Anti-refection coatings have proven to be one of the most important inventions in modern optics. By reducing the natural tendency of glass-to-air surfaces to reflect a portion of the incoming light, they ensure that the maximum amount of light reaches the film instead of being reflected away from the lens surfaces or, worse, bouncing around inside the lens. Thanks to modern anti-refection coatings we are used to brilliant images in almost all lighting situations on the film set.

History 

First developed in the Carl Zeiss laboratories in 1935, anti-refection coatings found widespread adoption after 1945. These first coatings were single layer coatings which optimized transmission for one color only, leading to an uneven transmission behavior across the color spectrum. A significant improvement was introduced in the 70s when multilayer coatings were introduced, offering a further reduction in reflectance from glass-to-air surfaces in a broader spectral range. A highly sophisticated version of this technology is the Zeiss T* coating used in the ARRI/Zeiss Ultra Primes and Variable Primes.

T* XP

Originally developed for the ARRI/Zeiss Master Prime lenses, the multi layer T* XP (Extended Performance) anti-reflection coating ensures maximum transmission in a wide spectrum of wavelengths. The T* XP coating has been optimized with respect to the spectral sensitivity of motion picture film and the sensitivity of the human eye.

Designing the proper coating formula is one part of the high art of anti-refection coatings. The second part is the equally tricky art of applying the coating in the proper and even thickness to the lens elements. Zeiss uses a carefully monitored, elaborate process in high vacuum where special optical substances are evaporated one after the other and deposited on the lens surface with precisely controlled thickness. For the T* XP coating this process was further refined to assure a perfectly even and symmetrical application. It assures uniform performance across the whole lens surface from the center all the way to the edges. This is especially important on lenses with large, strongly curved surfaces. When compared to conventional multilayer coatings, the T* XP coating has up to five times better transmission at the edges.

The result is higher contrast, deeper blacks and a great reduction of false light effects such as internal reflections, veiling glare, flare and narcissism. In combination with proper internal lens construction, the T* XP coating ensures that the lens can easily handle tricky lighting situations like strong backlight, sunsets or car headlamps. Lenses using the T* XP coating can catch subtle tones in the deepest shadows and fully utilize the high dynamic range of modern film stocks.

This graph shows the reflectance of a lens surface with a conventional Anti-reflection coating at different frequencies oft he light spectrum. Notice how the red line, which represents the reflectance at the edge of a strongly curved lens, indicates a severe increase in reflectance starting at 580 Nanometers.

This graph shows the reflectance of a lens surface with the T* XP Anti-reflection coating at different frequencies of the light spectrum. Notice how the red line, which represents the reflectance at the edge of a strongly curved lens, essentially follows the yellow line, which represents the reflectance at the center of the lens.

In order to achieve a high quality image even at wide open T-stops, lens designers are using new shapes for the surfaces of individual lens elements. These new shapes help eliminate optical aberrations and distortions, while keeping the lens’ size and weight down. One of these shapes is the aspherical lens surface, as used in the ARRI/Zeiss Master Primes, Ultra Prime 8R, Ultra 16 lenses and Lightweight Zoom LWZ-1.

Sophisticated Technology

Most lenses have spherical surfaces, that is their surfaces have a constant curvature, as if the lens surface had once been part of a sphere. Aspherical lens surfaces, in contrast, have surfaces that have a different degree of curvature at the center and the edges. The sophisticated technology for creating such lens surfaces was originally developed for lenses for computer chip manufacturing. it requires ultra-high precision in manufacturing and a complex and expensive holographic measurement process for surface accuracy verification.

Holographic Measurement

Aspherical lens surfaces offer excellent correction of spherical aberration (the inability to focus all light rays from a point source onto a point on film, directly affecting resolution performance), chromatic aberration (color fringes) and geometric distortion (keeping straight lines straight). By using aspherical lens surfaces, the optical designer can create a lens that is smaller, lighter and optically better than lenses using only spherical surfaces.

An aspherical lens element of extreme curvature is known as a “radical aspherical lens”.

Spherical Lens vs. Aspherical Lens Design

The above illustration shows the basic principle of how spherical aberration is corrected. Spherical lens elements suffer from an inability to focus all light rays from a point source onto a point on film. Therefore additional lens elements are needed to compensate, making the lens heavier and introducing other performance issues. The further away a light ray is from the optical center, the more pronounced this aberration becomes, making it a crucial issue for fast lenses with their larger diameter elements.

In order to achieve a high quality image even at wide open T-stops, lens designers are using new shapes for the surfaces of individual lens elements. These new shapes help eliminate optical aberrations and distortions, while keeping the lens’ size and weight down. One of these shapes is the radical spherical lens surface. These surfaces have a very strong curvature; some are almost half-sphere shaped. They are called ‘spherical’ since the curvature remains constant.

Improved precision manufacturing and new measurement techniques have made the creation and verification of these lens surfaces possible. Still, radical spherical surfaces are cutting edge technology; they are difficult to grind, tricky to polish and demand precise attention during coating. However, mastering these manufacturing techniques brings the reward of incomparable optical performance at substantially reduced weight.

Radical spherical lens surfaces are used in the ARRI/Zeiss Ultra 16 lense and in the ARRI/Zeiss Lightweight Zoom LWZ-1.

Rectilinear’ and ‘Fisheye’ are two different optical designs for wide angle lenses, resulting in very different looks. All modern ARRI/Zeiss wide angle lenses, including the Ultra Prime 8R, are of rectilinear design.

