AVS Forum banner
1 - 4 of 4 Posts

·
Registered
Joined
·
3 Posts
Discussion Starter · #1 · (Edited)
Hi All,

I have the chance to get both options, currently I have JVC x700 and anamorphic lens Prismasonic HD-5000M (prisms based lens).
Projecting with throw ratio of 6 meters projector to screen (166").

As there is no major differences between x500 and x700 projectors (besides plus of contrast on x700), I was wondering the idea to change x700 -> x500 and sell my prisms based lens HD-5000M to get a Cinomorph C-150R.

I have heard that cylindrical lenses have some pros and benefits:
  • sharper corner-to-corner focus.
  • less chromatic aberration.
  • it passes light a bit better through.
  • Pincushion effect maybe a bit smaller (?).

Besides above pros, I would get motorized lift stand version that lets move out lens when watching 16:9 content.

In short optical performance wise the Prismasonic C-series equals the Schneider/ Isco lenses at a much lower cost (2500 $ vs 8000 $).

Would it be worth the change? What do you think ? :)

Thanks!
 

·
Scott Horton, techht.com
Joined
·
5,746 Posts
I respectfully disagree that the P cylindricals are equal to the Schneider/Isco/XEIT. But you are right, that added quality adds substantially to the cost.

You said you thought there were pros and cons. Performance wise there should only be pros. Price being the only con. Was there something else you thought was a con?

The answer to your question though needs to factor in the full feature set of both PJ's and your room conditions. Personally I'd rather have a lens that I can move out of the way no matter which you choose, manual or otherwise. Also consider the XEIT www.xeitoptics.com.
 

·
Registered
Joined
·
3 Posts
Discussion Starter · #3 · (Edited)
I respectfully disagree that the P cylindricals are equal to the Schneider/Isco/XEIT. But you are right, that added quality adds substantially to the cost.

You said you thought there were pros and cons. Performance wise there should only be pros. Price being the only con. Was there something else you thought was a con?

The answer to your question though needs to factor in the full feature set of both PJ's and your room conditions. Personally I'd rather have a lens that I can move out of the way no matter which you choose, manual or otherwise. Also consider the XEIT.
I was referring to the idea to downgrade projector X700->X500 (main difference is drop of contrast) and upgrade from prisms lens to cylindrical. Huge benefits in lens and maybe some minor cons due to projector change.

Thanks for the suggestion, didn't know Australian XEIT. I have contacted with the seller and it seems pretty interesting, on pair with Isco IIIL.

I'm considering XEIT CM-4KR lens but it's expensive than Prismasonic cylindrical solution (Cinomoprh C-150R) besides taxes, even more including the shipping to EU. Differences ?

Thanks!
 

·
Registered
Joined
·
79 Posts
Prism-based lenses suffer from the following deficits, compared to some cylindrical lenses:

1. Utilising only plane surfaces, two prisms placed in a counter-rotated arrangement to produce anamorphic horizontal expansion (or vertical compression), have no degrees of design freedom available.

Hence they will laterally distort the image to a fixed amount, with a weak expansion (or squeeze) in the center of the image and exaggerated expansion (or squeeze) from about the half way mark to the edges of the image.

Some cylindrical lenses employing only two glass groups of elements (i.e. front and rear groups) do this as well. However, Xeit lenses have extra glass incorporated in the design, a third glass group, to even out this distortion, thus exhibiting the lowest lateral distortion of any lens on the Home Cinema market, by a factor of 50%.

Lower lateral distortion means more even light spread and a consequent reduction in hot spots, as well as being aesthetically more pleasing to the viewer.

2. Prism lenses have astigmatism problems. Astigmatism is defined as differing focal points in the vertical and horizontal planes.

Some prism lenses incorporate a very weak cylindrical element to bring either vertical or horizontal plane focus forwards or backwards so that the two focal planes coincide. These are normall called "corrector" lenses. Corrector lenses have been around since Panavision built their first prism-based system in 1951.

However, correctors have a set correction distance. They are a lens of fixed focal length. The user must specify the throw distance and a suitable corrector will be provided as an optional extra. Corrector lenses are NOT adjustable. They do NOT have a "range" of focal distances over which they work (although some manufacturers claim this). Unless the throw distance is the exact distance indicated by the strength of the corrector lens, the image will not be in focus as well as it can be.

In the case of 4K projection, with on-screen pixel sizes down to less than 0.5mm, the best possible focus is critical, if the image is to be seen as "pixel sharp".

Cylindrical lenses usually have a continuous focus adjustment, just like a real projection lens. Astigmatism can be focused out at any throw distance.

3. Simple prism lenses exhibit color aberration. While some prism lenses incorporate color-corrected prisms (two groups of two prisms, with each group being a cemented doublet made from a combination of crown and flint glass), and they thus avoid this problem.

