replace doublet with fresnel in projector head?

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SE_cwags

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Joined 2004
#1
Here's my situation. I have an Ovation 822c, and a 3M 9100 OHP. Out of the box, I can't get the OHP to focus to save my life. Mathematically, the OHP should project roughly a 1/1 ratio for screen size vs. projection distance (in feet); the doublet lens has a focal length of 14". But when I project, regardless of distance, I can never pull the panel image into focus. Using a cheap page magnifier in place of the doublet lens, the image is sharp and more of an excellent result then I had imagined. Any ideas on why the doublet is not focus correctly and why the fresnel does? I'm baffled, but since it does work, I'm planning on cutting the fresnel to the same size as the doublet and simply replacing it. Thanks for any answers, I searched, but didn't find any threads specific to this scenario.

P.S. When both lenses are used, with the doublet in the projector head and the fresnel held between it and the panel, I can also get the image to focus, as well as adjust screen size. (By raising or lowering the fresnel between the doublet and the panel.)
 
#2
just a fresnel will give you pretty bad quality vs. a doublet. The only reason I would see why your doublet wount focus is because either it can go close or far enough from the panel for focus, or the elements are damaged or incorrectly placed.
 
#3
I did some playing around with the OHP last nite after I posted. This is my first experience with them, and I'm wondering if maybe I'm missing an element. There is one lens on the bottom of the projection head facing the stage. Then of course is a mirror, and on the front, when it was shipped was a glass plate that I assumed was simply a dust cover. It was loose, and I have not replaced it since it's been a real pain to get back in. (It has to be forced in, and I don't want to break it.) This plate has no optical qualities (just a clear glass), is there supposed to be another lens on the front? It could be that simply my projector is not as complete as I thought it was supposed to be. Thanks.
 
#4
a doublet is a double element lens, and in overhead projector, also with a mirror. You should have another element for sure. Are you sure the glass has no optical qualities? put it infront of your eye, does everything blur, or become bigger/smaller? if not, that sounds like you got ripped off.
 
#5
I think I got ripped, the plate seems to be a support for a missing lens. Least I bought it cheap, so no big loss, and it still has it's uses.
As stated, the cheap fresnel worked gr8, but I know I can do better. I must be confused on the doublet lens as well. When I looked it up, I read that a doublet is one lens made out of two, with an air pocket between them. But it sounds to me like the doublet I'm supposed to have is two seperated elements, divided by the mirror. Is that correct?
Is my best solution to replace the missing element (size unkown) or to replace it with a different option. (and please don't say new projector, I want to play with this one some) Thanks
 
#6
You should have another element for sure. Are you sure the glass has no optical qualities? put it infront of your eye, does everything blur, or become bigger/smaller? if not, that sounds like you got ripped off.
Click to expand...

No, maybe its only got a singlet lens. By the sounds of it it seems that it is a doublet lens missing the front element, but lets not jump to conclusions as we dopnt have enough info yet.

Trev🙂
 
#7
But it sounds to me like the doublet I'm supposed to have is two seperated elements, divided by the mirror. Is that correct?
Click to expand...

Yess

Is my best solution to replace the missing element (size unkown) or to replace it with a different option. (and please don't say new projector, I want to play with this one some) Thanks
Click to expand...

I got spares here i won in an auction over a year ago on ebay if your interested, you will need to tell me the EFL and weather the lenses are square or round. diameter would help too.

Trev🙂
 
#11
Tetutigre,

I've been doing some testing of white matte screen vs lenticular silver screen, and this is what Ive come up with

The white screen has more smoother whites, and better corner illumination, better viewing angle, and no warm spotting (the last 3 can be cured on the silver screen by curving it)

The silver screen has better contrast, and maybe just a bit more brighness, and much better ambient light rejection.

Trev,
I got a 135mm diameter all glass triplet for about 10-15$
and a 175mm diameter all plastic triplet for about 30-40$
😀😀 😀 cant help my self, but to show off.😉
Thanks anyway:cannotbe:
BTW, just curious Ace, is Trev short for Trevor? or is Trev your full name?

I used the projector setup written in my SIG to do the screen testing.
 
#12
ancorp said:

Trev,
I got a 135mm diameter all glass triplet for about 10-15$
and a 175mm diameter all plastic triplet for about 30-40$
😀😀 😀 cant help my self, but to show off.😉
Thanks anyway:cannotbe:
BTW, just curious Ace, is Trev short for Trevor? or is Trev your full name?

I used the projector setup written in my SIG to do the screen testing.
Click to expand...


Nice where from?

MY name is Trevor, Trev for short.
 
#13
135mm triplet - Bellnhowell opaque projector, won on ebay for 9.99, with handling and taxes,about $15 for the whole projector.
It looks identical to the one at diylabs. Seems like a great deal,

175mm triplet - member on this forum - rcavictim, this triplet is not suitable for a lcd projecor, its designed to project a curved screen 14" tv, it wont focus the edges on a flat screen. I got the 14" tv, a big fs mirror, and the triplet for a trade in of my SoundBlaster Audigy sound card (the mouth of my computer😀, a truly amazing soundcard, still havent got another one, just runnin on onboard sound right now, its quite horrible....
 
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