With the recent interest in Zeiss lenses, we've read alot of posts regarding the T* coatings and their impact on image quality. On the buy/sell board we've seen MM version sell for more the AE version based upon the theory that the MM versions have improved or different coatings compared to their earlier AE versions. Since I haven't seen a any references to factual date supporting this argument, I did a little research and here's what I found:
Carl Zeiss T* anti-reflection coatings ensure the virtual elimination of unwanted reflections inside the lens. This means that only image-forming light reaches the image plane of the camera. Your photographs will have greater impact due to higher contrast, deeper color saturation, more shadow detail and cleaner, crisper colors.
Each of the seven layers of the T* anti-reflection coating is optimized for a portion of the total bandwidth of white light and together the seven layers cover the entire visible spectrum. The thickness of each coating layer is 1/4 wavelength of the frequency of light that the layer is dedicated to. Light is phase shifted 1/4 wavelength as it passes through the coating the first time. If the photon is reflected, it passes through the anti-reflection coating again and as it passes, it is phase shifted again 1/4 wavelength. Now, the reflected photon is 180 degrees out of phase and it is annihilated.
Carl Zeiss invented anti-reflection coatings and their continued dedication to the elimination of internal reflections is complete. T* optical coating represents the most advanced optical technology available. It is so good you can see the difference. And T* coatings are only available on Carl Zeiss lenses.
What is the definition of “T*” as it is used in Carl Zeiss T* Lens?
T* is the symbol Carl Zeiss chooses to use for its lens element coating technology. The coatings serve two purposes. First, the coatings reduce reflections from the internal lens element surfaces. The reduction in reflections increases the transmission of light through the lens thereby reducing internal flare. Carl Zeiss claims that their transmission standards are unsurpassed.
Second, the coatings are used to maintain a constant chromatic transmission between different Carl Zeiss lenses. Carl Zeiss claims they are the only lens manufacturer to do this. A constant chromatic transmission insures that the color spectrum produced by the CZ G 28-mm. lens, for example, is the same as those produced by any other CZ G lens. For that matter, the color spectrum is the same chromatic spectrum as that produced by any other CZ lens irrespective of lens mount or number of lens elements. No other lens manufacturer claims such a result.
The T* coatings are formulated in Germany taking into consideration the specific glass composition types used in each lens element of a particular lens. The coatings are then shipped to the lens manufacturing operations where they are applied only to the specified glass composition elements for that particular lens. Production examples of the assembled lenses are tested to verify the results of the coatings.
Comment: This pretty much sinks the idea that the MM categorically has different/better coatings than AE.
Color reproduction (Spectral transmittance)
Color transmittance of a lens is an important factor in determining lens quality along with resolution and contrast measurement. Every Carl Zeiss T* lens has the same spectral transmittance and the color does not change even if the lens itself changes. It is a closely held trade secret how Carl Zeiss accomplishes this task. The T* coating is applied in Japan to the Carl Zeiss lenses by prescription generated in Oberkochen, Germany. This coating material is sent from Germany to be applied to each lens as it is produced.
A lens is inspected and assembled under standards that dictate the spectral transmittance of each surface of each lens element from 360nm to 700nm corresponding to the range of visible light. Then the overall transmittance of the assembled lens is tested again, and this measurement must meet the specifications generated in Germany for each production batch. There is no difference between a Japanese manufactured lens and a German lens. Any other manufacturer would say it is impossible to assemble a lens from different types of glass having different spectral responses and then correcting this mixture by varying the coatings to obtain a uniform result.Carl Zeiss does this continually on a production basis. No other optical manufacturer in the world is capable of this performance.
Many people believe that the color of their lens multicoating layer indicates the inherent coloration of the lens. That isn't true. The color of the multi-coating layer on the front of the lens indicates only the reflected light coloration from the lens coating layer(s). We are concerned with the light transmitted through the lens. See our Lens Recoating FAQ for more on lens coatings.
