What’s the difference? A Porro assembly consists of two prisms, each like a right-angle triangle. Traditional binoculars use Porro prisms, while some modern types use roof prisms. Light gathered by the objectives travels down the barrels to the prisms, where it is reflected a couple of times.

The coating is fairly, which is why it's important to know how to clean binoculars properly (without damaging them.) Prism break Roof prism binos (see below) may also have a phase correction coating to reduce internal loss of light. Your lenses will be described as either coated (a single coat on at least one surface), fully coated (single coat on all major surfaces), or multi-coated – meaning more than one layer on several surfaces. A strong green color might indicate the coating is too thick and will have poor performance. A blue color usually indicates a single magnesium fluoride coat, while two-tone (blue/purple or purple/amber for instance) points to multi-coating – a sign of better quality.

Manufacturers keep their coatings recipes a closely-guarded secret, but they're usually based on magnesium fluoride or calcium fluoride.

Good quality coatings reduce this to a fraction of one percent per surface, meaning the whole system may lose only 5% or so.Ī blue/ purple sheen on lenses may indicate that they have multiple layers of coating. Multiply this by the numerous surfaces – each side of each lens, plus the prisms – and as much as a third of the total incident light might be wasted. These make a huge difference, but transmissivity can never be 100%.Īn uncoated lens surface loses around 4-5% of the light passing through it. Lenses are treated with special coatings to reduce reflections. Each optical surface causes light to be lost: some is reflected back, and a little is simply absorbed. Not all the light that enters the binoculars will make its way to your eyeball. If you're really keen on maxing out things like aperture and magnification, then you may want to opt for one of the best telescopes on the market instead. As a side note, while those 70mm lenses are wonderful for wildlife watching, they're a little unwieldy if you want to point them at the sky – which is why we reckon that the best binoculars for stargazing usually have 50mm lenses.

Prisms for photography