Just got my French Prisms - Page 5

We can make bigger prisms but it will require at least 50 to be made.

If you can spark some interest and get at least 25 people to commit to an order, we can have them in production.

We would need detailed specs with a drawing if possible. From there a sample would be made, sent to you for approval, then a 25% deposit would be needed to start production.

Our email is: surplushed@aol.com all specs would go there.

We saw there were quite a few people looking to get the prisms and we just wanted to let them know where to get them. Sorry for any misunderstanding.

Be careful, because Sam got banned for the Same thing. Did you check your PM?

Mark

So what exactly is the type of glass used.

Any reason why the prism ruins the focus of the image. I agree that these
prisms could be expected to cause distortion to the edges of the image but at least if they do not affect the focus it might be worth as a cheap alternative for CIH.

BTW: I still have one more pair left. It has a little chip at one corner but it
does not affect the image. I bought this through Sam for $126 (yes it was shipped
from surplusshed, PA)

Asking $95 + S&H. I sold one to a guy in Canada. The UPS cost to Canada was $12.
I would presume it would be under $10 within US.

I am sure the prisms that you buy from Surplus Shed would also have chips
like others here had reported :-)

BK 7 @ 20 degrees...

Mark

You shouldn't have to refocus the image if your moving the lens in/out of the
light path, but once focused, you may notice that the image is sharp in the centre and then progressively becomes less sharp towards the edges. Some of the loss in focus could be due to the CA that is virtually 0 in the centre and increases towards the edges.

Only corrective elements can fix this...

Mark

And again, only corrective elements can fix this...

Mark

Nope I havent tried other lens systems. I had the Mallison prisms but
I returned them as they caused too much reflection.

With the French prisms I had to adjust the focus a bit. Not that the image was blurry but it wasnt as sharp as the image without the prism. The image without the prism is so nice that I could not live with the image with the prism even though it is only $126. So I decided to get rid of them. I have already sold one pair. The second one is up for sale.

I guess people should stop selling all these cheap crap claiming that they can be used as anamorphic lens. There is no way a DIY lens can beat the image from the commercial lens.

If the size of the 2.35 image is the only thing one cares about, then these cheap options may be ok but just bigger isnt better for me. I would rather watch a small but prestine image straight from the projector than a big but lousy image through the prisms.

well you and I think they are BK 7. what is the guarantee that those pieces of glass are in fact BK 7. They could be just as bad as any other $8 prisms they are selling. I also doubt if they are coated considering how much they reflect. They arent as bad as mallison prisms but for coated prisms they put out an awful lot of reflections.

BK7 is pretty much the same as Crown Glass and is optically clear and not that expensive which is why it commonly used. Allot of the final image quality depends on the finish of the faces and the finer the finish, the better the result, but the more you pay. Having received quotes as high as $10.5K for four BK 7 prisms, would suggest that $126 is indeed entry level. It is not to say that those prisms could not be polished to much higher spec, just the price would go up dramatically.


No, one should not expect ISCO III performance from just 2 prisms that cost just $126.00. The 2 prisms lens works to a certain level (entry) and has been well documented to have faults - CA and some focus). It has just 2 elements after all, and it needs at least 5 (CA correction as well as focal correction) to qualify as "high end" product and you can be assured that even from a DIY end, it will cost way more than $126.00 in parts alone...

Mark

Wow. Pretty high expectations for $100 anamorphic lens.

The fact is, these CAN be used for an anamorphic lens. If you want commercial quality, please buy a commercial lens--they have all kinds of correction features.

But these prisms in the DIY system get you MOST of the way there at a fraction of the cost.

(As an aside, Mark did a shootout against a commercial lens, and the DIY lens fared pretty well).

Anyway, I am sure my standards are a heck of a lot lower than yours. Personally, I don't really put all my faith in testing grids that demonstrate focus issues or CA. My standard is at the "movie watching" level. Once a movie starts, do I notice the slight focus issues or the slight CA? With the masking, do I notice the pincushion effects? Hardly. Am I pleased with the DIY lens? As punch.

In fact if you could find the screen shots Sam posted, you could see the defocus at the edges...

Mark

You mention defocus due to CA toward the edges. I am planning to buy prisms for a diy constant width setup. I am using an IN72 480p unit to do the scaling. If the image resolution is increased by using all of the pixels, shouldnt the defocus be virtually undetectable from normal seating distances? As well as CA for that matter.

I get so confused reading all the post, one guy says he can live with CA and the other says he cant because it's so bad, yet they never tell the distnace they are making these judgements by. Is one guy looking at the screen within 12" and the other 12' ? I think every opinion should state setup, distance viewed, size of screen and equipment used. Of course a DIY lens will look better on a 1080p rs1 compared to a 480p in72. Just think it needs to be a little more structured comparison wise before you bash it or praise it.

A real big factor for the amount of CA in the image is the zoom setting of the projector lens. For the best results you need to position the projector as far from the screen as possible and set the projector lens to the smallest zoom setting. The less angle that the light leaves the projector lens the better.

Randy

Or if you can, calculate a pre-set TR - say 2.0:1 (screen height x 1.78 x distance from screen to projector). That way you know that CA will be reduced...

Mark

Hi Folks. I'm a new member with interests in anamorphic screening since I just acquired a Panny AE2000. I found this thread and was hoping for an easy way out with the prisms mentioned. I worked as a projectionist in my early days and used a couple of the high end prism based anamorphic add-ons for Cinemascope films (these went on the front of the main projection lens). These were simple coated prisms similar to what is being discussed in this thread BUT they were achromatic prisms. Each of the two prisms consisted of two slabs of glass cemented together (Crown and Flint) to correct for color dispersion, which may be largely responsible for the poorer performance some of you mentioned in this thread (sometimes what appears to be defocusing is really a color dispersion problem). Positioning of the prisms relative to the front element of the projection lens is also important to get the best results.

I started work designing a pair of cylindrical mirrors to accomplish the anamorphic squeeze or stretch (without the achromatic errors refractive elements introduce) but that is really only in the early planning stage right now. If I ever get on with that, will report my results. Just thought I'd mention the former though since non-color corrected prisms are famous for what they're used for - to disperse the colors in a light beam and make rainbows.

OMAN and the rest, sorry I have been away so long but my life has been in a bundle. I was rear ended in my postal vehicle and was off luckily a week then the holidays...I am still working on it and was sidetracked building my curved screen(which I love) SO I plan to get to my bud's place and see what he could do.

Is there any place to get DIY plans for an anamorphic lens? Just got my HC 4900 up and running and was thinking about giving it a try. Just doing zoom now and picture looks good to me. I would only imagine that a lens would make it that much better.

Thanks Oman. How do you figure out what the angle of the prisms in relation to each other should be?

I went to "Surplus Shed" web site & found nothing when I searched for "French prism". Is there a part number or have stopped selling them ?

Thanks!

I think they call them something like "large anamorphic prisms pair".