Unfortunately, prism lenses (usually "budget" products) made from a single glass type ALWAYS exhibit this lateral chroma aberration. It is a myth that using "optical quality glass" eliminates chroma aberration. The qualoty of the glass is not the problem. The Laws Of Refraction are.

No matter how good and flawless the glass, single glass systems will generate the "rainbow effect", which is best illustrated by observing a vertical, nominally white, single-pixel grid line projected onto the screen, which has separated, due to color aberration, into individual red, green and blue lines. Closer to the center of the image there will be some overlap of these lines, thus leading to an observed "blurring" of horizontal detail, as the three colors do not quite superimpose.

This cannot be cured by the addition of any correction lens. The only way to fix it - and then only a little - is to get involved in some complicated menu function that differentially adjusts RGB registration to different degrees at different parts of the screen. I have seen these systems at work on Sony 4K machines, and they are underwhelming and very complicated to control. Of course, when the anamorphic prism lens is removed, the image is out of registration, as the adjustment only works when the anamorphic prisms are in the path of the beam. Let us not forget that the price of Sony 4K projectors is not cheap. It seems an awful long route around the problem to spend a small fortune on an expensive projector so that you can compensate for a cheap prism lens's aberrations. And THEN you have the astigmatism problem to solve... and lateral distortion and all the rest of a prism system's inherent deficiencies...

There are even some "color-corrected" prism systems that exhibit color aberration. These are the "adjustable" systems where each prism element is individually adjustable (rotatable) to achieve the desired expansion ratio. However, each prism MUST be adjusted to precisely the correct degree of rotation, for any given expansion ratio, or else color aberration rears its ugly head again.

4. Horizontal shift is also exhibited by prism lenses.This applies to ALL prism lenses, not just the cheap ones. As the projection beam passes through the prism system it is shifted laterally (horizontally) an appreciable distance, sometimes several centimetres. If the user has a carefully aligned image on the screen, exactly fitting a masking arrangement, then this shift will mean that the image goes into and out of alignment as the lens is placed into and out of the beam. The only way to solve this is to enlarge the image so that the shift is less than the overlap into the masking.

5. Prism systems are generally heavier than (roughly) equivalent cylindrical systems. This due to the huge thicknesses of glass required at the base end of the prism. Optical glass is heavy. Due to the infinitely greater degrees of design freedom available to the cylindrical lens designer, thickness can be more tightly controlled, thinned out, and light more functionally "bent" by utilising curvature to achieve the same effect that crude thickness and plane surfaces achieves in a prism system.

6. Ghost images (internal reflections that can sometimes spill onto the screen as multiple images) are more prone to occur with prism systems, due to the plane surfaces used, compared to the curved surfaces used in cylindrical systems, which tend to disperse internal reflections.

There is potential for ghost imaging in any optical system. You only have to consider the "Panavision Effect", where a strong, isolated small light source (such as a car headlight) in a movie can cause a large blue ghost image right across the screen, or multiple versions of the image to be displayed in odd locations. A perfect example of this is the "Tent in the jungle" scene in Indiana Jones And the Kingdom of the Crystal Skull where the tent Jones is being tortured in (by the nast Russians) is full of point sources of light: candles, lamps etc. There are ghost images everywhere.

However, with cylindrical systems, ghost images (if any) will be far less likely to happen, far less in number and far less distracting than with an equivalent prism system.

Additionally, multiple reflections between planes of glass in prisms systems can induce a "wash" of light over the image, a highly blurred version of the original image, resulting in a reduction in contrast of brightly lit scenes.

7. Conclusion: while prism lenses were the only option available during the early 1950s for Cinemascope projection, they quickly were superceded by cylindrically-based systems that eliminated the many problems and limitations of prism systems: weight, aberrations (color and geometric), astigmatism performance and lateral shift.

True, good cylindrical systems are more expensive than prism systems (although not necessarily as much as some here seem to think), but the performance benefits available from a good cylindrical system far outweigh those of the inexpensive, but cruder prism systems, especially in these "4K" days.

Although Home Cinema applications may appear to be "amateur" compared to commercial cinema applications, in many ways the reverse is true. Home Cinema has comparatively short throws and large beam angles, the better to achieve a (relatively) large image size in a small(ish) room. The long throws and relatively high throw ratios of commercial cinemas are much more forgiving of aberration and potential for distortion than the critically confined parameters of Home Cinema. This is why a lens tailored for Home Cinema applications, utilising the infinite range of design freedom provided by the curved surfaces of cylindrical lenses as opposed to the plane surfaces of prisms, has greater inherent potential for excellence in image quality than a prism system.

It is far simpler to just purchase a high quality cylindrical lens that works out of the box - delivering low distortion, lower weight, no color aberration, even, adjustable focus, far less chance and seriousness of ghost images, and homogenous light distribution across the screen.
 
1 - 4 of 4 Posts
Top