Interest in 22" EFL w/ 5" Diameter Opaque PJ lens?

[Guy Grotke]

If it has an AR coat, it is pretty worn out by now. The surface looks a bit dirty, but I don't see the typical green or violet tinge in reflected light at an angle. So if it ever was coated, then I think it needs to be done over.

But you bring up a good point about the surrounding area. I usually glue some black felt around the lens, but I have not done that for this lens mounting board yet.

[slizm]

I have a few of these lenses, from the 18" and 22" opaques. I also have 2 lenses that are the same sizes as these but they are labeled, "Projecta-vision". Any way to find out the specs on these? I haven't had much luck searching on my own.

[Guy Grotke]

The first thing to discover would be the focal length. Go outside at noon with a yardstick (or even better a meter stick). Raise and lower the lens until you form the smallest image of the sun on the ground. (Like kids do to incinerate ants.) Then measure the distance from the ground to the middle of the lens. Flip the lens over and measure that distance. They should both be about the same distance, which will be the focal length.

The second interesting thing is the lens diameter. That tells you how much light you can get through it with a normal DIY projector lamp.

The third interesting thing is harder to measure. The Field Of View is the diameter of an object (like an LCD) that you can project through the lens using the real application throw distance. You could make a mock-up of a 17" LCD using cardboard and some printed transparency film. Then setup a spotlight, the LCD mockup, and the lens on a table. (I use stacks of books and masking tape to hold stuff in place.) The lens should be about 12 feet from a white wall. Adjust the lens position until the center of the projected image is in focus. Then move only the spotlight to see how much of the "LCD" you can get to the wall in focus. (That's where the printing on the transparency film comes in.) if you can get the entire 17" LCD area on the screen in focus with equal brightness, then you have a great lens. If it can only do 15" diameter, then you have a lens for a 15" LCD, etc. Don't be upset if it can only focus a 10" or 7" area of the "LCD". Lots of people build very nice projectors using 7" Lilliput or 8" Hami LCDs.

[slizm]

ok here's what I got with the first 2 tests:
lens is 4 inches in diameter..................................
focused point using both sides of lens out in the sun was about 11.75 inches(focal length)........................................... ...........

still trying to set up something to do the 3rd test.........................

[Guy Grotke]

That's a 300 mm fl, which is very convenient, because you can use it with the very standard 330 mm fl field fresnel. It is exactly the same focal length as a standard overhead projector lens, but at 4" in diameter it would give you more light.

If its FOV covers a 15" LCD, then you would get a 100" diagonal image with a throw distance of 90". If you use it with a 7" LCD, then this would work as a long-throw lens: 100" diagonal image from 179".

[AustynSN]

Just thought I'd let everybody know, but I checked with the supplier listed at the beginning of this thread about trying to get one of the lenses and he said he'd scrapped all the projectors.

Bummer.

[Guy Grotke]

Did you ask him for a projector, or just the lens? Last I read, he was asking $100 each for just the lenses. I would be surprised if he scrapped the lenses, if he thought he could get $100 each for them!

I could see removing the lenses and recycling the old projector bodies for the metal value. I doubt anybody was willing to pay for shipment of an old opaque projector.

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Note to Rox: I did add the black felt around the lens, and it is better. I also did a contrast estimate with the LuxMeter: I found the point where a black image gives me just over 1 Lux. When I measure that same point of a white image, I see over 450 Lux. The 1 Lux reading is somewhere between 1 and 2, so the contrast ratio is between 450:1 and 225:1. This agrees pretty well with the LCD's rating of 300:1, so I don't think that removing the AG layer decreased the contrast ratio.

[Rox]

nice news GG (how did you know i was subscribed to this thread ? )

I was not saiyng the antiglare removing decreases the contrast... I was saiyng it could be increased anyway...

mmm, your 1 lux reading, is it the highest you could read? could you place it closer to the lens so a higher black level is meassured on your luxmeter? (I don´t like the luxmeter value when it is working near its limits... i gues you understand what I mean)

if you could repea it for let's say 8-16 lux value (if posible) would be very welcome for me...

thanks.

[slizm]

I live near a surplus that has a bunch of these opaque lenses if anyone's interested. Most of them are the 18" but there are a few with the 22"

[Guy Grotke]

Are they selling just the lenses? How much are they asking?

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Rox: I was replying to your post #44. It just took me a while.