basic question about coating in multi-element groups

[John Schneider]

From a curiosity viewpoint, are the inner surfaces of lens elements that will be cemented together always coated? And from a practical standpoint, I have a 19" Dagor wherein only the two outer air-glass surfaces are coated -- how much more/less contrast will this have than either an uncoated example or one that has been coated on all glass surfaces (seeing that I don't have any of those to compare)?

[Gem Singer]

I have always been told that lens coating was only necessary on glass to air surfaces and not for cemented surfaces.

Is this true?

[Mark Sawyer]

The very outside surfaces (front and rear) are coated for efficiency reasons, as uncoated surfaces lose about 4% to reflection. Interior surfaces are coated for both efficiency and to reduce flare, (internal reflections). In most modern lenses, all surfaces are coated, although the glass-to-air surfaces are far more critical.

BTW, the Plasmat is also called the "air-spaced Dagor" because it's the same design but with a cemented doublet separated to allow more freedom in the design. In the early Dagors (pre-coating), it was more important to reduce the flare than wring out the last little bit of performance. Coatings made the concern moot, and the Plasmat became the prefered design.

[Oren Grad]

http://theonlinephotographer.typepad...310fb762c4970c

>> ...But how it was described to me is that most lenses are coated "strategically." That is, depending on their spec, price, and technical requirements, various surfaces might be multi-coated, single-coated, or even uncoated. <<

Mike was specifically talking about 35-format lenses in that discussion thread; I don't know whether/how it applies to LF lenses. Perhaps Bob Salomon will know about Rodenstock.

[Arne Croell]

The loss for an uncoated surface depends on the difference between the refractive indices (n) on each side of that surface - for air (n≈1) and a standard optical glass like BK7 with n≈1.5, that works out to the 4% Mark mentioned (for 90° incidence, more for other angles). Now in a cemented pair it depends equally on the indices of the glasses and of the cement - that is why some cemented interfaces are easier to see in reflections than other. Normally, these interfaces are not coated, that is right.

[Paul Fitzgerald]

" Now in a cemented pair it depends equally on the indices of the glasses and of the cement - that is why some cemented interfaces are easier to see in reflections than other. Normally, these interfaces are not coated, that is right."

Another thought, the early AR coatings may not have been hard enough to withstand bonding the elements together, separation problems that were not really needed.