THE ROLE OF MULTI-COATINGS

The Carl Zeiss T* multi-coatings used Contax, Carl Zeiss and Hasselblad camera lenses play an important part in a lens’ color and optical performance. Lens makers employ coating to reduce flare, ghosting and other unwanted lens characteristics. Flare occurs when light bounces off the lens elements; the multi-coatings help tame stray surface reflections. Ghosting are the faint circles in the shape of the aperture blades - sometimes a desired effect. 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.

The lens coatings also influence color. Many photographers characterize Zeiss’ T* colorings as cooler; I prefer to say the Canon lenses are warmer with strong(er) tendencies in the red channel. Canon’s reds work well for sunset pictures, but the reds often cause issues (for me) with skin tones. I feel the Canon lenses have a hyper-saturated look in the red, yellow and green channels. The Contax colors are more controlled in the reds and yellows; overall the Zeiss colorings tend to come across as more blue. This is most apparent when looking at the same scene taken with a Contax and a Canon lens. For portraits the Zeiss colors look more neutral to my eye.

Rollei had their HFT coatings and these a variation of the Zeiss T* “recipe”. More information on Rollei coatings is available here. And as FYI, Canon refers to their multi-coatings as “‘Super Spectra’ lens element coatings”; click here for more information via Canon’s U.K. website. Of course Zeiss doesn’t publish the T* recipe - that is a trade secret. The following excerpt from an essay written by Alexander Lee does provide some insight into multi-coatings without going too deeply into the topic.

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 very concise 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.

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.

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

If you would like to read more about manual lenses, the http://forum.manualfocus.org/ website may be helpful.  Also, another great source of information about bokeh are the following sites - http://bokehtests.com/Site/About_Bokeh.html and www.vanwalree.com/optics/bokeh.html. While bokeh has little to do with T* coatings, I’m guessing if you’re reading about the Zeiss T* coatings, then bokeh will probably be an interest too.