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4. Twilight factor (TF) (1 Viewer)

The twilight factor is often used to compare the suitability of binoculars for use in twilight, when less light is available.

The larger the objective lens, the more light enters the binoculars, and the higher the magnification, with the same image brightness, the more details can be observed. So when calculating the twilight factor we must use both objective diameter and magnification.

It is calculated by multiplying the objective lens diameter by the magnification, and obtaining the square root of the result:

TF = √ (magnification x objective lens diameter)

However, the twilight factor is a mere mathematical parameter. It is not possible to include in the calculation the really important aspects of binocular performance such as image quality (including resolution and contrast), light transmission, colour rendition etc...

Every pair of 8 x 42 binoculars, for example, has the same twilight factor – 18.2 – regardless of how good or bad it is in practice, regardless of the brand or price!

The twilight factor can also be completely misleading as shown in the following example: two binoculars, 8 x 56 and 56 x 8 (such a model does not exist but would be theoretically possible), have an identical twilight factor of 21.2. While an 8 x 56 model is ideal during twilight, a 56 x 8 pair would be totally unusable, even in daylight. Therefore the twilight factor alone does not offer any kind of useful guidance.

A large exit pupil is always the most decisive and important factor for twilight use. Ideally, it should be at least as large as the pupil of the user. Anything with an exit pupil smaller than 4 mm is, therefore, unsuitable in twilight right from the start – even if the twilight factor is high.

Finally, if the binocular is of high optical quality, and the exit pupil is at least 4.0mm, and the observer is able to hold the binocular steady or have it supported in some way, a 10 x binocular will reveal more details during twilight than an 8x model.

Just, when comparing binoculars of the same quality and the same objective diameter, i.e. 8x42 and 10x42, this will reveal that although the twilight factor of the 10x42 – 20.5 - is higher, the image of the 8x42 indeed is brighter.

The reason for this is simple to explain: With the same objective size, both binoculars gather the same amount of light, but with the higher magnification, this light is distributed on a larger area of the retina, and thus the light intensity for the single receptors of the eye is lower.
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