T* Coating

Technical Details

T* coatings consist of 7-15 alternating layers of various materials such as magnesium fluoride (MgF₂), titanium oxide (TiO₂), and tantalum pentoxide (Ta₂O₅) with layer thicknesses of 50-150 nanometers. Spectral transmission reaches peak values of 99.7-99.9% at a wavelength of 550nm. Modern T* systems use Ion Beam Sputtering for more uniform layer thicknesses with tolerances below ±2nm. Variants include standard T* for visible light (400-700nm) and Broadband T* for extended spectral ranges (350-1000nm).

History & Development

Carl Zeiss introduced T* coating for their Contax lenses in 1972 and transferred the technology to Master Prime series cinema lenses in 1975. The development was based on military riflescope coatings from the 1960s. In 1983, Zeiss expanded the system with multi-color interference coatings, followed by moisture-resistant top layers in 1995. Since 2010, Zeiss has been using plasma-assisted coating processes for even more homogeneous layer distribution.

Practical Use in Film

T*-coated Master Primes were used in "Blade Runner 2049" (2017) to minimize stray light in LED-intensive night scenes. The coating reduces ghosting effects by up to 85% compared to standard coatings and enables higher contrast images in backlight situations. In "1917" (2019), cinematographers used T* lenses for the long, continuous takes, as the low internal reflection ensures consistent exposure values across different lighting situations.

Comparison & Alternatives

While standard multi-layer coatings exhibit 1-3% reflection loss, T* coating is below 0.5% per surface. Cooke's S4/i series uses similar "Panchro/i" coatings with comparable transmission values. Arri/Zeiss Ultra Primes rely on modified T* systems with additional UV-blocking layers. For extreme wide-angle lenses (below 14mm), cinematographers often prefer uncoated front elements to avoid color shifts caused by layer interference.