Thanks to Richard Knoppow, we learn:
You can get some idea of what kind of coating is on a lens by the color of the residual reflection. Single coated lenses typically have either a magenta color (when the coating is peaked in the green) or a straw color (when the coating is peaked in the blue). Some multi coated lenses have peaks at the two ends and look green. A truely broad band coating whould have little residual reflection and it would be pretty neutral in color.
Again, the color of the light reflected by the multicoating layer is not related to the color filtering of the light which travels through the various optical glass elements to reach your film. While most optical glass used in lenses is pretty close to neutral in coloration, there is a slight but detectable lens color effect in many lenses.
I'm not reading anything here which supports the theory that MM lenses categorically have different or better coatings than the AE lenses. I'm sure Zeiss has changed their formula over the years, but those changes can't be linked to MM change - which is just a simple mechanical update to control the aperture from the camera.
Even if you do see a differences between the same AE and MM lens, you can't assume it's the coatings. It could be differences in the glass composition, optical alignment, optical design, changes in the barrel design (ie refraction).
You're right, John. AE or MM denotes the mechanical change and not the optical formula or coating. Interestingly the color of the coating looks very different: the 50 f1.4 is clear, while the 35-135 looks very green, and there are ones that look a bit purple!
The coatings are optimized for each lens - so comparing a 50 to 35-70 is apple & oranges. T* ensures a constant coloring across the Zeiss lenses. It's not a constant formulation unviersally applied to all Zeiss lenses.
The closest thing you could compare are two of the same lenses - say a 50/1,4 AE and 50/1,4 MM and see if the front optic reflects differently. But, that only tells about the front optic (and rear) optic - not the internals. Each optic within the lens is also coated.
I suspect if you took the same MM lens that was in production for 10 years and compared one from year 1 to year 5 to year 10 - you'd see differences.
Guess I should repost the following pic. As it shows in part, color of the reflection varies with each lens design and with each air-glass interface. The only design I have in duplicate (P50/1.4 MM) shows identical coloration. FWIW.
We can read as much as we want but there are 2 basic principles as to why there are differences in T* coating between type of lens and between the MM versus AE lens of the same focal length lens. The question is what is the aims and purposes of differences in T* multicoatings of Zeiss coating for each type of lens and what can Zeiss do to improve upon their already famous T* coating so that they can stay in competition or in this case on top of the competitions.
1) Simply to have every lens in the Carl Zeiss lens in the lineup to yield the same color for the output. It does not matter what optical elemental formulas that the light must transmit through in the lens whether short or long, fast or slow lens but the out put must yield the same color rendition. Thus the goal of Zeiss or any manufactory is to produce the same looks in the photographs. They do not like to have one lens produce more yellowish and other more neutral. Thus you see that most of Zeiss lens produce the similar color quality. This explains why longer tele lens with anomalous dispersion glass such as the ApoSonnar 200mm F2.0 has a different coating then the 50mm F1.4 and so on. The different in color coating is to compensate so that all lens from Zeiss will yield the same looks.
2) Improvement of technology and the quest of research for better optics is the second goal. Thus resulting in changes in the multicoating of the famous T*. The MM versus AE is not about the coating but it is about the extra black plastic fin at the end of the rear lens to allow the Contax camera be able to do shutter priority. After this mechanical conversion the coating technology also changed. Thus there are MM lens that has the same T* coating for the AE mount but no AE mount has the later MM lens coating. Zeiss as any other optic companies continue to do research and with the advance in technology has been able to improve upon their original famous T* coating. However, the improvement is there for some folks to see and for other it is difficult to see since Zeiss older T* coating is so advanced that the image quality is so good that the newer techonogy T* coating is not as noticeable among Zeiss user. Come on the reality is that with the internet and the more testing of these lens on digital cameras and the sharing of information we are now noticing it more of the information. To me the older T* coating is very good as well as the newer T* coating.
As it shows in part, color of the reflection varies with each lens design and with each air-glass interface. The only design I have in duplicate (P50/1.4 MM) shows identical coloration. FWIW.
