Vignetting occurs because of the reduction of light in the periphery compared with the center of the image.
With binoculars, vignetting is mainly caused by the prisms.
No matter how much light the objective lenses collect, vignetting prevents us from seeing the full brightness. So, it’s important to understand how it occurs and how to choose binoculars with less vignetting.
First, let’s talk about the prism material.
How to tell Bak4 prisms from BK7
In the specifications (and sometimes on the cover-plate), “Bak4 prism” is printed, which may sound cryptic. This indicates the material used for the internal prisms.
Bak4 (Barium Crown glass) is superior to BK7 (Borosilicate Crown glass) as it has a higher refractive index. So, what makes the refractive index so important?
The prism reflects light from the objective lens. With a low refractive index, Bk7 will lose some light during transmission. When there is not total internal reflection of the peripheral rays of light from the objective lens, vignetting occurs.
Compared to Bk7, Bak4 has a high enough refractive index to get total internal reflection of the light, which produces a bright image without vignetting.
It‘s easy to tell which glass material is used for your binoculars. Holding the binoculars about 30cm away from your eyes and looking through the ocular lenses, you can see bright circles in the middle.
These are called the exit pupil. If your binoculars have Bk7 prisms, you’ll see a blue-grey frame in the exit pupil, which is caused by the loss of light due to non-total internal reflection (see below).
On the other hand, a Bak4 prism has a perfectly round exit pupil (see below).
In daylight, our pupils shrink to restrict the amount of bright light. So, the difference between Bak4 and Bk7 is negligible.
At night, however, the difference in brightness is essential, especially for astronomers. Bk7 prisms are often found in lower-priced binoculars, whereas Bak4 prisms are built into better pairs.
Also, the prism housings can cause vignetting. As you can see in the picture below, the edge of the prism housing truncates the exit pupil, which causes a reduction in brightness.
Recently, Sk15, which have dispersion between Bak4 and Bk7 yet a higher refractive index than both, are used for well-made binoculars. Looking through Sk15 prisms, you can get a very clear image with high contrast.
The material of the prisms is just one factor that determines binoculars’ brightness. Here are other factors related to brightness:
Since binoculars with Bak4 prisms are available at a reasonable price, I recommend that you steer clear of those with Bk7 prisms.