Rodenstock lens multicoating question

[jetcode]

Does older multi-coated glass provide as much contrast as the newer coatings?
Is APO a function of optical design and/or multi-coating?

[Dan Fromm]

Beats me.

The two have nothing to do with each other. Nothing at all.

[jetcode]

Beats me.

The two have nothing to do with each other. Nothing at all.

I take it those are two answers in sequential order. So APO must have to do with the way the glass is shaped for color correction while the multi-coating has to do with eliminating flare and internal relfections.

Thanks Dan - Joe

[Ted Harris]

APO - short for apochromatic. Very short version of the text book definition. The three light primary colors all come to the sharpest focus at the same distance.

You will find lengthy discussions of the way the term apochromatic is applied in modern lens naming and manufacture. Search the Forum.

[Oren Grad]

So APO must have to do with the way the glass is shaped for color correction

Also the choice of glass types.

[Robert A. Zeichner]

Achieving apochromatic performance in photographic lenses can entail a number of things. The use of different types of glass with distinctly different indexes of refraction and dispersion characteristics is certainly important. Using different shapes of elements in proper combination enables the necessary compensation of the bending and spreading of rays on their journey through the lens to the focal plane. If you consider the simplest of lenses, a biconvex element that looks like this (), you can imagine that rays from a single point on the subject striking various points on the front surface will do so at varying angles resulting in varying amounts of bending. The exiting rays all need to meet at the same relative point on the film plane unscathed. Since white light is made up of varying frequencies (colors) of light which bend at differing degrees, the trick is to get all these dispersed components to be reconstituted by the time it reaches the film and all in focus. This is not an easy thing to accomplish. One can appreciate the genius of optical designers who managed to calculate all of this with nothing more than a slide rule and some tables with which to do it. The development of computers and more recently the software that enables creating virtual models has made this task much much easier and a great deal faster. One other consequence of computer aided design and machining is the ability to create aspheric elements (having profiles with varying curvature). These have helped enormously in reducing the size and number of elements needed to correct these chromatic problems. This is by no means meant to be a detailed technical explanation, but rather an overview of what's involved. It's fascinating stuff to read about if you are so inclined.

[Nathan Potter]

Robert. the holy grail of lens design and manufacturing is the use of graded index glass. That is the index of refraction can be changed incrementally from the center to the edge of the lens element. A pretty challenging feat to accomplish during the glass casting I imagine. Are you aware of any recent implementation of this technique in a practical LF lens?

Nate Potter

[Robert A. Zeichner]

Robert. the holy grail of lens design and manufacturing is the use of graded index glass. That is the index of refraction can be changed incrementally from the center to the edge of the lens element. A pretty challenging feat to accomplish during the glass casting I imagine. Are you aware of any recent implementation of this technique in a practical LF lens?

Nate Potter

Wow, that's new to me. Sounds almost impossible to believe. My understanding is that the glass melt has to be mixed pretty thoroughly to achieve homogeneity. Is what you are saying that somehow more than one formula can be blended in such an accurate fashion as to create a cylinder of material that has a gradually changing RI from center to edge? Holy cow! I wonder how you could do that. I know that lens formulation has undergone quite a revolution in recent years due to RoHS compliance and I suppose some techniques might have been developed that might change the way casting is done, but I would sure like to learn about the technique to which you are referring. If you could point me to some site with detailed info, I'd be appreciative.

[Oren Grad]

Robert, the idea has been around for a long time. Google "GRIN lens" and see how much turns up - I was pretty amazed myself to see how much more progress has been made in practical application of the concept in the years since I last read about it.

See also the Wikipedia entry on "gradient-index optics" - especially the last paragraph about production via ion-exchange methods. Very cool.

I'm not aware of any ordinary photographic applications, though. Maybe there's one lurking somewhere in that Google search.

[Gene McCluney]

There were Apochromatic lenses "Apo" made long, long before lens coating was invented. Apochromatic len design has nothing to do with lens coatings. Lens coatings help minimize flare and increase contrast. Many if not most process lenses since at least the 1920's were apo lenses.