Optical Coatings Prisms Explained

For all you number crunchers, and anyone that is interested, here is some technical information on lens coatings.
Lens coatings are there to help give you a clear image and to stop as much of the light escaping or reflecting away.(uncoated lenses lose 50% of the light gathered.)
Most binocular lenses are coated to level one standard which,in daylight is more than adequate.
This makes the binocular cheaper to produce, but less quality here will impact directly on the image you view.

The more light gathered means a better image so Level IV, whilst the most expensive coating, gives the best image in lower light conditions, not all binoculars are the same.

Level I: Magnesium Fluoride coating on 16 glass-to-air surfaces: 4 for two objectives, 12 (6 per side) for 6 eyepieces (3 per side). The four long sides of the four prisms are not coated - it costs more to coat a prism than an objective or an eyepiece. The coating is only one layer.

Level II: Broadband multi-coatings (5-7 layers) on the 4 glass-to-air surfaces of the two objectives, and 4 surfaces of the two outmost (ocular) eyepieces (one per side). Magnesium Fluoride coating on the other surfaces: 4 long sides of the 4 prisms and 8 glass-to-air surfaces of the 4 eyepieces (the lens in-between the outmost eyepieces and the prisms). To sum up, broadband multi-coatings on 8 surfaces, one layer Magnesium Fluoride coating on 12 surfaces.

Level III: Broadband multi-coatings on all the surfaces except 4 long sides of the 4 prisms one which there are still Magnesium Fluoride coatings.

Level IV: Broadband multi-coatings on all the surfaces (listed above) including 4 long sides of the 4 prisms. This is the ultimate coating solution.




PRISMS
A brief word on prisms, there are two types of prism used in the construction of binoculars today, BAK 7 and BAK 4.
Of the two the BAK 4 type is more dense and therefore more costly to produce.
BAK 7 are more commonly used as they are cheaper to produce but offer slightly less capability than the more expensive BAK 4 prism.
Again both are more than adequate at what they do, but BAK 4 processes more light giving you clearer sharper images.