I've been running an (admittedly cruddy) OHP based projector for several months. (A hideous cardboard and duct tape contraption.) Last week, I decided to give it a proper upgrade, remove unneeded parts, clean up the design, put it in a case, etc. Everything was going perfectly, i was getting a better image than the previous incarnation... until I shattered my Fresnel.
I'm trying to see this as an opportunity rather than a setback. My old Fresnel was warped and scratched, it was never perfectly in focus, and left me with several visual artifacts. I'd always planned to get a better one, but I'm sure that anyone reading this can identify with avoiding a new parts purchase for "just a few more weeks."
At last, this brings me to my question, focal lengths. My current main lens is a triplet from my old 3M OHP. Thus far it's done a great job. For the obvious reasons of cost, I'd like to keep it. But since I'm replacing the Fresnels anyways, I was thinking of swapping the second lens from a for one of the 550mm Fresnels. (Thus allowing me to push the projector further from the screen) I'm wondering about compatibility, will my current 3m lens work at 550? Do both lenses have to be swapped to get a longer throw, or can I do it piecemeal (one lens at a time)? What about brightness?
I'm also wondering if my information is wrong, a bunch of posts mention a 550mm lens from 3dlens.com, but the one on their site is listed as 5500mm(!), am I barking up the wrong tree? Is a 550mm Fresnel even available? Is this just a typo?
I imagine that there's quite a bit of n00b mixed into my questions. I've done a goodly bit of research before posting, but have some conflicting reports about the rear focal length of triplet lenses. A conclusive answer could save me from having to buy three Fresnels, so any help you could offer would be greatly appreciated.
I'm trying to see this as an opportunity rather than a setback. My old Fresnel was warped and scratched, it was never perfectly in focus, and left me with several visual artifacts. I'd always planned to get a better one, but I'm sure that anyone reading this can identify with avoiding a new parts purchase for "just a few more weeks."
At last, this brings me to my question, focal lengths. My current main lens is a triplet from my old 3M OHP. Thus far it's done a great job. For the obvious reasons of cost, I'd like to keep it. But since I'm replacing the Fresnels anyways, I was thinking of swapping the second lens from a for one of the 550mm Fresnels. (Thus allowing me to push the projector further from the screen) I'm wondering about compatibility, will my current 3m lens work at 550? Do both lenses have to be swapped to get a longer throw, or can I do it piecemeal (one lens at a time)? What about brightness?
I'm also wondering if my information is wrong, a bunch of posts mention a 550mm lens from 3dlens.com, but the one on their site is listed as 5500mm(!), am I barking up the wrong tree? Is a 550mm Fresnel even available? Is this just a typo?
I imagine that there's quite a bit of n00b mixed into my questions. I've done a goodly bit of research before posting, but have some conflicting reports about the rear focal length of triplet lenses. A conclusive answer could save me from having to buy three Fresnels, so any help you could offer would be greatly appreciated.
Well, you are on the right track, but have a little ways to go. Selecting the right focal length (FL) for your fresnel is a little more than just picking and choosing. Everything is connected to everything. These are how the components are related:
(Distance = Throw = Distance from Projection lens to Screen)
1 / (Projection Lens Focal Length) = 1 / (Distance from LCD to projection Lens) + 1 / (Throw)
In a split design the field fresnel is about 20mm in front of the LCD screen (between the LCD and projection lens). So whatever your "Distance from Projection Lens to Screen" is, subtract 20mm and that's the focal length of the field fresnel.
In an unsplit design (field fresnel goes behind the LCD), same procedure except you ADD 20mm and that's the focal length of the field fresnel.
So you need to find the FL of the projection lens, your desired throw (distance from projection lens to screen), and how big you want your screen to be. With these numbers, you can figure out what you're looking for. Good luck!
An externally hosted image should be here but it was not working when we last tested it.
(Distance = Throw = Distance from Projection lens to Screen)
1 / (Projection Lens Focal Length) = 1 / (Distance from LCD to projection Lens) + 1 / (Throw)
In a split design the field fresnel is about 20mm in front of the LCD screen (between the LCD and projection lens). So whatever your "Distance from Projection Lens to Screen" is, subtract 20mm and that's the focal length of the field fresnel.
In an unsplit design (field fresnel goes behind the LCD), same procedure except you ADD 20mm and that's the focal length of the field fresnel.
So you need to find the FL of the projection lens, your desired throw (distance from projection lens to screen), and how big you want your screen to be. With these numbers, you can figure out what you're looking for. Good luck!
Thanks for replying! 🙂
This information is extremely useful for calculating the final size of the screen, and is a much more detailed version of the formulas I found elsewhere on the forum. But while this does provide clear information about the final screen size, it doesn't answer my primary concern:
If I change the focal length of the field Fresnel, will my projection lens still be able to focus (provided that the projection lens is placed at the proper distance from the Fresnel)?
When I ran the other formula I found earlier, I determined that I wouldn't be able to achieve my desired throw with the 550mm lens alone. However, the distance is still better than with my previous (effectively 330mm) Fresnel. Eventually, I plan to replace my current projection lens, but I'd rather buy one part now and another later.
