What is the twilight index of a pair of binoculars?

Twilight index is a technical term frequently encountered when choosing binoculars. It represents the binoculars' ability to reproduce details in low-light conditions, particularly at twilight. A high twilight index indicates that binoculars are better able to distinguish details in dark environments.

Understanding the twilight index

Thetwilight index of binoculars is a measure of their effectiveness in low-light conditions, such as twilight or overcast conditions. In France or Germany, where twilight lasts a long time, it is essential to choose binoculars with a high twilight index for optimum observation. On the other hand, in tropical regions where twilight is very short, the light intensity or clarity of the binoculars may be a higher priority.

How to calculate the twilight index?

To calculate the twilight index of a pair of binoculars, simply multiply the magnification by the lens diameter, then take the square root of the result. For example, for 8x42 binoculars, the calculation is as follows:

• Multiply magnification by objective diameter: 8 x 42 = 336
• Take the square root of the result: √336 ≈ 18.3

In this example, the twilight index of the binoculars is 18.3. The higher this index, the more effective the binoculars will be at observing details in low-light conditions.

Twilight index and light intensity

It's important to note that the twilight index should not be the only criterion to be considered. The relative brightness of a pair of binoculars is also crucial in assessing their performance. Relative brightness is calculated by taking the diameter of the exit pupil (e.g. 5.25 mm for 8x42 binoculars) and multiplying it by itself:

• Exit pupil: 42/8 = 5.25 mm
• Relative brightness: 5.25 x 5.25 ≈ 27.6

Higher relative brightness means the binoculars are able to capture more light, which is essential for low-light observation.

Choosing binoculars for day or night viewing

As a general rule, binoculars with a twilight index above 15 are suitable for daytime use. For night-time use, binoculars with a relative brightness above 50 are preferable. However, these values are not absolute rules, and it is important to also take into account the exit pupil and other technical characteristics according to your specific needs.

In conclusion, understanding and calculating the twilight index will help you choose the best binoculars for your observation conditions, whether for hunting, astronomy or twilight nature observation.