Hi all,

I recently tried using the RPTV lens from Tgreenwoods PSone pj. Now as we know this lens cannot be used with a 7" screen because the screen is too big. However I tested using a reverse fresnel lens to shrink the screen and it worked. The fresnel reduced the 7" screen down to 5" for the lens. This worked with CFL lighting.

Ill post some test pictures when I get a chance.

C

Hooray!!!!!

Can't wait to see the pictures, both construction and screenshots.

Way to go!

Tgreenwood

Man, can't wait to see those pictures...

how 'bout some details???? Which fresnel did you use.... How far away is it from the lens??? C'mon, I'm chomping at the bit here

Really now, glad to hear that this is working for you... I too can't wait to see some pictures....

Congrats...

I also cant wait to see some pictures. Also, this means you could use the delta II with a 10.6" HD screen and because the Delta II has such a large f-stop it would let a lot of light through giving a very bright picture.

If this works realy well it will realy cut down on the cost of my next projector. I wouldent have to buy a new triplet.

Christof,

I too am using a 7" LCD, and a reverse fres. I've been busy so I haven't been able to post my build yet. Should be soon though.

Good to hear I'm not the only one trying the fresnels with the CF bulbs. I've placed a fresnel behind the screen like in a regular Lumen Lab build.

The fresnel behind the LCD seems to even out the light a bit more. Forcing the light to pass through the LCD at more of a perpendicular angle, rather than from multiple angles (reducing light refraction).

The fresnel on the front, then condenses the image and all of the light to a size that fits the 5" Delta field of vision.

Can't wait to see your results too.

Chears,

Movie Freak

I soon as I get my friends camera Ill post pics. Ill try for sooner rather than later.

Moviefreak, perhaps you could post some of your results to tide everyone over ?

Thanks
C.

Cristof, you are a really fast worker when it comes to trying new ideas. Thanks a lot for giving it a try. This is just wonderful. I only told you about the negative focal length fresnels on May 24th here..........Front Fresnel With Rptv Lens, and that they should work to make a larger LCD look small enough. Man, you're quick.

Is it safe to assume that you are using a negative focal length fresnel when you say "reverse" fresnel? (I would be really embarrassed if I'm wrong). Did you get yours at an Auto Parts store? Was it thin and floppy, or thicker and fairly rigid? Did you like the quality, or do you think it would be better to get a negative fresnel from an optics store, like anchoroptics.com?

How far away from the LCD did you have to put the negative fresnel? Did it have to be mounted separately from the TV projection lens, or could you put it right on the projection lens? Can you tell that I am extremely curious?

Sorry about all of the questions, but I am psyched about this.

MovieFreak, these questions are for you, too. Even if you don't have pictures yet, tell us all about it!


Tgreenwood

Hey no problem, questions get things done !

The focal length was negative (-320mm).

It was from an auto store, thin and floppy. The quality was quite poor and a optical quality fresnel is definitely needed.

As for distance, about RPTV lens about 1mm away from the fresnel. A better lens also may give a sharper picture. So far Ive got moving video focused nicely, but computer text is a little burred, readable, but blurred. GThe panel is XGA, so no problem there.

Cheers
C

Interesting !

From a quick search most wide angle neg fresnel lens (auto use) have designed differing angles.

Pulled from 3dlens

"Focal Length: -300mm
View Angle: upward:13º side:25º down:27º "

This must have some effect on the result ..have you noticed ?

Or perhaps the lens you guys are using are diff. ?

Hey all,

I know that with the holiday coming up and all that, if you're like me things are a little on the busy side... just wondering if you ever got a chance to take some pictures of your setup.... I really would like to see what your projected image looks like (focus, how even the light is across the screen, etc...)

Not that I'm trying to pressure you or anything....

Helloooooo?

Anybody have any pictures?


Tgreenwood

I "discovered" something called Negative Meniscus Lens.
It is a reducing lens.

Could one of these be used instead of a negative fresnel to make an image small enough to go through a triplet with a small FOV?

Or even reduce the image from a large LCD in order to icrease the throw of a standard triplet?

I have read about them but i'm having a hard time understanding their real world applications.

Any negative focal length lens theoretically should shrink the image in the same way, negative fresnel, negative meniscus, biconcave, plano concave are all negative lenses.

Here are some ray traces to show how we are hoping to use negative lenses. I used this webpage to do them, Geometric Optics Animation.

Click to view attachment
In the first one you can see that the negative lens creates a "virtual image" of the arrow at half the size of the original, and projects it. The second picture shows how the same lens projects the smaller arrow all by itself. They act the same.