Once I have a chance to re-measure, I'll post the results.
This information is extremely useful for calculating the final size of the screen, and is a much more detailed version of the formulas I found elsewhere on the forum. But while this does provide clear information about the final screen size, it doesn't answer my primary concern:
If I change the focal length of the field Fresnel, will my projection lens still be able to focus (provided that the projection lens is placed at the proper distance from the Fresnel)?
When I ran the other formula I found earlier, I determined that I wouldn't be able to achieve my desired throw with the 550mm lens alone. However, the distance is still better than with my previous (effectively 330mm) Fresnel. Eventually, I plan to replace my current projection lens, but I'd rather buy one part now and another later.
Once I have a chance to re-measure, I'll post the results.
Actually, you can figure it out with these equations. Example:
Projection Lens FL = 457mm (18")
Throw = 100"
LCD Diag = 17"
So with these numbers I get a screen size:
(95 - 18) * (17) / (18) = 72.7"
1 / 18 = 1 / (Distance from LCD to projection lens) + 1 / 100
Distance from LCD to projection lens = 22.2" (564mm)
So this LCD has to be placed 22.2" from the projection lens. Therefore, if you use a split design (fresnel between the LCD and projection lens), then you need to place the fresnel around 15 - 20mm from the LCD (ie subtract 15 - 20mm from 564mm). A 550mm fresnel would work just fine for this. In an unsplit design the fresnel goes between the LCD and the light. Therefore you add 15 - 20mm to 564mm. A 580mm fresnel would work in this case.
Projection Lens FL = 457mm (18")
Throw = 100"
LCD Diag = 17"
So with these numbers I get a screen size:
(95 - 18) * (17) / (18) = 72.7"
1 / 18 = 1 / (Distance from LCD to projection lens) + 1 / 100
Distance from LCD to projection lens = 22.2" (564mm)
So this LCD has to be placed 22.2" from the projection lens. Therefore, if you use a split design (fresnel between the LCD and projection lens), then you need to place the fresnel around 15 - 20mm from the LCD (ie subtract 15 - 20mm from 564mm). A 550mm fresnel would work just fine for this. In an unsplit design the fresnel goes between the LCD and the light. Therefore you add 15 - 20mm to 564mm. A 580mm fresnel would work in this case.
the simple answer
superdaveeumo gave you all the equations you need to figure out fresnels and projection lens focal lengths, but he did not say what happens if you screw those up:
With badly mismatched fresnels & projection lens, you will still be able to focus an image on the screen. But if the fresnels are not sending most of the light into the projection lens, then you will get a very dim image!
You might be able to make some adjustments of the arc image location by fiddling with the lamp to condensor fresnel distance. But you will be much better off by starting with a matched set. Get a 220 & 330 mm fl set of fresnels to go with your 3M lens, or get 220 & 550 mm fl fresnels to go with a 450-480 mm fl projection lens.
BTW: The 3dlens.com fresnel is a 550 mm fl, and it is terrific for a long-throw projector. Don't pay for the quickest shipping. Mine took only a few days with the standard airmail shipping.
superdaveeumo gave you all the equations you need to figure out fresnels and projection lens focal lengths, but he did not say what happens if you screw those up:
With badly mismatched fresnels & projection lens, you will still be able to focus an image on the screen. But if the fresnels are not sending most of the light into the projection lens, then you will get a very dim image!
You might be able to make some adjustments of the arc image location by fiddling with the lamp to condensor fresnel distance. But you will be much better off by starting with a matched set. Get a 220 & 330 mm fl set of fresnels to go with your 3M lens, or get 220 & 550 mm fl fresnels to go with a 450-480 mm fl projection lens.
BTW: The 3dlens.com fresnel is a 550 mm fl, and it is terrific for a long-throw projector. Don't pay for the quickest shipping. Mine took only a few days with the standard airmail shipping.
Measurements, questions, guesses, and screwed up math.
Sorry for the long delay in reporting back, but I haven't had the time until now to properly measure everything. Having gone through all my math again, I'm faced with a few more questions.
First and foremost, does the FL in the above equations refer to the FL of the Projection Lens, or the FL of the field Fresnel?
I finally found the exact focal length of my Projection Lens, it's 330mm. Gods bless you Google, is there anything you can't find? Along with this information, I went back and got exact measurements of all the other variables.
Vital stats (all in cm):
Projection lens FL: 33
Field Fresnel FL: 33 or 55 (depending on which I buy)
Screen size (diag): 38.98 (39) (Standard 15 inch lcd)
Ideal Screen size (diag): 200
Ideal throw: 286 (it's a small room)
Distance from LCD to Projection Lens: 20-60
Depending on my understanding of the equations, plugging in these numbers yields two results:
330mm fresnel = 279 cm screen diag
550mm fresnel = 156 cm screen diag
In both cases, not what I want, though the 33mm result can be corrected by moving the projector forward. (As it is right now.)
