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.

[Robert A. Zeichner]

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.

That is pretty amazing, although most of the application-related material I found has to do with fiber optics. The Ion-exchange process is very cool. It seems though that it would be an incredible feat to create the equivalent of a conventional aspheric element using this technique.

[Struan Gray]

I have and use some GRIN achromat singlets for coupling a laser into and out of an optic fiber. They work very well, and removing one interface helps reduce backscattering. But, using the bigger ones as loupes, or projecting an image onto a sheet of paper shows a lot of aberration around the edges of a very restricted field. The ones I have at least are not good for wide-field imaging. They, and most of the others I have seen, are designed to focus a beam of parallel rays onto as tight a spot as possible. The rest of the focal plane is not a priority.

I'm no expert, but my impression is that a GRIN element is like an asphere, it allows you to eliminate a surface elsewhere in the design. For laser applications it also has the nice property that the front and rear faces of the lens can be flat, which simplifies handling, mounting and alignment.

[Robert A. Zeichner]

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.

Indeed. When you think about it, most of what is known about lens design was understood before the turn of the century..... the previous one, that is. Before 1900, The Zeiss company offered 57 varieties of optical glass (thanks mostly to F.O. Schott). Designs like the Tessar are still produced and I believe that formula is nearly a century old.

Apochromats were necessary for graphics arts reproduction, even before color lithography was commonplace, this because if you wanted to reproduce a color painting, for example, you still needed to have a way to bring all visible wavelengths of light into focus on the same plane. And yes, before the Red Dot Artar, came the plain old Apochromatic Artar (and others) with no coating.

The business of multi-coating has been around since the 1970's. The idea was that if a single coating could reduce reflection at the air/glass boundary by a couple %, perhaps using a couple of different types of coatings could reduce the remaining bit of reflection to a fraction of that. By reducing the reflections at all of the air/glass boundaries within a lens (particularly a complex design with lots of them), you could prevent these reflected rays from bouncing back and forth and increasing the flare that results from it. Still, you are talking about the last little bit of correction. A lens design has to embody good contrast characteristics to start with. That accomplished, I imagine the very latest multi-coatings would make some difference. And there is no substitute for efficient shading, particularly with lenses that have coverage that far exceeds the film area. I have seen differences (subtle, but noticeable) on identical negatives shot with and without efficient shades and made with lenses that are brand new multi-coated types.

[Arne Croell]

Indeed. When you think about it, most of what is known about lens design was understood before the turn of the century..... the previous one, that is. Before 1900, The Zeiss company offered 57 varieties of optical glass (thanks mostly to F.O. Schott). Designs like the Tessar are still produced and I believe that formula is nearly a century old.

Apochromats were necessary for graphics arts reproduction, even before color lithography was commonplace, this because if you wanted to reproduce a color painting, for example, you still needed to have a way to bring all visible wavelengths of light into focus on the same plane. And yes, before the Red Dot Artar, came the plain old Apochromatic Artar (and others) with no coating.

Indeed again. The term apochromatic is slightly older than apochromatic process lenses and was first coined by Ernst Abbe, and employed for microscope objectives designed by him in 1886. See:http://micro.magnet.fsu.edu/optics/timeline/people/abbe.html

[IanG]

My Multi Coated 90 f6.8 Grandagon isn't marked as an "Apo" lens, its coating are superb, I can shoot into the sun with no flare, and I mean with the sun in the image.

I've owned Multi coated lenses (Hoya) that were terrible (not LF) because not the all air/glass surfaces were coated, but the two Rodenstock multi-coated lenses I own are on a par with my SMC Pentax lenses and are outstanding.

Should add so are my Schneider Multi Coated lenses.

Ian

[Bob Salomon]

My Multi Coated 90 f6.8 Grandagon isn't marked as an "Apo" lens, its coating are superb, I can shoot into the sun with no flare, and I mean with the sun in the image.

I've owned Multi coated lenses (Hoya) that were terrible (not LF) because not the all air/glass surfaces were coated, but the two Rodenstock multi-coated lenses I own are on a par with my SMC Pentax lenses and are outstanding.

Should add so are my Schneider Multi Coated lenses.

Ian

None of the Grandagon lenses are APos. The Apo-Grandagon 35, 45 and 55mm are.

And your 90mm is probably a Grandagon-N not a Grandagon.