Twilight Factor, Transmittance and Brightness

This question question comes from someone looking to find out a little more about the Twilight Factor of a pair of binoculars:

Have A Question For Me?Question:

I’m still not clear about the twilight factor of binoculars, I cannot see how 10x50 which have a factor of 22, would be worse in low light conditions than 16x50 that have a factor of 28, using the twilight factor figures? I keep reading the chapter on twilight performance and it still does not register in my brain, can anyone come up with something I can understand?


Many thanks for your question and I admit that twilight factors can sometimes be a little confusing and as in the example you give, don’t seem to make sense, but I will try and explain as I understand it and to the best of my knowledge:

It is true that larger magnifications should in theory perform less well in low light, because of the thicker glass is needed and so less light makes it to your eyes, but remember twilight factor is the amount of detail you see in low light and is not a term used to indicate observed image brightness so you cannot assume that a binocular with a higher twilight factor will seem brighter.

According to the equation, the larger the twilight factor, the more detail you can see in low light (With a TF of 17 or more usually required for reasonable low light use ) – The way I understand it, the binocular with the larger magnification is in theory better because you are looking at objects with more power (closer) and therefore see more of their details – However I would go on to say that the twilight factor does exaggerate the importance of magnification and that quality of lenses, prisms and coatings are probably more important these days when it comes to image details in low light.

The Twilight Factor of a pair of binoculars was far more important in the past, before things like anti-reflective coatings were commonly used. These days the brightness of a pair of binoculars has a lot to do with the quality of glass used and the coatings and not just the size of the magnification and objective lenses.

Also I feel that it is important to stress again that twilight performance is not a measure of brightness, but a guide to the amount of detail can be resolved in low light. If you are worried about how bright a binocular is, then this relates to their transmittance as well as the size of objective lens.

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If you have a question regarding a pair of binoculars, or want to know about a particular technical term or feature that I have not already answered or written about, I would love to try and answer it for you: Ask Me Here

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