When you reduce the LCD image you are trying to project, it should have the same effect as using a smaller LCD to increase the throw of a triplet for the same screen size.

I don't know yet how to figure what focal length to use, I guess it'll take some experimentation.

Tgreenwood

These negative lens are cheap at online surplus stores, cheaper than negative fresnels.
I would love to use a Delta Lens with a bigger LCD so I think i'll add one of these to my shopping list.

Hey Guys,

Sorry for being away for so long. Been really busy.

Anyway, I found the whole negative fresnel thing still too blurry to watch.

However, after returning to the forum to find that negative lenses are being discussed with the use of the CF projectors, I did a little digging of my own.

I found a number of reduction lenses at http://www.anchoroptics.com/catalog/product.cfm?id=34.

The largest one is 152 mm in diameter (5.98 inches), Grade 1, FL 355mm (13inches seems too long) and only $29.00 US dollars.

The one lens just a little smaller than the one mentioned above is 127mm in diameter (5 inches), Grade 2, FL 192mm (7.5 inches seems much more compact), and is only $18.50 US dollars.

Does anyone think these would work in condensing a larger screen to project through the Delta, or the Pro triplet at that

Maybe the reduction lens from a salvaged photocopier would work? Thoughts?

Later,

Movie Freak

Well, if the negative fresnel worked, I don't see why a negative focal length glass lens wouldn't work. They both do the same thing.

Glass lenses are better for imaging, we just use fresnels in standard projectors because they are a whole lot lighter and way cheaper than a glass lens of the same size. (And the floppy rear view fresnel from the auto parts store isn't exactly stellar quality).

For use with the RPTV lens, we need a negative focal length lens with the same or larger diameter as the projection lens.

The focal length of the reverse fresnels are around -300mm to -320mm, so you might want to get a negative lens of about the same focal length since you know what that focal length can do.

About the blurriness, just to check, see if the blurriness improves when you remove just the reverse fresnel. The blurriness may be caused by another issue.

How do (or did) you have everything set up, distances and stuff? I'd like to know the distances between the LCD and reverse fresnel and projection lens.

Boy, I sure hope that you have some pictures of your setup. Would love to see them.

Tgreenwood

Sorry guys, I'm such a perfectionist , that after I found it blurry, I totally tore it down to rebuild and experiment some more with all the parts.

I live in such a small space that I couldn't have it laying out in the open unless it was finished. I've got a 2 year old that gets into everything, so most of my stuff is locked away.

I could put it all back together and snap some photos, but it may be a while. Again Sorry.

Later,

Movie Freak

Hey, don't worry about it.

Anything you could remember would be helpful, like if the reverse fresnel was really close to the lens or a few inches away, and about how far away the LCD was from the reverse fresnel. Don't worry about being precise, just a general guess would be good.

Tgreenwood

Ok, if memory serves me right, I had the negative fresnel almost touching the back of the Delta lens, there was still about a millimeter gap between the lens and the negative fresnel.

And then both the negative fresnel and the Delta were placed in front of the LCD about 2 or 3 inches.

Did anyone else experimenting with the negative fresnel have similar spacing?

I'm guessing that the quality of my negative fresnel may have been the cause of the fuzzyness. It was marked "Experimental" from the supplier.

Later,

Movie Freak

Today I got a rear view fresnel lens, and put it in the base of my slide projector lens,

I am able to project a 5.5" transpernecy image (printed using color laser printer), pretty well,

but the fresnel is very low quality and keeps bending .....

I think a negative glass lens (instead of a square fresnel) will be the right choice for me,

what features should I look for when shopping for a negative lens ????

If this works , i can make a really small projector that can project 40+ inch image.....

I stumbled upon a negative focal length fresnel that is rigid (I think) not floppy.

Here's the picture....

Click to view attachment

It looks pretty decent and is 10 inches in diameter. Don't know what the focal length is, though.

Found here....The Minimizer Lens at ScienceKit for $7.45.


Tgreenwood

I found out why the viewing angle is different on the automotive rear-view fresnels.
The optical center of the lens isn't in the actual center of the piece you get. The optical center (where all the rings converge to a dot) is about one-third of the way up (or down ) the piece. They just cut it off-center to manipulate the view angle.

Tgreenwood

I am not to familar with these type of lense but would it have a sorta like builtin lens shift?

So, did this seem to work? I have come late to the game here and "Delta II" lenses are impossible to find - seems like this may be an alternative.