i need your guys help with this lens!

18" focal length

No, that means you could focus a spot of sunlight 18" from the lens. Or focus an LCD 18" from the lens on the surface of the moon! A real screen will have to be more like 12 feet away. Then the simplified equation is:

1/fl = 1/LCD to lens + 1/lens to screen

So:

1/18" = 1/LCD to lens + 1/144"

LCD to lens = 20.57"

You can try plugging in other screen distances to calculate the LCD to lens distances. Other useful equations:

Magnification (M) = (lens to screen) / (LCD to lens)

Screen image size = M * LCD size
 
Mikey p: you got ripped off

Sounds like you had a good case for eBay buyer protection. 18" versus 12" is a pretty big misrepresentation.

But if you look at the other photos of corryn00's lens, the 18" focal length is printed right on the front retainer ring. I think it will work fine as a long-throw projection lens. There are just two unknowns:

1. How wide the field of view will be. If it was from an opaque projector, then it should be able to handle an 8.5" by 11" object. A 15" 3:4 LCD is 9" by 12", so it might work okay for that size LCD. For anything smaller, it should be fine.

2. Is it color-corrected (chromatic aberration)? Some early projection lenses were not, but it should not matter much for an LCD projector. It just would not be as sharp around the edges as one that is corrected.
 
lens auction finished

I see that the big lens sold for $44 US (54.08 C). This is right around where I have seen several opaque projector lenses sell recently on eBay.

The nice thing about this auction was that they posted the focal length. Most of the others did not.

If somebody reading this thread won it, please let us know how well it works in your projector.
 
18"lens

I just received my opaque projector. This thing is huge. The lens is clearly marked 18" EFL. it's about 5 in diameter. I powered up the projector, after the big fan has blown a tone of dust I got a pretty clear and colorful image. tomorow I'll be taking it aparts to check how it works. I'll keep you posted.;)
 
opaque projector lens

I have one of those eBay 18" lenses on the way, too. I am really interested to see if these lenses are corrected for chromatic aberration. The easiest way to tell is to project an image of something with a black background and some white text around the edges. If the letters have multi-colored fringes, then it is not color-corrected. If they look sharp, then it is.

I think opaque projectors are pretty simple: They just throw as much light as possible on the object surface. That means a high-wattage lamp, fan, and reflectors. The diffuse light from the object is usually bounced off a front-surface mirror, so the object can lay horizontally in the tray. Then the light goes through a great big lens to get the image focussed on the screen. The lens has to have a large diameter, since the light from the object goes in all directions. Only a small part of that light gets to the lens, so the projected image is not very bright.

I think you could remove (or just ignore) the lamp & fan, open up the bottom so you could put an LCD exactly where the copy tray is, and then add a MH lamp, reflector, fan, and fresnels under the LCD. It would not be difficult to make a nice looking box for the stuff under the opaque projector. I think it could look very professional. The only problem might be that the tray area, mirror, and lens FOV might not be quite large enough for a 15" LCD. You might be limited to a 14" or smaller. Actually, an 8" Hami, or a 7" Lilliput would work fine, to give you a long-throw projector you could put at the back of the room.
 
I am really interested to see if these lenses are corrected for chromatic aberration. The easiest way to tell is to project an image of something with a black background and some white text around the edges.

Yes , the lens are corrected. I have 2 18"EF Opaque projector lenses , they have about 10 years of difference in age, did the test with them with very good results.

The only problem might be that the tray area, mirror, and lens FOV might not be quite large enough for a 15" LCD. You might be limited to a 14" or smaller. Actually, an 8" Hami, or a 7" Lilliput would work fine, to give you a long-throw projector you could put at the back of the room.

As I already said here in the forum , the lens have a enough fov to work even with a source of about 20". But the mirror that´s inside the projector is very small , I´m sure that it will not work with big lcd displays.
 
18'' fl lens

Bad news for me guys, I took my recently acquired opaque projector apart and found out that the triplet lens has some spots that looks like fungal growth. So I decided to take the lenses apart and try to clean them. In the process of dismantling it, I chipped the edge of the first PCX. After many curses and hair pulling. I continued and tried to gently clean the lenses with dish soap and a soft sponge. After drying them I found out that when I put the projection side PCX lens to the light I can see like a thin crackle effect on the curved side of the lens. My guess is the coating layer of the lens some how with age deteriorated. The projector is an old one (1957). Does anybody knows how to get rid of the coating altogether. Then polishing it to a smooth clear surface?!
 
You're talking about trying to salvage a 48 year old lens. That's a delicate situation. If I were you I'd be looking up "optical" in the yellow pages and explaining the situation to a professional to see what the advice might be. Better to hear from someone who knows then from a forum where you don't know if it is sound advice or just somebodys best guess. No sense taking a chance on wrecking a lens that you really want.
 
18'' lens

Thanks for the advice of consulting a professional. It's just seems to me unlikely that professionals would care about like you put it a 48 years old lens! I know this is a long shot but I've read somewhere in the forum that it is possible to recoat lenses. So if someone knows a company that would do this (on a diyer budget) that’ll be great. Thanks again.
 

Rox

Member
2004-07-25 10:06 pm
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supahcool said:


Yes , the lens are corrected. I have 2 18"EF Opaque projector lenses , they have about 10 years of difference in age, did the test with them with very good results.



As I already said here in the forum , the lens have a enough fov to work even with a source of about 20". But the mirror that´s inside the projector is very small , I´m sure that it will not work with big lcd displays.


mm, here are we again talking about the lens performances ;D.

well, some of you did test it and decided (trust) the performances are somehow good for you. I will never tell you that this is not a good enough lens for you but what i can tell you is what the field angle is stated on that lens. In fact, i don´t need more info to know if it suits me or not.

Anyway, I hope GG will post his sensations about this lens when he test it on his projector.
 
135 mm lens

Rox, I've read your posts about your experience with that lens and the 17in LCD. First I wonder if the opaque projector lens (18" EF) that I bought has the same specs like the 135 mm from DIYprojector Company. But even if it is the same lens the projector is designed to project a picture of a document no more than 11" by 8.5". The size of the lens is good only for maximizing the light collection not for viewing bigger objects than what it was designed for. As long as you’re using a 14.1" max LCD screen or less you should be able to improve the brightness of the overall picture over the regular picture produced with the 60mm or 80mm triplets. Remember the huge size of the arc produced by the light source for a DIY light source. Anyway that’s my own conclusion from what I’ve read so far and tested with the opaque projector before I took it apart. I’m a new comer to the field and just wanted to get my two cents in. Thanks guys for all your efforts and help.