Birdwatching Dot Com


Birding FAQ

Birding Tips







The Binocular Advisor

Optics Myth 9:

"Twilight factor" is key to performance in dim light.


REALITY - The quality of the coatings is much more important.

You sometimes see "twilight factor" listed in a binocular description, as a measure of the resolving power in dim light. This term was more important years ago, before modern optical coatings were invented, than it is today.

Twilight factor is a mathematical formula that shows how both the size of the objective lens and the magnifying power contribute to a binocular's ability to show detail in dim light.

Twilight Factor MathThe twilight factor is the square root of the product of the diameter of the objective lens and the magnifying power of the binocular. [For example, an 8x32 binocular would have a twilight factor of 16, and a 10x42 would have a twilight factor of 20.5.]

However, in a modern binocular, performance in dim light depends more on the quality of the optical coatings than on the twilight factor formula. Good coatings can double the amount of light that gets through the binocular.

If you pick up an old binocular from the 40s, or a cheap, low-quality binocular, you'll see very poor performance in dim light. Then look through a modern, top-quality binocular with the same magnification and lens size, and see how much brighter and clearer the image is, despite the fact that both binoculars have the same twilight factor rating.

So if you're looking for good image quality in twilight conditions, you shouldn't just rely on the twilight factor number. You have to take the quality of the coatings into account.

Copyright 2006 Michael and Diane Porter


Sharp-shinned Hawk

Optics Myths and Misconceptions