not too much money, then you could try making a duplet with them. But I think maybe the aspheric lenses would cost too much.
I don't recommend you use an aperature. If you adjust the fresnels right, they give you the same effect with no loss of light. An aperature would be more important if you were using the lens for photography, where you can sacrifice light for even exposure.
If you can't get the arc image into the lens, then some of your light will not get through the lens. Maybe the distance you need is just too far for the fresnels you have. I think lumenlab.com has some longer fl fresnels. I have one of their 790 mm fl fresnels, but I think their new ones are 650 mm fl. But even if you don't have the arc image in the best location, that should not affect the focus of your screen image. Just the evenness.
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