i thought it was close to 220 (only 8cm away). I didnt try it, but will do as soon as i can. Yes, the fresnells arent very good lens but if they somehow can be usable as projection lens, i guess using it as condenser is easier for the lens.
the only problem i see is that the effective angle from the light bulb is smaller since the lamp is farder. A propper condensor lens would fix this problem.
about the simetry of the proyection lens, triplets are not symetric so why would doublets be?
the only problem i see is that the effective angle from the light bulb is smaller since the lamp is farder. A propper condensor lens would fix this problem.
about the simetry of the proyection lens, triplets are not symetric so why would doublets be?
symmetry
Symmetrical douplets are symmetrical just because that's how they are built: Two identical positive meniscus or PCX lenses, one on each end of the tube. But you can build a duplet with non-identical lenses. I think the distortion may be higher. With the identical lens design, you can add an stop aperature to insure the image has even brightness. (This is more important for photography. With a properly converged condensor system, the stop doesn't really do much.))
Symmetrical douplets are symmetrical just because that's how they are built: Two identical positive meniscus or PCX lenses, one on each end of the tube. But you can build a duplet with non-identical lenses. I think the distortion may be higher. With the identical lens design, you can add an stop aperature to insure the image has even brightness. (This is more important for photography. With a properly converged condensor system, the stop doesn't really do much.))
OHP doublet
Did you actually measure the focal lengths of the two lenses from the OHP? There is no reason the second lens could not be a bit smaller diameter than the first, if that saved some money.
I think the individual lenses I used are well-centered, but I did not setup a laser on a precision testbench to measure them. The duplet I built with them works fine and the alignment is what I expected. It does certainly have some aberrations and distortion, but the total of all of those is much smaller than the pixel size in the screen image. (Verified by both OSLO and direct observation.) So all they do is make the screendoor lines have lower contrast. Which is pretty meaningless from more than two feet from the screen!
Triplets are terrific for DIY projection, and probably necessary for shorter focal lengths. I think they are overkill for cases where the focal length is over 500 mm.
Did you actually measure the focal lengths of the two lenses from the OHP? There is no reason the second lens could not be a bit smaller diameter than the first, if that saved some money.
I think the individual lenses I used are well-centered, but I did not setup a laser on a precision testbench to measure them. The duplet I built with them works fine and the alignment is what I expected. It does certainly have some aberrations and distortion, but the total of all of those is much smaller than the pixel size in the screen image. (Verified by both OSLO and direct observation.) So all they do is make the screendoor lines have lower contrast. Which is pretty meaningless from more than two feet from the screen!
Triplets are terrific for DIY projection, and probably necessary for shorter focal lengths. I think they are overkill for cases where the focal length is over 500 mm.
yes, i measured the focal of each lens and found one was larger than oher, like 6cm larger.
as you said, one is larger diameter than the other (one is square and the other circle). the lens facingo to the trasparencies is the shorter focal, the one facing the wall has longer (if it helps you).
when you say, you can move a litle the separation, you are doing a varifocus lens, could you tell me the separation range as well as focusing range? (if you do it so much you lose quality or what happens exaclty?
as you said, one is larger diameter than the other (one is square and the other circle). the lens facingo to the trasparencies is the shorter focal, the one facing the wall has longer (if it helps you).
when you say, you can move a litle the separation, you are doing a varifocus lens, could you tell me the separation range as well as focusing range? (if you do it so much you lose quality or what happens exaclty?
symmetric duplet
I just adjusted the spacing from 30 mm to 150 mm with the OSLO modelling program, to see the effect of spacing using available eyeglass lenses. In that range, the resulting spot size performance does not change by much. The focal lengths change by about 5%. Since the lenses I am trying to buy are available in 1/12th Diopter steps, this means that I could make 10 different focal length duplet projection lenses in the 500 mm to 800 mm fl range, just by using 5 different lens strengths and two different tube lengths:
508 mm fl 17" LCD from 10' throw = 85" image
541
553
586
608
640
674
706
758
789 mm fl 17" LCD from 18' throw = 101" image
I just adjusted the spacing from 30 mm to 150 mm with the OSLO modelling program, to see the effect of spacing using available eyeglass lenses. In that range, the resulting spot size performance does not change by much. The focal lengths change by about 5%. Since the lenses I am trying to buy are available in 1/12th Diopter steps, this means that I could make 10 different focal length duplet projection lenses in the 500 mm to 800 mm fl range, just by using 5 different lens strengths and two different tube lengths:
508 mm fl 17" LCD from 10' throw = 85" image
541
553
586
608
640
674
706
758
789 mm fl 17" LCD from 18' throw = 101" image
I have an existing DLP projector that I need to extend the throw on. Very expensive to just go out and buy one new, either a long throw projector or a lense to extend the throw of the projector I already have.
This is a little different from designing the whole projector optical system form scratch. I need to just about double the throw, the image is about twice as big on the screen diagonally as I would like it to be.
What kind of lenses would I need to come up with to accomplish that?
This is a little different from designing the whole projector optical system form scratch. I need to just about double the throw, the image is about twice as big on the screen diagonally as I would like it to be.
What kind of lenses would I need to come up with to accomplish that?
extending your throw
You need an achromat with a large negative focal length, and it has to be wide enough to not obstruct the cone of light coming out of your existing lens.
Achromat so you don't add a lot of chromatic aberration to your screen image. Negative focal length to make the effective focal length of your lens larger.
I just looked through the lens list at surplus shed. They do have achromats in the -1000 mm to -4000 mm fl range, but mostly they are small diameter (32-37 mm wide). If you supplied some parameters like the focal length of your existing lens, the throw and image size you have now, and the throw and image size you want, then we could calculate exactly the fl of the lens you need.
But keep in mind, the resulting image will not be quite as sharp as the one you have now.
You need an achromat with a large negative focal length, and it has to be wide enough to not obstruct the cone of light coming out of your existing lens.
Achromat so you don't add a lot of chromatic aberration to your screen image. Negative focal length to make the effective focal length of your lens larger.
I just looked through the lens list at surplus shed. They do have achromats in the -1000 mm to -4000 mm fl range, but mostly they are small diameter (32-37 mm wide). If you supplied some parameters like the focal length of your existing lens, the throw and image size you have now, and the throw and image size you want, then we could calculate exactly the fl of the lens you need.
But keep in mind, the resulting image will not be quite as sharp as the one you have now.
Re: extending your throw
By the way is an achromatic lense really important? In searching a couple of places today (Surplus Shed & Anchor Optics) I see there are lots of plain old double-convex lenses but very few achromatics ...
40 mm seems nowhere near big enough, I'm thinking I will need a lense around 75 mm or larger.
By the way is an achromatic lense really important? In searching a couple of places today (Surplus Shed & Anchor Optics) I see there are lots of plain old double-convex lenses but very few achromatics ...
40 mm seems nowhere near big enough, I'm thinking I will need a lense around 75 mm or larger.
Guy Grotke said:
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