When choosing binoculars for bird watching, lens coatings and glass quality play an important role in image clarity, brightness, and overall viewing experience. Understanding the different types of coatings and glass technologies can help you select a pair that delivers the best optical performance.
ED glass helps reduce chromatic aberration (colour fringing), which occurs when light passing through a lens is not focused correctly, causing different wavelengths of light (colours) to focus at different points. This can result in a visible halo of colour, typically red or blue, around high-contrast objects. It is especially noticeable at higher magnifications or when the light conditions are challenging.
Chromatic aberration is caused by the way traditional lens materials bend light. Since different colours of light refract by different amounts, some colours can get “spread out” as they pass through the lens, leading to this fringing effect. ED glass is specifically designed to address this issue. By using special materials that have a lower dispersion rate, ED glass brings all the wavelengths of light into focus at the same point, reducing the occurrence of chromatic aberration.
By minimising this distortion, ED glass enhances clarity and sharpness in the image, particularly at the edges of objects. This improvement is particularly beneficial for bird watching, where fine details are important for identification. The reduction in chromatic aberration also improves brightness, as the image is more uniform, allowing you to see subtle details in low light conditions or high-contrast settings.
Phase correction coatings are specifically used in roof prism binoculars to address a phenomenon known as phase shift. Roof prisms, commonly found in more compact, straight-line binocular designs, work by reflecting light through a series of prisms to ensure the image is oriented correctly before reaching your eyes.
However, when light passes through these prisms, it can undergo a slight phase shift, which essentially means that the light waves are altered in a way that can cause a subtle misalignment between the two channels of light (the left and right eye). This misalignment can result in a reduction of image quality, particularly in terms of contrast, sharpness, and overall resolution.
Without phase correction, this phase shift can lead to a slight blurring of the image and a reduction in clarity, especially at the edges or fine details of objects. The light that reaches each eye is no longer perfectly in sync, which can diminish the overall visual experience, especially at higher magnifications where clarity and sharpness are crucial.
Phase correction coatings are designed to correct this phase shift by realigning the light waves passing through the roof prisms, ensuring that the left and right images are perfectly matched before they reach your eyes. The phase-corrected prisms are coated with special layers that adjust the phase difference, restoring the image to its intended clarity and sharpness. As a result, you get a much crisper, more detailed view with improved contrast and resolution, particularly in low-light conditions or when viewing subjects with fine, detailed features.
Anti-reflection coatings reduce light loss and glare, improving brightness and contrast. The level of coating varies across different binocular models:
Coated: A single anti-reflection coating applied to some lens elements. This improves light transmission compared to uncoated lenses.
Fully coated: A single anti-reflection coating on all air-to-glass surfaces, offering better brightness and contrast than a partially coated lens.
Multi-coated: Multiple layers of anti-reflection coatings on some lens elements, allowing even more light to pass through.
Fully multi-coated: Multiple anti-reflection coatings on **all** lens surfaces, maximising light transmission for the brightest, clearest image—ideal for birdwatching.
Dielectric coatings are applied to the prisms in high-end binoculars to enhance light reflection and produce vivid, high-contrast images. This ensures colours appear as natural and bright as possible, closely resembling what you see with the naked eye.
While not directly related to optical quality, coatings that make binoculars waterproof and fog-proof are essential for bird watching in diverse weather conditions. They prevent moisture from entering the binoculars, protecting the lenses and internal components, ensuring clear vision in rainy weather and preventing fogging caused by rapid temperature changes when moving between warm and cold environments.
Some binoculars come with scratch-resistant coatings on their lenses, which is particularly important when handling them outdoors. They protect the lenses from scratches or damage due to accidental contact with rough surfaces, ensuring your binoculars last longer and maintain image clarity.
Premium binoculars may feature hydrophobic and oleophobic coating, water-repelling and oil-repelling coatings that make it easier to keep your lenses clean by preventing water droplets and fingerprints from sticking to the lens.
While lens coatings help to optimise light transmission and reduce distortions, the lens material itself has a direct impact on the clarity, sharpness, and colour accuracy of the images you see through your binoculars.
High-index glass and fluorite glass are two premium materials used in the construction of high-end binocular lenses. Both materials offer superior optical qualities that significantly enhance the performance of the binoculars, making them ideal for bird watchers who demand the very best in image quality.
High-index glass is engineered to have a higher refractive index than standard glass, which means it bends light more efficiently, helping to reduce the overall thickness and weight of the lenses while maintaining excellent optical performance. Because of its ability to bend light more effectively, high-index glass can reduce chromatic aberration and distortion, resulting in clearer, sharper, and brighter images. High-index glass is particularly beneficial for binoculars with higher magnifications, to maintain sharp focus and clarity.
Fluorite glass is an even more advanced material that is known for its exceptional ability to transmit light with minimal distortion. As a result, it effectively reduces colour fringing and chromatic aberration, even in low-light conditions, producing images that are not only sharp but also incredibly true to life. Fluorite glass lenses allow more light to pass through, enhancing the overall brightness of the image and improving the contrast between light and dark areas which makes it ideal for observing birds in shaded or low-light environments.
This page contains affiliate links. For more information please see our disclosure here.
