Lens coatings let the light in
Videographers buying a new camera often refer to the lens as the "glass," as in "what kind of glass are you going to put on that camera?"
High-definition and electronic-cinema videography have increased the performance requirements of new lenses, and while the types and quality of glass used remains vitally important, the coatings lens makers apply to that glass has become just as important.
Lens coatings were invented in the 1930s, and are the lens makers' secret sauce-closely guarded proprietary formulas. Technically, lens coating is a vacuum-deposition process that forms an extremely thin transparent film on the surface of lens elements.
Because of the importance of various coatings, lens manufacturers are not specific in explaining what goes into them.
Typical is this response about lens coatings from Fujinon Marketing Manager Dave Waddell.
"Many different proprietary coatings are used, depending on the type of glass and the desired effect," he said. "Some of these include magnesium fluoride and zirconium oxide."
Panavision Inc. Executive Vice President of R&D and Optics, Iain Neil, generalized the need for lens coatings.
"The main advantage of lens coatings for a lens maker is that they enable the lens designer to achieve complex lens systems in terms of both total number of lens elements employed and glass types utilized," he said.
There are several reasons coatings are employed, and primary among them is the ability to minimize reflection off element surfaces.
"In general, when light enters an element, approximately four to 10 percent of the light is reflected back at each element surface," said Gordon Tubbs, assistant director at Canon USA Broadcast and Communications Division.
LIGHT LOSS REDUCTION
That amount of light loss, on both entry and exit of a lens element, compounded over as many as 40 or more elements in a video or film lens, can severely limit the light that ultimately passes through the lens.
"You can see that if an uncoated lens reflects up to 10 percent of the light per surface, [with enough elements], there would be no light reaching the sensor," said Fujinon's Waddell.
Thales Angenieux U.S. Sales Manager Chris Beauparlant agrees.
"Without coatings, it would be impossible to make a lens with more than five elements because of the loss of transmission, ghost images and increase of flare due to the reflection of the glass," he said. "With coatings, the amount of elements is no longer an issue."
It becomes critical, therefore, to decrease the amount of light reflected from a lens element's surfaces, which results in more light passing through the element.
Because lens makers take advantage of the properties of many types of glass to build their lenses, different coatings must be used for each type of glass.
Canon's Tubbs said that his company uses several types of "multilayer" coatings, "optimized according to the index of refraction of the lens element to be coated."
Kornelius Mueller, marketing manager for the camera dvision of Carl Zeiss, puts a number on the light transmission performance lens coatings can bring.
"Single layer optical coatings can usually reduce this loss down to around one percent per surface," he said. "Well designed multilayer coatings, with three to ten layers, can reduce the loss to 0.3 or even 0.1 percent per surface."
What does that mean in terms of the light transmission improvement through a complete lens?
Panavision's Neil said, "Ignoring light losses by glass absorption, a contemporary 24-element cine zoom lens would have a transmission of 29 percent [without coatings] and 88 percent [with coatings]."
That equates to a nearly two-stop increase in performance.
Minimizing reflection off element surfaces has other positive effects on lens performance, Tubbs said.
"Repeated reflections between the element surfaces may cause flare or ghosting in the image," he said.
Mueller said well-designed coatings also help to minimize flare and ghosting, as well as an effect called Narcissus, named for a mythological character who was attracted to his own mirror image.
Coatings can also serve as protection for certain highly polished surfaces of exotic lens-element materials that need to be sealed against moisture or other corrosive influences.
Finally, light waves outside the visible spectrum-ultraviolet above and infrared below-can affect the image focused on a camera's sensors, causing them to be less sharp.
In Angenieux's case they use a dichroic coating that can take as many as 50 layers.
"Dichroic coatings are used to filter one part of the spectrum," said Beauparlant. "[They] have a very sharp edge on the spectral bandwidth."
As an example, an IR filter can be fully transparent at the low wavelength spectrum (400nm to 750nm) and a very sharp cut off at 750nm.
So while videographers may still refer to their lenses as "glass," the coatings on that glass will continue to play an important role in the design and performance of those lenses.