An introduction to
anamorphic cinema optics

Featured

The three steps to Home Cinema ‘scope images:
Image On Disk + V-Stretch + Anamorphic Lens

Anamorphic cinema is the process of shooting a movie by optically squeezing a wide panoramic view onto a standard film format, and then projecting it through an expanding lens to restore the image to its correct geometric proportions.

Without anamorphic optics, trademarked anamorphic systems include Cinemascope, VistaVision, Technirama, Panavision and Ultra Panavision, “2.35:1″, and many others that have come and gone would not have been possible.

This ability to optically widen a projected image allows Home Cinema users to view “Cinemascope” (TM) images in the way the producers intended them to be viewed: a panoramically wide screen, the same height as the native projector screen, but over twice as wide as its height.

Anamorphic cinema would not be possible without special optical lenses called anamorphic adapters.

Anamorphic adapters are optical devices used to either compress or expand a beam of light. The adapter is placed in front of either a camera (to squeeze the image) or a projector (to expand the image) and a focused optical beam is passed through them and comes out wider or narrower, but still in perfect focus.

Optical squeeze is usually used in cinematography. Optical expansion used only in cinema projection.

Cinema Projection Adapters
In the case of cinema projection, anamorphic adapters are used to widen an image by a fixed factor (the Home Cinema expansion factor is 1.33x, Digital Cinema 1.26x and Commercial Cinema 2.0x) while keeping image height constant (“CIH”, or Constant Image Height).

Figure 1. Anamorphic adapter aligned with an angled projector beam

The projector is first focused, then the adapter is placed in front of it and is adjusted to maintain the projector’s focus. The main difference between adapter-in and adapter-out is: the final image is wider.

Figure 2. 2x and 1.33x Anamorphic expansion

The reason images were squeezed and then expanded, rather than simply filmed wide in the first place with an ordinary camera lens, was because there was already an existing heavy investment in film format, standard and equipments… film and cameras were expensive.

If a stupendous panorama could be squeezed into an image of standard width on 35mm film, and then expanded in the cinema by reversing the squeeze process, this would save a lot of money. Cinemascope (TM) was a relatively cheap way of bringing “wide-screen” grandeur to cinemas and their audiences, in even the most remote one horse town in the country – as long as it had a cinema.

Prism-based adapters
Early anamorphic adapters were constructed from two prisms, aligned top-to-bottom and counter-rotated. Enhancements such as anti-reflection coating (AR Coating), color aberration correction (CA Correction) and correction lenses for improving focus helped prism-based adapters to deliver images of reasonable but limited quality, if used at the long throws typical of commercial cinemas.

Figure 3. Typical layout of a color-corrected prism adapter

But the big restriction is that prisms have only one type of surface – flat (or plano). There are only so many design “tricks” that can be employed to improve prism system performance, especially in the areas of distortion and image clarity.

Astigmatism
a mismatch in focus between horizontal and vertical image planes – can never be eliminated without the use of extra “corrective” lenses tacked onto a prism system.

Ghost images
due to internal reflections between internal plano surfaces are common.

Horizontal displacement
a shifting of the image sideways from the axis of the projector beam, is another (see diagram above).

Unequal optical path lengths
between the left and right-hand sides of the screen cause further loss of image symmetry.

These problems are lessened at throws over, say, 60 feet (20 metres), but in the Home Cinema environment, where throws can be as short as just a few metres, they become damaging to image quality.

In short, for the short throws and relatively wide screens typical of Home Cinema, prism lenses, with their restrictions on design freedom, cannot ever deliver the highest levels of optical performance.

Enter the cylindrical lens adapter…

Cylindrical Lens-based adapters
The highest quality anamorphic lenses achieve image expansion by using lenses made up of curved surfaces that are sections of cylinders, not sections of spheres, as in ordinary optical lenses. These lenses are called “cylindrical lenses”. The faces of cylindrical lenses are curved in only one direction (see Figure 4. below).


Figure 4. A cylindrical lens is a section of a cylinder

An anamorphic adapter made for cinema projection consists of a special combination of cylindrical lenses: at least one positive (converging) cylindrical lens at the entrance and at least one negative (diverging) cylindrical lens at the exit. These are arranged so that the air-gap between them “cancels” out their two focal lengths, resulting in an optical combination that has “infinite” focal length.

Figure 5. Basic form of an anamorphic adapter
“Classic” arrangement of one positive lens and one negative lens. (Viewed top-down)

The extra degrees of freedom in design afforded by the almost infinite number of curvature combinations that can be used in cylindrical lenses makes them ideal for the typically short throws and relatively large screens encountered Home Cinema.

Like a pane of glass (another object which has “infinite” focal length) a properly configured cylindrical anamorphic adapter does not alter the focus of the beam passing through it.

The ratio between the two focal lengths determines the expansion factor of the anamorphic adapter. This expansion only proceeds in the direction of the curvature of the lens. Hence an anamorphic adapter does not change image height, only image width.

In an ideal world with perfect lenses – and unfortunately there is no such thing – the air gap would never vary, but in practice the distance between the two lenses needs to be slightly varied to achieve focus of the adapter across the screen.

The phenomenon where anamorphic lenses may not focus both horizontal and vertical image planes evenly across the screen is called “astigmatism”. Hence, another name for focusing an anamorphic lens is “astigmatism correction”.

Prism-based adapters generally do not have the ability to be continuously focused in this way, at best relying on fixed “corrector” lenses that work best at a “sweet spot” throw distance.

Cylindrical adapters, however, do employ continuous focus (or astigmatism correction) allowing the sharpest image to be attained no matter how close or far the projector is placed from the screen.

Figure 6. Correcting astigmatism with a continuously focusable anamorphic adapter

Anamorphic adapters are not necessarily limited to two lenses. There may be additional lenses added to the “classic” positive and negative lens anamorphic combination. As a general rule in optics, the more complicated a design, the better it performs.

XEIT CM-4KR advanced anamorphic adapter
The XEIT CM-4KR is one of these advanced adapters. Its ground-breaking design incorporates extra lens elements above and beyond the simple “one positive and one negative lens” format illustrated above.

The extra lenses in the CM-4KR are there to enhance image quality in ways such as:

• Reducing distortion,
• Reduction of illumination hot spots,
• Capturing unwanted stray light,
• Maintaining edge-to-edge sharpness,
• Aspect Ratio better-maintained as focus changes,
• High contrast performance

These are enhancements that simpler designs cannot achieve without sacrificing basic quality.

The XEIT CM-4KR will provide a positive enhancement to any Home Cinema experience. The CM-4KR delivers state-of-the-art optical performance, allowing you and your friends to better appreciate the thrill of watching movies truly “as the director intended”.

Best to all,

TONY DUMMETT
XEIT CM-4KR Lens Designer and Manufacturer,
Sydney, Australia.