ED glass binoculars and lens coatings

What ED glass does

Nikon says chromatic aberration is caused by a difference in light wavelength. Different colors bend by slightly different amounts, so they do not land on the same spot.

Nikon adds that ED (extra-low dispersion) glass reduces chromatic aberration to a remarkable degree compared with achromatic lenses. In practice, that means less colored fringing along branches, rooflines and the edge of the moon.

Only 146 of the 554 models list ED glass, so it is a minority feature. You can find them with the binoculars from $200 to $500, where the makers' documents name it most often among the mid-price bands.

Where ED glass pays off

Fringing shows most at high magnification and in high-contrast scenes. A 12x50 such as those in the 12x50 size group magnifies the defect more than an 8x42 does.

Celestron describes ED glass by its effect on a visual defect, not by a brightness gain. Treat it as a sharpness and color-accuracy feature, and do not expect a brighter image from it.

If you mostly glass wide scenes at 8x, the gain is smaller. The 8x42 models have a median field of view of 393 ft, which already gives a calm, easy view.

What the coating words mean

Celestron defines four terms by how many surfaces and layers carry coatings. Coated means one or more surfaces have optical coatings applied.

Fully coated means all air-to-glass surfaces have optical coatings applied. Multi-coated means one or more surfaces have multiple layers applied.

Fully multi-coated means all air-to-glass surfaces have multiple layers applied. The RSPB shop agrees that fully multi-coated means every lens element, and it calls that ideal.

Why layers matter

Nikon says anti-reflective coating goes on both surfaces of each lens. It comes as a single layer or as multilayer coating, usually 3-5 layers.

Nikon says multilayer coating cuts reflected light that a single layer cannot remove. Less stray reflection means more light reaches your eye and a cleaner, higher-contrast image.

Makers do not print a per-surface light figure in these documents, so the words on the box are the only comparison you get.

Prism coatings on roof prism models

Roof prism is the most common prism type in the chart. Nikon says a roof prism surface can shift the phase of light and lower resolution, and a phase-correction coating limits that loss.

Celestron says phase coating goes on one surface of the shorter light path half of a roof prism. The RSPB shop notes it does not apply to Porro prism designs. For the difference between the two designs, see roof and Porro prism binoculars.

ZEISS says it introduced phase correction at the end of the 1980s to prevent wave-optical effects.

Dielectric coatings

A dielectric coating is a high-reflectivity prism coating. Celestron says it gives high reflectivity across the entire range of visible light.

Nikon states that its dielectric multilayer coating on roof prisms has reflectance that exceeds 99%. ZEISS says its dielectric mirror coating consists of more than 70 layers applied in a vacuum.

These figures are the makers' own, printed for their own coatings. They do not let you compare one brand with another.

Prism glass: BaK-4 and BK-7

Prism glass is a separate choice from coatings. Celestron says BaK-4 is generally preferable to BK-7 or K9 because it has a higher refractive index.

It also says BK-7 and K9 cost less than BaK-4. Celestron warns that the claim of a not-perfectly-round exit pupil from BK-7 should not be trusted absolutely.

How to read a spec sheet

1. Look for the words ED, HD or low dispersion next to the objective lens. Only 146 of 554 models in the chart list ED glass.
2. Find the coating term and match it to the four Celestron definitions: coated, fully coated, multi-coated or fully multi-coated.
3. On roof prism models, check for phase-correction and dielectric prism coatings.
4. Check the prism glass type, BaK-4 or BK-7, as a separate line.