Welcome to Optics Trade debates. In each episode, we talk about a different topic and try to answer the most common questions we receive about it. Today, we are going to talk about twilight factor.
The twilight factor used to be an important piece of information but it got less and less useful after time. Twilight factor is a geometrical term that defines the brightness of an optical product.
It was a reliable piece of information before WW2 when there were no coatings and all manufacturers used the same type of glass.
The parameter is the square root of the product of the magnification and the diameter of the objective lens.
Twilight factor is still used in catalogs even though it is no longer useful. Two different pairs of binoculars can have the same twilight factor, but when they’re compared in low light situations, the difference in actual brightness can be astonishing.
If an optical device has multiple levels of magnification, it has a minimal and maximal twilight factor. The minimal factor is calculated with the smallest magnification, and the maximal factor is calculated with the highest magnification.
This feature shouldn’t be used anymore and things like exit pupil diameter and light transmission rate that would be measured on every wavelength on the visible spectrum should replace this outdated specification.
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Explanation of the term on our website:
In the past, the twilight factor was an important value in determining the brightness of the optics. The manufacturers were using the same kind of technology and materials of the lenses, therefore, the optics were comparable. Nowadays, they use different types of lenses and modern coatings so the twilight factor has lost its meaning, because the brightness of the optics depends more on the quality of the coatings than on the twilight factor.
Twilight factor is calculated by the square root of multiplying magnification and lens diameter.
The twilight factor of 8×42 binoculars is a square root of 336, meaning 18.33. All the binoculars with this kind of magnification and lens diameter have the same twilight factor, but not the same brightness. If you look through an old pair of binoculars made in the 1950s and a new pair with the same magnification and lens diameter you could see the difference in brightness even though they share the same twilight factor. The new pair is significantly brighter due to better lens materials and coatings.
Though manufacturers still specify the twilight factor, we recommend you to ignore it as it’s not important.