– Super Ikonta B 532/16 – Zeiss Ikon | –
This is my second Super Ikonta B. It is newer that the first one and has a coated lens. In this post I will focus in this aspect of anti-reflection lens treatment. The coating is a thin layer of metal covering the glass surface. It can be multicoated going up to several layers . The physics involved was studied by John William Strutt, 3rd Baron Rayleigh (1842-1919), an English physicist. As an industrial process, it was first developed by the German firm Zeiss (Zeiss Ikon produced cameras and Zeiss is the optical concern).
It may sound odd to think of metal covering glass and rendering it more transparent than before, but that is what really happens. It is, of course, a very thin layer going to atomic dimension. It is a vapor of several metals that is deposited over the glass surface. To understand how this layers help in admitting more light into the glass I will refer to a macro experience that everyone is acquainted with. Haven’t you ever tried throwing flat stones over a calm lake water surface? What happens? Stones are bounced back when they hit the water surface. The game is about trying to make it bounce several times before loosing speed and finally sinking in. Well, we may say that due to different reasons light does the same when trying to penetrate the glass. But as there is nothing like gravity pulling it down, what happens is that part penetrate and part is reflected and go away. Like the stone it is more noticeable when the light direction gets more parallel to the glass surface. Now, what if we could have over the water some layers softer than water in order to make the transition less abrupt? The bouncing effect would be reduced. Imagine we could have some foam, for instance. The metal layer over glass does what the foam would do in the stone air/water transition. To light, the metal layer is softer than glass and has an effect of reducing the bouncing effect. The harder the transition, the more light is reflected. Hardness, in this case, means going from a medium with a certain refractive index to another with a much higher refractive index. Having thin metal layers with gradual increase of refractive indexes will lead to more light penetrating than being reflected. A reduction of 50% is standard in current lens treatment. But is important to bear in mind that for angles normally used in photographic lenses the reflexion is only a minimal part compared to transmission. Most of the light passes. So 50% means half of something that is already small.
Bottom line is that the lens becomes more transparent, darker in a camera, when we look at it, because there is less light being reflected to our eyes. In the picture above we have the same light condition, the same lens, and the one coated, on the right, allows us to see through it in a better way. More light penetrates the optical assembly so we marginally gain in actual f stops with the same geometry. Depending on the lens construction it can be a dramatic increase. Less light gets lost (uncontrolled reflections) inside the optical assembly, so less undesirable light reaches the film, thanks to lens coating. We should expect pictures with more contrast with coated lenses, in theory, but I never read a comparison about it. Modern zoom lenses with 15 elements would be impossible without coating. In this case, not only improved transmission is a goal, but also destroying what is reflected is also considered. For that, the “intereference” of waves going forward an backward is arranged to kill the already minimised reflected part.
In old Tessars, with 4 elements in 3 groups the coating effect is not that visible in not so contrasty situations. To identify whether or not a lens is coated it is enough to see if there is some colour on it. Normally there is a light deviation to blue or yellow.
Below pictures taken in a condition in which the coating for sure didn’t make any difference in the final result.