Natural Geometric Relationships

When a lens projects a three-dimensional scene onto a two-dimensional piece of film, not all geometric properties of the original scene can be maintained. This is essentially the same problem as mapping the shape of the continents of our three-dimensional globe onto a two-dimensional map. The choices of lens design, focal length and distance to the subject determine the character of this mapping, which is commonly referred to as perspective, one of the cinematographer’s most important tools. For wide angle lenses, the lens designer must make a choice between a rectilinear or a fisheye lens design, with different consequences for perspective. The most obvious differences can be seen by how straight lines and objects at the edge of the frame appear.

Since the human eye judges distance by the way elements within a scene diminish in size and the angle at which lines converge, most lenses are designed to duplicate those “natural” geometric relationships on film. This is called a rectilinear perspective, and to achieve it the lens will stretch the image so that vertical, horizontal and diagonal lines that we perceive as being straight are reproduced as straight lines on film.

Straight Lines or Curves

There is, however, a limit as to how wide a lens with a rectilinear perspective can be, based on the limited amount of space available in front of the camera, and on various optical problems that get increasingly unwieldy as the angle of view increases. The 114° horizontal angle of view (for the Super 35 format) of the Ultra Prime 8R lens is already at the limit, making it a unique and unusual lens in the cine, video and still photography fields.

Because it is so difficult to design an extreme wide angle lens with a rectilinear perspective, many extreme wide angle lenses are designed as fisheye lenses. A fisheye lens can have a wider angle of view than a rectilinear lens, but it maps the scene to film differently than we perceive the world around us, because the focal length is actually changing within the image. The farther a straight line is from the center of the frame, the more it will be rendered as curved, with objects at the edges of the frame heavily distorted by a fisheye.

A rectilinear wide angle lens on the other hand renders all straight lines in the subject as straight lines in the image. To achieve this, though, there is linear stretching applied to the image that increases as an object gets closer to the frame edge. This effect tends to exaggerate perspective, i.e. it will make rooms appear larger than they are, enhancing the illusion of depth, or making speed appear greater if the camera moves. However, a circular object, like a ball or a person’s head, located near the edge of the frame will appear to be somewhat enlarged and will have an oval shape. Neither fisheye nor rectilinear wide angle lenses represent reality in quite the same way as we see it. They provide two different ways to manipulate perspective, to change the illusion of space and distance.

Diopters are optical elements that can be placed in front of a prime or zoom lens, offering a fast and convenient way to increase the lens’ magnification. In practical terms they move both the close and far focus distance of a lens closer to the film plane; on one hand the lens can focus closer than its native close focus distance, but on the other the far focus moves from infinity to a closer value. The strength or magnification of a Diopter is expressed in the ‘diopter power’ number. Common diopter powers are +0.5, +1 and +2.

Uses for Diopters
When a static shot of something small is needed, but the close focus distance of the lens is insufficient. For best optical quality at the highest magnification, use a single low power diopter with a long focal length prime lens. This will also leave ample room for lighting.

Using a diopter on a wide angle lens reduces the close focus distance by only a small amount but throws the background substantially out of focus, thus achieving a unique wide angle look with shallow depth of field.

A shot where the focus moves from something very close to the lens to a medium distance or vice versa.

Special effects work.

Advantages of Diopters

Having a set of diopters is standard procedure on many projects. They are used instead of macro lenses because they are less expensive and can thus be rented for the whole show. If an extreme close up is planned, a macro lens is rented, but often the situation arises where an unplanned close up is needed, and that is when a diopter can save the day. One cinematographer interviewed said: “A craftsman never knows when he needs a hammer, but he always carries one. It is the same with diopters.”

Some cinematographers like diopters because they match the look of the prime lens more than the traditional macros in terms of color balance and in terms of focal length. Also, traditional macros need more light than regular primes, and thus a scene would have to be re-lighted for a macro shot, which costs time and money and again results in a different look. Diopters need no exposure compensation, so a T1.3 lens stays a T1.3 lens.

The use of diopters is subject to local shooting styles just like the use of all other film equipment. Cinematographers on one continent, for instance, prefer to use them with long primes, as that gives a higher magnification ratio and allows for more space between subject and lens. Another continent is currently going through a phase where “close and wide” is popular, so they use diopters with wide angle primes and short zooms.

Diopters come in two basic types: regular Diopters that consist of one simple glass element, and achromatic diopters that consist of two or more glass elements. The optical quality of regular Diopters is poor, mostly due to chromatic aberration, also known as color fringing. They also show spherical distortion, geometric distortion, flare, poor contrast and poor edge resolution. Achromatic Diopters consist of at least two glass elements and are much higher in quality.

An achromatic assembly is a group of lenses designed to compensate for various unwanted optical effects. Achromatic assemblies are used in most modern cine lenses. They are also used in the +1 and +2 ARRI/Zeiss Master Diopters.

Achromatic Lens Assembly

An achromatic lens assembly ensures that all colors are focused at the same point. Achromatic assemblies are particularly good at compensating for chromatic aberration, an optical effect that can be seen as color fringing in the image, usually at high contrast edges. They also improve spherical aberration, an effect where a white point is projected not as a point but as an out of focus blurb.

In a two lens achromatic assembly, one lens is compensating the dispersion introduced by the other. For such purpose, one lens is usually made in a crown glass with low dispersion, while the other is made of a flint glass with high dispersion. The crown lens performs the desired optical effect (for example magnifying an image) and introduces some dispersion, and the flint lens aims at balancing this dispersion while having the least possible optical influence itself.

Optical Dispersion of a Single Lens

A single lens bends different colors at different angles.

When a beam of light passes through a single lens, the different colors are bend at different angles based on their wavelengths (called dispersion), and the lens will focus the different colors at different points. The result is chromatic aberration, an unwanted color fringing that will lower the lens’ resolution and that is also problematic in special effects matting and blue and green screen work.