DIY anamorphic lens

[Michael F T]

Does anyone know the specs for the solid versions of the prisms?

What are the Prismasonic dimensions and angles?

There is a company in the UK (and probably one in the US) that will make solid glass prisms based on specs provided.

But what specs?

Is there a formula that could be used?

[tahustvedt]

Here's a downloadable program for calculating optics:

http://www.astrion.de/download/download.html

I downloaded it but couldn't make much sense of it, but I haven't tried very hard either.


Tor Arne

[Michael F T]

What do you think of the concept of using a solid prism over a water/oil prism.

If I had a 2 special blocks of glass that could be cut with a standard knife and then cut them at some angle, is that a prism?

Is it just the glass that is bending the light?

If so, what about using a optically clear liquid plastic and form a prism?

Or have a prism manufacturer create the prisms?

Is it all about the angles involved or are the materials special?

Last question, have you ever tried putting the prisms on their sides and see if that makes an expansion lens?

[rwhitley]

The prisms on their sides do make an expansion lens.

You can use solid glass for the prisms, it is just extremely expensive to do so, and the angles would be different.

Rick

[woneill]

The reason that two prisms are normally used (oil/water) is the same reason that good quality optics use doublet lenses (glass/flint).

It is to correct for chromatic aberation.

The prism is the simplest way to demonstrate this - hold a classic glass prism up to white light and it will disperse it into its constituent colours. A lens is just a curved prism, and will also disperse light into its constituent colours. The shorter the focal length, the more the dispersion.

This is due to the refractive index actually being different for different wavelengths of light.

In optics, an achromatic converging lens is produced by matching a glass converging lens with a flint diverging lens. If the lenses are matched for chromatic distortion (the flint cancels out the chromatic distortion of the glass), the overall produced lens is still converging (glass has a more consistent overall refractive index than flint).

Thus, you get the classic achromatic doublet.

That is all that is going on with the two prisms (oil and water): their chromatic distortion properties cancel while allowing for an overall bending of the light.

You could do it with any two refractive media - as long as a balance can be achieved where the two prism materials cancel out the chromatic dispersion while giving an overall converging effect.

Thus if you could get the correct formulae and find appropriate materials from amongst the liquid plastics, transparent epoxies, glass etc. you would be able to make your prisms without TOO much trouble (people are able to grind their own reflectors for telescopes, which are FAR more difficult to do acurately than plane surfaces...)

Bill.

[Michael F T]

Thanks for the response, makes sense.

The transmission of the light would probably be lower too through a plastic. However, maybe not too much.

So the trick is getting the correct data to apply to a prism of different refractive indexes.

I still don't understand why 2 glass prisms would be all that expensive. I just can't figure out the sizes and angles for them.

I was hoping someone would have documented the details much like the Oil/Water version.

I'm still a little uncomfortable with liquid being so close to the projector.

[woneill]

Generally, prisms are made to very accurate tolerances for optical instruments. Anything specialised is very expensive.

If you wanted to make them yourself, or have blanks made to rough spec and finish them yourself, the price and quality would not be anywhere near the commercial ones, but that is not necessary for projectors.

Literally, with the correct materials, they would not be TOO difficult or expensive to make (or have made...)

Bill.

[ap0the0sis]

Could someone explain to me what a anamorphic lens is does and what its used for?

I read on a site that it is used to squeeze a 4:3 image to a 16:9 but wouldn't that distort the image? Doesn't make much sense to me right now....

I have a 4:3 projector, would i benefit from this?


ap0the0sis

[Glenn Refling]

ap0the0sis,

The anamorphic lens does indeed squeeze the image. The reason they're used by owners of 4:3 projectors is that they allow all of the pixels in your projector to be used in generating the image.

The trick to avoid distortion in your squeezed image is to pre-apply a distortion in the opposite direction. There are two means generally employed to do this. One way is to use an external scaler or PC to re-scale a 16:9 image to fill your whole 4:3 panel.

If you viewed this image without an anamorphic lens, it would show people tall and skinny and all things stretched vertically. Now placing an anamorphic lens in front of your projector causes the image to be squeezed back to 'normal' geometry. The big advantage is you are now using many more pixels to create the picture giving you a better resolution.

The second way of doing this is using a DVD player that allows aspect ratio settings. If you tell the player the aspect ratio is 16:9 and tell the projector it is 4:3 then the picture takes on that tall and skinny people look.

Hope that helps.

Regards,
Glenn

[ap0the0sis]

ahh, i see. make some sense to me now. i dont think this lens setup would work well for someone that is switching between tv viewing (4:3) and movie (16:9) right? This is more like one time setup and leave it for movies


got another question. how come these lenses are filled with fluid? Wouldnt it be better to use a solid piece of plexiglass and grind the prizm shapes from that? This way you can get the right curves to compensate for distortion?


justa thought

ap0the0sis