THE ROLE OF MULTI-COATINGS

The Carl Zeiss T* multi-coatings used on Contax, Carl Zeiss and Hasselblad camera lenses play an important part in a lens’ color and optical performance. Lens coatings reduce flare, ghosting and other unwanted lens optical issues. In the 1940’s and 1950’s single coatings were common. In the 1970’s multi-coating became popular. Today just about everything is multicoated - even our filters.

Many photographers characterize the Carl Zeiss T* colorings as cooler; I prefer to say the Canon lenses are warmer with a stronger red channel. Canon’s reds work well for sunset pictures, but the reds can cause issues with skin tones. At times the Canon lenses have a hyper-saturated look in the red, yellow and green color ranges. The Zeiss colors are more controlled in the reds and yellows; overall the Zeiss colorings tend to come across as more blue. For portraits the Zeiss colors look more neutral to my eye.

Rollei had their HFT coatings, a variation of the Zeiss T* - more information  here. Canon refers to their multi-coatings as “‘Super Spectra’ lens element coatings”, more information at Canon’s website. Zeiss does not publish the T* recipe since it is patented trade secret. The following essay written by Alexander Lee provides some insight without going too deep -

ALEXANDER LEE’S ESSAY

In 1999 Alexander Lee published some of research on the web concerning the history of Carl Zeiss camera lenses. One section has some information about the T* coatings and how the single and multiple coating lenses effect lens performance. The full article can be found at this link, the following excerpt is the section I found most interesting -

“Before coating, each transmission surface resulted in about a 4% to 8% loss of light to reflection depending on the refractive index of the glass. So an uncoated Dagor or Protar with four transmission surfaces looses 15% to 29% of the light to flare. An uncoated Tessar looses 22% to 40% of the light to flare. An uncoated Planar with eight surfaces looses 28% to 49% of light to flare. The flare would exhibit itself on the film as unfocused non-image forming light which reduced the contrast of the picture.

1. •Single Coating - After single coating, this dropped to about 2% to 4% loss of light per transmission surface. Applying the coating at quarter wavelength thickness could greatly increase the effectiveness of the coat, but it could completely block some wavelengths of light and partially block others. Typically blue-green wavelengths were suppressed with an amber coat, or green wavelengths with a purple coat.
   

2. •Multi-coating - Multi-coating was first done as two separate coats at different wavelength thickness on different transmission surfaces to balance the color of the light transmitted to the film. Later, multi-coating as we know it, one coat stacked on another (first used on a production lens by Leitz) reduced the light lost to diffraction further to about 1/2% to 1% per transmission surface. The classic second coat was bismuth oxide again applied at quarter wavelength thickness for a different wavelength, typically orange-yellow for the second coat and green-blue for the first coat giving a faint green reflection. A multi-coated Planar could now only loose about 4% to 8% of the light to flare, quite a difference.
   

Coating and multi-coating allowed designers to use more complex designs with more air spaces which allowed easier design for correction of spherical aberrations. The difference between uncoated lenses and coated lenses are great, the difference between single coating and multi-coating is visible, but not nearly as great as the first leap from uncoated to coated. Coating and multi-coating opened the way for many otherwise unfeasible modern lens designs, such as complex wide-angle lenses, big multi-element zooms, and lots of marketing hype. Coating still won't save you from nasty flare in certain lighting conditions, such as shooting into the sun, so make sure to use those lens shades!”  - Alexander Lee, 1999.

ADDITIONAL INFORMATION

For some related information on lens bokeh - bokehtests.com and www.vanwalree.com. In April 2010 Dr. H. H. Nasse from Carl Zeiss published a lengthy white paper describing DOF and bokeh as well.