Calculating the angles of the lens

[Tarzan_91]

Hello!

I'm quite new to this forum, but now I'm posting my question here because i couldn't find it anywhere else, and if i would have posted it on the forum i'm normally on I wouldn't have got any answers at all i guess...

How do i calculate all the angles of the prisms and the angle betweene them in a horizontal strech lens?

I've studied physics for about a 1½ year so i know "n1 sin v1 = n2 sin v2" and how to use it, but i don't really know how the "light beams" are (or goes?) inside the lens.

As you can see, I'm not english (or american), so a answer in a quite easy language is prefered.

thx

[John Martin]

I mocked up my prisms on a table and marked out the screen on the wall with painters tape. I then adjusted the prisms until the edge of the image touched the painters tape on both sides and that is what I went with. Worked perfectly. Can't tell you the exact angle since your prisms are likely different than mine.

John

[CAVX]

The angles also vary based on prisms materials and their angles...

Mark

[Tarzan_91]

So the angles of the prisms can be compensated by the angle betweene them?

If so, it leads to that I could just pick som angles (realistic ones) and make the prisms, and then experimentally find out the angle betwene them? That would be nice...

[CAVX]

So the angles of the prisms can be compensated by the angle betweene them?

Yes. The angles for displacment for the "French" prisms at 20 degrees are less than what is needed for mine at 14.5 degrees and so on. My original 30 degree water prisms did not have much displacement at all due to their steep angle.

The best way to aligne the prisms is in the light path. You need to centre the 16:9 rectangle of light on the 2.37:1 screen, then adjust the prisms so that the light is optically stretched to each side...

Mark

[Tarzan_91]

And now the two last questions
1 If I make 2 prisms whith different angles, does it matter which one is nearest to the projector? I does not, does it?

2 (a little OT...) I know a curved screen is better for CIH but the radius of the curve, how long is it? Double the distance to the projector? Or is this just to try and see what suites one's lens?

thx a lot for your replies

[Steve Scherrer]

And now the two last questions
1 If I make 2 prisms whith different angles, does it matter which one is nearest to the projector? I does not, does it?

2 (a little OT...) I know a curved screen is better for CIH but the radius of the curve, how long is it? Double the distance to the projector? Or is this just to try and see what suites one's lens?

thx a lot for your replies

Don't know the answers to your question. But I am curious - how do you plan on making the prisms?

[Tarzan_91]

I Have some pieces of glass. Four of them are museum glass which reflects light less than ordinary glass, but was way more expensive... (Those 4 sides ar of course those which the light will go through)

I will just glue them together, of course first I will glue 4 sides then fill the prism with the glycerin and then put on the last one...

[CAVX]

Well good luck with that, but if you were to read over the DIY work on this site or DIY Audio, you would notice that we all went to solid BK7 prisms that you can buy quite cost effectively these days.

The French team that did the gliserine prisms used 20 degrees...

Mark

[Tarzan_91]

Well... I've allready got the glicerin and the glass so...

I'll try making two 20 degree prisms then.

[CAVX]

They used 20 degrees, but I think you can use less...Good luck with that :)

Mark

[LightsOff]

OK yeah

[bgosselin]

If you look at my signature you will find a link that goes to Tor Arne website. He is a fellow AVS members that experiment on this a few years back. He give a lot of details about angle and material he use.

Good luck with your project.

Bruno

[CAVX]

His angles were 20 and 24 degrees. The interesting part about prisms is that they can be further offset to increase or decrease the stretch, so having the exact angle is not always important. They are "forgiving" for lack of a better word...

Mark

[Tarzan_91]

But why would he (tor arne) bother making the prisms with 2 different angles if it wasn't necessary?

[CAVX]

But why would he (tor arne) bother making the prisms with 2 different angles if it wasn't necessary?

The angles I listed were wrong - 24 and 30, not 24 and 20. The angles were based on a few things -
1. mediums with different Refrective Indecies - they used water and oil.
2. prism displacement.

Tor simply followed what others before him had done. What made Tor such a legend was his ability to craft the prisms, then document everything for others to follow.

However, if you read the very long thread at DIYAudio, you see that several people end up having positive results using the same angles and even the same medium - hence why the BK7 prisms work.

The original lens Tor made was also a VC, where most of us made HE lenses.

The key point is that whilst two prisms (mostly) cancel out CA, different mediums are needed to full correct this - hence why they used oil and water.

The same medium can be used in both prisms. As for the angles, different medium will have different angles in order to obtain the correct amount of expansion or compression, however, changing the angles of the displacment can also be done when using two of the same prism...

Mark

[bgosselin]

What made Tor such a legend was his ability to craft the prisms, then document everything for others to follow.

At that time Anamorphic lens were not that frequent. He started a thread on this forum that was one of the most viewed and read thread at that time. He comments often and the thread was like a blog almost. Some manufacturer menace AVS of law suite so they had to erase the thread. That also help create Tor's popularity. :)

[Tarzan_91]

OT: Why was there a law suite?

[bgosselin]

Not sure they were a law suite. PM sent.