The Lake County Astronomical Society

Breaking the Lens Coating Code

Some lenses are touted as "fully coated" or "multicoated", while others are said to be "fully multicoated". Far from being just advertising gibberish, there is a difference. All refer to thin antireflective coatings of magnesium fluoride (MgF₂). Astronomy author, Philip Harrington, explains it as follows:

Coated = at least one optical surface is coated

Fully coated = all optical surfaces are coated

Multicoated = at least one optical surface is coated with multiple layers

Fully multicoated = all optical surfaces are multicoated

All air-to-glass surfaces coated = all open glass surfaces are coated (but this can be deceptive: in cheap telescopes some of the lenses may be made out of plastic)

Al Misiuk of Sirius Optics added that each lens to air that is left uncoated results in a 4% transmission loss. A single coating is of little use, but it can fool people into assuming all surfaces have been coated. Magnesium-fluoride coatings are excellent, and a little extra transmission can be gained with multi-layer anti-reflective coats. Obviously, fully multicoated is the best. Read the specs on different eyepieces to see what they say and check your own eyepieces (sometimes the coating type is printed on the barrel). You now can decipher the "coating code".