Why are prism and lens coatings important to birding binoculars?

Prism and lens coating are important to reduce the loss of light through reflection.

When the glass surfaces reflect away less light, more usable light actually travels through the optical instrument. Therefore, the better the resulting image at the end of the sequences of lenses.

How important is light transmission in birding binoculars?

As much as 5% of light reflects away every time light enters glass. And with such a high number of surfaces where this can happen, binoculars can lose light at each point.

This indirect light gets lost inside the binoculars. As it reflects around within, it can cause haziness around the image that you see.

Prism and lens coatings help to minimize this problem. The specialized coatings aim to bring the figure of lost light down to about 1%.

What is prism coating?

In addition to taking account of the type of glass used for the prisms in your binoculars, it is also helpful to be aware of the coatings that manufacturers apply to the prisms. They coat prisms to further minimize internal reflections and to increase the effectiveness of the light transmission.

In production, up to eight different coatings might get applied. These correct the focusing of the whole spectrum of visible light.

How do prism coatings work?

Thin layers of metal oxide are vapor-deposited on the prism glass. The oxide reduces the unwanted aberrations and reflections that detract from the final quality of the image.

The coatings improve image quality, but also increase production cost. The more layers of anti-reflective coatings the binoculars have, the better the image will be. Hence, too, the more expensive the price.

What is dielectric prism coating?

There are a variety of coatings on the market. The very best are called dielectric coatings.

The dielectric coatings are valuable because with this the prism directs almost all of the light available. Consequently, dielectric-coated prisms give exceptionally good brightness and contrast.

What is phase correcting?

Phase correcting is another prism coating. It is specific to binoculars using the roof prism configuration.

Roof prism binoculars experience something called phase shift. You only see phase correction coatings on roof prisms.

Porro prism binoculars do not have this, therefore their prisms do not require phase correction coatings.

Porro prism vs roof prism binoculars: which is better for birding?

What is phase shift?

In roof prism binoculars, as the light enters the objective lens, it splits into two beams. By the time both beams have moved through the prism system and arrived at the ocular lens they are fractionally out of phase with each other.

When one beam reaches the ocular lens very slightly before the other in this way, it is called phase shift. Phase shift affects color and contrast.

Phase-correction coatings solve this issue by slowing the faster beam of light. Then the two beams are back in phase, and there is no difference in their arrival time at the ocular lens.

What are lens coatings?

The reason to coat a lens is to reduce the reflectivity of its surface.

With optical instruments that have a series of glass lenses, we want as much light as possible to go through the lenses, rather than get reflected off them.

What are the effects of lens coatings?

• Increased light transmission
• More contrast
• Vibrant color
• Sharper, clearer images
• Less glare

As a general rule, the more coatings the better - as long as the coating is designed to actually do something.

What are the different types of lens coatings?

Uncoated

• No special coating at all.

The four most common lens coatings are:

Coated

• A thin single layer of anti-reflection coating, usually of magnesium fluoride. This is applied to one side of one of the lenses.
• The most basic option enables more light to be transmitted than without any coating at all.

Fully coated

• At least one anti-reflection coating on all air to glass surfaces. This means the lenses are coated on both sides, as is the long side of the prism.
• Covers most light-transmitting surfaces, so preferable to a single coating.

Multi-coated

• The multiple anti-reflection coatings cover a wider range of wavelengths of light.
• The ‘multi’ can mean:
  coatings will be applied to multiple surfaces.
  multiple layers of coatings have been applied
• A higher percentage of light will be transmitted to you.
• There is a theory that a single coating is more hard-wearing, and so some of the top models have a single coating. In this case, a single coating is put on the outside lens surface, which doesn’t reduce contrast.

Fully multi-coated

• Multiple anti-reflection coatings on all inner and outer surfaces of the lenses and prisms.
• The multiple anti-reflection coatings target a wider range of wavelengths of light. With this coverage of so many different refractive indices, the instrument is optimized for light transmission.
• The top choice because more light is transmitted to you. Color rendition, contrast and clarity will be at their best.
• This method is used on high-end models of optics.

Ruby coated binoculars, amber coated binoculars, blue lens: what color lens is best in a pair of binoculars?

Neutral lens coatings are the best choice for birding binoculars. Bird watchers need to see the true colors of the birds that they are looking for in order to identify them correctly.

Why do binoculars have red lenses?

Binoculars with red lenses were aimed at hunters. The idea was that the ruby coating makes it easier for hunters to spot a target among foliage. They serve little purpose for a bird watcher.

Why are my binoculars blue?

The objective lenses may appear to have a blue tint to them if they have been coated.

If you observe them in artificial light, the reflections may appear to have green, yellow or purple hues. This is an indication that there is a coating on the surface.

What you need to know next

The hi-tech coatings that manufacturers apply to the glass prism and lenses in binoculars are crucial because they direct the light most effectively. The more light that reaches your eyes, the better the image that you see.

As well as the coatings on the outside of the glass components, the actual glass itself is really important. Find out what ED glass is and why it's worth investing in a pair of binoculars that feature this.

What is ED (extra low dispersion) glass in bird watching binoculars?