The basic problem with simple prism systems is that it is impossible to achieve simultaneous horizontal and vertical focus of the image at any one place on the screen.
The first illustration below says it all.
Shown are three computer modelled versions of a 16-pixel (4 x 4, black/white) checkerboard test pattern projected through a set of non color-corrected prisms.
To emphasize the aberrations seen, the test pattern is placed slightly towards one side of the final on-screen image. In the exact center of the screen the color aberrations shown would be minimized, and at the far edges the aberrations are so bad you might think I'm exaggerating them! So a compromise position was selected.
Each set of 4 x 4 pixels represents an on-screen area of 5.4 x 7.2 millimetres (about 1/5th x 1/3rd of an inch). The image is 1920 x 1080 pixels, projected onto a 3500mm wide (150") anamorphically stretched screen. Thus, the sets of 16 pixels shown are about 0.00075% of the full screen area.
Note: a basic prism lens cannot be focused. All focusing must be performed va the projector's focus mechanism.
Image #1
shows the projector focused to achieve the sharpest side edges of the pixels (best horizontal focus). However, the top and bottom edges of pixels are extremely blurred. Note also: because the prisms are not color-corrected, the three colors - Red, Blue and Green - are laterally displaced from each other at this point
Image #3
shows the converse situtation. This time the projector's focus has been changed to sharpen the top and bottom edges of the pixels (best vertical focus). However, the penalty for fixing the these has been to now blur the side edges. Color aberration is unchanged, as it is not primarily dependent upon focus of the system.
Image #2
shows the projector focused a third time. On this occasion, a "compromise" focus has been reached. Neither vertical nor horizontal focus is ideal, but at least some outline of the pixels can be seen. If you stand back a little from your monitor and squint you can make out a slightly soft, but discernable checkerboard pattern. Some of the color aberration also disappears... but only if you continue squinting!
What must also be remembered is that even this compromise focus can only be achieved over a limited area of the screen. At the pixel level, if the centre is compromise-focused vertically, then the sides will be blurred, and vice versa.
Contrast with cylindrical lens
The illustration below depicts a computer simulation of the Horizon lens's focus and color performance at the same point on the screen.
Note that both vertical and horizontal focus coincide. This has been brought about by first focusing the projector (without the Horizon lens in the projection beam) to achieve best focus across the screen.
Now, simply interposing the Horizon into the beam may not be enough to maintain vertical focus (see Image #3 above). But this is where the Horizon's adaptability comes into play: the Horizon has its own focus mechanism. This focus mechanism only affects horizontal focus. It has no (or extremely little) effect on the projector's focus in the vertical direction.
As the Horizon's focus mechanism is adjusted, vertical focus (via the projector) remains unchanged, but horizontal focus is improved until it matches the projector's focus. Now both horizontal and vertical focus is excellent.
This relationship is based only on the throw distance. The Horizon, once focused, is focus-synchronized with the projector. Focus will be maintained through all zoom settings of the the projector, as long as the throw is not changed (changing throw is usually a rare event anyway).
If the projector goes out of focus when the zoom is changed, all that will be needed will be a readjustment of the projector's focus only. The Horizon's focus will still be synchronized with the projector.
Another aspect of the Horizon's design is that, during the focusing procedure, image size will not change appreciably. Because the Horizon uses weakly refracting internal elements to focus (not the main anamorphosing elements) the aspect ratio and basic image size will remain the same throughtout the focusing procedure.
Color performance has also improved. There is some very slight color aberration, but it is much less than a quarter of a pixel in width. In resolution terms, this slight color aberration is more than four times finer than the resolution at which any image will be viewed by the audience. It will thus be invisible at all except microscopic levels of observation (for example, from about six inches from the screen) and then it will be difficult to pick out amongst the projector's native aberrations and distortions.
Color-Corrected Prism Systems
Color correction is achieved by using two types of glass for each prism element, with each prism element thus being made up of two prisms cemented together.
Many HT buffs believe that color-correcting prism system eliminates both color-aberration and astigmatism. This is untrue. Color-corrected prisms still exhibit roughly the same level of astigmatism as non color-corrected prisms.
This is illustrated by removing the color aberration from Image #3 above and presenting it as Image #4 below.
Note that the color aberration has been fixed but, because color aberration and astigmatism are unrelated, the modelled prism system in Image #4 still exhibits the differential focus problems discussed above in relation to Images #1, #2 and #3.
Color-corrected, compensated prism system
To fully correct a prism system for both astigmatism and color-aberration, a cylindrical element must be added either before or after the prisms to bring the entire screen area into focus, while maintaining good color-correction performance. In effect, this type of lens is a combination prism-cylindrical system (although the extra astigmatism correction element is only a very weak powered cylindrical lens).
However, due to the extra weight involved in using prisms (especially including heavy, color-correcting flint glasses), and the extra size required to maintain throw ratio performance, any such prism system will be quite heavy compared to a basically equivalent cylindrical system.
Above is a size comparison of the Horizon and an equivalent color and astigmatism corrected prism system that I designed for the purposes of seeing which was bigger and by how much. The prism system is approximately the same height, but nearly twice as long as the Horizon. It weighs 2.5 times as much (9.5 pounds v. 3.8 pounds).
In Conclusion
The discussion above does not claim that any cylindrical system must perform as well or better than any prism system. Design and manufacturing tolerances still need to be maintained to produce an excellent cylindrical system that clearly outperforms a prism system.
However, this much is true: the design flexibility afforded by using the complex curved surfaces of a cylindrical system, will always present better potential for astigmatism, distortion and color correction than can possibly be afforded with a prism system. This is due simly to the greater freedom of design and curvature options that a cylindrical system affords.
