WJC
Well-known member
Although I have been back on BF a relatively short time, I’ve come to see that being thorough in my comments, or having considerable practical experience in a subject, is not enough to keep me from having my assertions questioned, denied, altered, or unnecessarily extrapolated upon. Still, while I don’t intend to get into a peeing match about it, I would like to comment on coatings, since it seems to be a topic of never ending interest and conversation.
Few features in binocular ads are touted more than the effectiveness of the company’s proprietary lens coatings. Anti-reflective or AR coatings were developed by Zeiss in Germany, and Bausch and Lomb in the United States in the mid-1930s, and kept secret from the outside world throughout the Second World War.
Created to make images brighter and more contrasty, it’s been said that at the beginning of the WWII, only about 50% of the light striking an objective lens made to through the instrument. The rest was absorbed into the glass, or reflected outside the primary light cone by the optics or internal surfaces.
Some companies try to illustrate the benefits of their proprietary coatings by showing a photograph, half of which is sharp and contrasty, with the other half looking as if it were shot through a piece of frosted glass. This isn’t a fair comparison, as some photographs show a greater difference in clarity than one would find when comparing an uncoated lens system to the finest fully multi-coated example.
The bottom line? Switching from a binocular with magnesium fluoride coatings—which have been the industry standard for more than 80 years—to one with today’s multi-coatings on the same optical surfaces, an observer may experience an increase in light transmission of around 12%-14%. When comparing the multi-coatings of two leading manufacturers, however, you’re splitting hairs with an axe. The difference is below the perception threshold of even the most seasoned observer, and should certainly not be used exclusively when making a buying decision.
Those observers who note a difference are usually seeing differences in:
— Figuring
— Polishing
— Baffling
— Blackening lens and/or prism edges
— Prism shields
— Prism type
— Prism slotting
— Field stop positioning
— Field stop knife-edge
— Reflections from the rear eyelens
— Positioning of the eye relative to the exit pupil
— Psychology of performance
— Others
Finally:
Some observers profess to tell the effectiveness of AR coatings by the color of the fluoride deposited onto the lens. This is not a valid gauge. In the late 1950s and early ‘60s, binoculars with deep blue tints were popular, and many people supposed they were seeing better because of them. However, in conversations with the late Humphrey Swift, of Swift Instruments, I learned many of those beautiful coatings were a by-product of magnesium fluoride being deposited at a temperature short of ideal for the task, as a production shortcut.
Those coatings didn’t adhere well to the glass, and came off with the acetone used by professional opticians during repairs and adjustments. This gave rise to the persistent notion that “acetone will take off lens coatings!” This assessment—usually made by armchair opticians—is simply not true. Poorly applied, those coatings were removed during cleaning by inordinately light abrasion.
Some people have also speculated that the purpose of lens coatings is to “protect” the glass. This belief is, at best, based on poorly interpreted information. It’s true that magnesium fluoride is harder than the glass beneath—575 on the Knoop hardness scale vs. 520 for Bk7 glass. At a thickness of four millionths of an inch, however, it offers little by way of protection. This is something to consider the next time you start cleaning the optics of your best binocular with a dirty shirttail.
Few features in binocular ads are touted more than the effectiveness of the company’s proprietary lens coatings. Anti-reflective or AR coatings were developed by Zeiss in Germany, and Bausch and Lomb in the United States in the mid-1930s, and kept secret from the outside world throughout the Second World War.
Created to make images brighter and more contrasty, it’s been said that at the beginning of the WWII, only about 50% of the light striking an objective lens made to through the instrument. The rest was absorbed into the glass, or reflected outside the primary light cone by the optics or internal surfaces.
Some companies try to illustrate the benefits of their proprietary coatings by showing a photograph, half of which is sharp and contrasty, with the other half looking as if it were shot through a piece of frosted glass. This isn’t a fair comparison, as some photographs show a greater difference in clarity than one would find when comparing an uncoated lens system to the finest fully multi-coated example.
The bottom line? Switching from a binocular with magnesium fluoride coatings—which have been the industry standard for more than 80 years—to one with today’s multi-coatings on the same optical surfaces, an observer may experience an increase in light transmission of around 12%-14%. When comparing the multi-coatings of two leading manufacturers, however, you’re splitting hairs with an axe. The difference is below the perception threshold of even the most seasoned observer, and should certainly not be used exclusively when making a buying decision.
Those observers who note a difference are usually seeing differences in:
— Figuring
— Polishing
— Baffling
— Blackening lens and/or prism edges
— Prism shields
— Prism type
— Prism slotting
— Field stop positioning
— Field stop knife-edge
— Reflections from the rear eyelens
— Positioning of the eye relative to the exit pupil
— Psychology of performance
— Others
Finally:
Some observers profess to tell the effectiveness of AR coatings by the color of the fluoride deposited onto the lens. This is not a valid gauge. In the late 1950s and early ‘60s, binoculars with deep blue tints were popular, and many people supposed they were seeing better because of them. However, in conversations with the late Humphrey Swift, of Swift Instruments, I learned many of those beautiful coatings were a by-product of magnesium fluoride being deposited at a temperature short of ideal for the task, as a production shortcut.
Those coatings didn’t adhere well to the glass, and came off with the acetone used by professional opticians during repairs and adjustments. This gave rise to the persistent notion that “acetone will take off lens coatings!” This assessment—usually made by armchair opticians—is simply not true. Poorly applied, those coatings were removed during cleaning by inordinately light abrasion.
Some people have also speculated that the purpose of lens coatings is to “protect” the glass. This belief is, at best, based on poorly interpreted information. It’s true that magnesium fluoride is harder than the glass beneath—575 on the Knoop hardness scale vs. 520 for Bk7 glass. At a thickness of four millionths of an inch, however, it offers little by way of protection. This is something to consider the next time you start cleaning the optics of your best binocular with a dirty shirttail.
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