What the four labels count
Coated means at least one surface has a single coating layer. Fully coated means every air-to-glass surface has one. Multi-coated means at least one surface has several layers. Fully multi-coated means every air-to-glass surface has several.
The progression is not a scale of quality words; it is a count. A design with ten air-to-glass surfaces and one coated surface is honestly described as coated, and the nine untreated ones each reflect light away and scatter some of it back.
Alongside the coating labels sit prism glass designations — BAK-4 and BK-7 are the two seen constantly — which describe the glass a prism is cut from rather than anything applied to it. That distinction matters: a coating is added to a surface, a glass type is a property of the whole prism, and only the second decides whether the exit pupil comes out round.
What none of them promises
Transmission. No label states a percentage, and two fully multi-coated instruments can differ measurably. Where a maker publishes a transmission figure, that figure is worth more than any of these words.
They promise nothing about the optical design either. A poorly corrected eyepiece with excellent coatings is still a poorly corrected eyepiece, and coatings cannot recover a field that the design smears.
And they say nothing about durability. A coating can be soft enough to mark during cleaning, which is a reason to clean less rather than to distrust the label.
What they work with
Coating labels appear wherever light crosses glass: eyepieces and their sets, Barlow lenses, diagonals, binoculars, monoculars, spotting scopes and finders. The more air-to-glass surfaces a design has, the more the distinction matters.
On this site they are reported as the maker writes them, with the count they imply spelled out where it changes a comparison.
The questions that keep arriving
Is fully multi-coated always worth paying for?
On an instrument with many air-to-glass surfaces — a wide-field eyepiece, a pair of binoculars, a zoom — the difference in transmission and in scattered light is real and visible. On a simple two-element design there is much less to gain.
What do BAK-4 and BK-7 mean?
They are optical glass designations used for prisms, and the difference is the refractive index. BAK-4 is a barium crown glass of higher index than the borosilicate BK-7, which gives it a smaller critical angle for total internal reflection: light arriving at a wider range of angles is still reflected rather than lost. The visible result is that a BAK-4 prism delivers a fully round exit pupil where a BK-7 prism cuts its edges into a squared-off patch, dimming the periphery of the view.
Can I check a coating myself?
Partly. Look into the front of the instrument at an angle in daylight: coated surfaces show coloured reflections, and a surface with no coating shows a bright white one. Counting those reflections tells you more than the label does.
Last reviewed 17 September 2026