I'm hung up on the lcd to lens equation right now, mostly because I'm tired. The whole projector was built with variablity in mind, so both lens and lamp can be adjusted simply by sliding their mounts. (That's why there's a range of 20 to 60 cm in the distance to the LCD.) I'm planning to keep the lenses unsplit, because the sharpness of the final image is my primary priority.
As it currently stands, neither lens is ideal. It's clear that my current projection lens isn't perfectly up to the task of a longer throw. I can't afford the Fresnels AND a projection lens at the moment, but any suggestions you could offer for my eventual purchase of a new projection lens would be appreciated. For now, I'll probably buy three lenses, thus giving me room to experiment, along with the opportunity for upgrading the projection lens later.
P.S. - Thank you both for all your help thus far. The numbers certainly don't come out how I like them, but better to be armed with the information than to make blind assumptions.
Sorry for the long delay in reporting back, but I haven't had the time until now to properly measure everything. Having gone through all my math again, I'm faced with a few more questions.
First and foremost, does the FL in the above equations refer to the FL of the Projection Lens, or the FL of the field Fresnel?
I finally found the exact focal length of my Projection Lens, it's 330mm. Gods bless you Google, is there anything you can't find? Along with this information, I went back and got exact measurements of all the other variables.
Vital stats (all in cm):
Projection lens FL: 33
Field Fresnel FL: 33 or 55 (depending on which I buy)
Screen size (diag): 38.98 (39) (Standard 15 inch lcd)
Ideal Screen size (diag): 200
Ideal throw: 286 (it's a small room)
Distance from LCD to Projection Lens: 20-60
Depending on my understanding of the equations, plugging in these numbers yields two results:
330mm fresnel = 279 cm screen diag
550mm fresnel = 156 cm screen diag
In both cases, not what I want, though the 33mm result can be corrected by moving the projector forward. (As it is right now.)
I'm hung up on the lcd to lens equation right now, mostly because I'm tired. The whole projector was built with variablity in mind, so both lens and lamp can be adjusted simply by sliding their mounts. (That's why there's a range of 20 to 60 cm in the distance to the LCD.) I'm planning to keep the lenses unsplit, because the sharpness of the final image is my primary priority.
As it currently stands, neither lens is ideal. It's clear that my current projection lens isn't perfectly up to the task of a longer throw. I can't afford the Fresnels AND a projection lens at the moment, but any suggestions you could offer for my eventual purchase of a new projection lens would be appreciated. For now, I'll probably buy three lenses, thus giving me room to experiment, along with the opportunity for upgrading the projection lens later.
P.S. - Thank you both for all your help thus far. The numbers certainly don't come out how I like them, but better to be armed with the information than to make blind assumptions.
330 or 550
Do not buy the 550 mm fresnel if you don't have a 450 mm or longer FL projection lens. the 3M lens you have is only 330 mm FL you have to go with 330 mm fresnel for a proper match or you are going to be loosing lots of light (picture very dim)
Do not buy the 550 mm fresnel if you don't have a 450 mm or longer FL projection lens. the 3M lens you have is only 330 mm FL you have to go with 330 mm fresnel for a proper match or you are going to be loosing lots of light (picture very dim)
long throw lens
If you are looking for a long throw projection lens, keep in mind opaque projector lenses. Lots of these are 18" fl (457 mm), and 5" in diameter. They work very well with the 550 mm fl fresnel and a 15" LCD. You can look for old opaque projectors at office equipment surplus places, school district junk sales, yard sales, etc. I have seen one of these lenses go for as little as $12 on eBay! The highest I have seen was about $65. Much cheaper than buying a long-throw lens from an online DIY projector store.
Don't buy an artist's opaque projector on eBay: These are only good for LCDs around 7" diagonal. You need the lens from a full-page (8.5" by 11") opaque projector.
If you are looking for a long throw projection lens, keep in mind opaque projector lenses. Lots of these are 18" fl (457 mm), and 5" in diameter. They work very well with the 550 mm fl fresnel and a 15" LCD. You can look for old opaque projectors at office equipment surplus places, school district junk sales, yard sales, etc. I have seen one of these lenses go for as little as $12 on eBay! The highest I have seen was about $65. Much cheaper than buying a long-throw lens from an online DIY projector store.
Don't buy an artist's opaque projector on eBay: These are only good for LCDs around 7" diagonal. You need the lens from a full-page (8.5" by 11") opaque projector.
Thank you both for your patience and attention. I ordered the lenses (220 and 330) over the weekend, and am now awaiting their arrival. I've decided to hold off on the 550 and the Opaque projector lens for now, rather than biting off more than I can chew.
In the coming months, I may purchase that particular upgrade. As is often the case with DIY projects, there's always something worth improving. For now, I'm content to build a stand that lowers the projector below seated eye level, and work on modifying my screen to compensate for keystoning. I'll be sure to post results when I eventually have them.
In the coming months, I may purchase that particular upgrade. As is often the case with DIY projects, there's always something worth improving. For now, I'm content to build a stand that lowers the projector below seated eye level, and work on modifying my screen to compensate for keystoning. I'll be sure to post results when I eventually have them.
- Status
- Not open for further replies.