Perhaps, when choosing binoculars, you often encountered two concepts: ED-glass (with extra-low dispersion) and BaK-4 prism glass. Both of these characteristics affect the image quality, but in different ways. Let’s figure out today what is responsible for and how it affects the quality of optics.
ED-glass means extra-low dispersion glass
What is ED Glass?
ED stands for “Extra-low Dispersion”. That is, this is glass with an ultra-low light dispersion index. The lower the dispersion of light in the optical element, the less distortion the image will have. One of the types of distortion is chromatic aberration, a phenomenon in which colored contours appear around the image. Low dispersion glass improves contrast and color accuracy.
The effect of chromatic aberration occurs when a beam of light passes through the objective lens, refracts and breaks down into spectral colors (from red to violet). All colors have different wavelengths in length, so each will have its own angle of refraction, which ultimately leads to the appearance of various kinds of distortions in the photograph: color stripes, glare or rings.
ED glass is mainly used for lenses, but some high-end models can also have prisms made from it.
The main pluses of ED glass in binoculars are:
- clearer images due to reduced chromatic aberration
- improved colors
- improved contrast and sharpness of images, especially at high magnifications
What is Bak-4 Glass?
Bak-4 glass (barium crown) is a high-quality optical material used in binocular prisms. Prisms are, so to speak, the heart of the optical system of binoculars, they play a critical role in the quality of the image. They have a complex shape, and the beam of light passing through the objective is reflected many times from their surface on the way to the eyepiece. It is very important that the prisms are made of high-quality glass to minimize light loss. Bak-4 prisms have a higher refractive index and higher light transmittance than Bk-7 prisms (another popular glass for making prisms). They form a clear and bright, correctly oriented image.
Key Differences Between ED Glass and Bak-4 Glass
| Feature | ED Glass | Bak-4 Glass |
|---|---|---|
| Purpose | Reduces chromatic aberration | Improves light transmission and image clarity |
| Location in Optics | Used in objective lenses | Used in prisms |
| Effect on Image | Enhances color accuracy and sharpness | Increases brightness and reduces internal reflections |
| Impact on Low Light Performance | Minimal improvement | Significant improvement due to better light transmission |
Is Using Binoculars with ED Glass Worth It?
ED glass and Bak-4 prisms play an essential role in improving binocular performance, but their impact depends on what observation priorities you have:
- If you need sharp, color-faithful images with low chromatic aberration, ED glass is important.
- Bak-4 prisms are an important feature if you often use binoculars in dark areas or if you need a bright image.
- For the best overall viewing experience, high-end binoculars often combine both ED glass and Bak-4 prisms to produce sharp, bright, high-contrast images.
Both ED glass and Bak-4 prisms play an essential role in improving binocular performance.
To sum-up
ED and Bak-4 glass are most often used in different parts of binoculars: the former in lenses, the latter in prisms. As such, they perform different but complementary functions: ED glass reduces chromatic aberration, improving image sharpness and color accuracy, while Bak-4 prisms increase light transmission for brighter images. When choosing binoculars, consider where and how you will use them to determine which feature is more important. Ideally, choose binoculars that offer both for the best optical performance.
