• Welcome to BirdForum, the internet's largest birding community with thousands of members from all over the world. The forums are dedicated to wild birds, birding, binoculars and equipment and all that goes with it.

    Please register for an account to take part in the discussions in the forum, post your pictures in the gallery and more.
Where premium quality meets exceptional value. ZEISS Conquest HDX.

Daft question about low-light performance (1 Viewer)

There have probably been a gazillion threads about low light but what very few attempt to define in them is - what does low light actually mean? Civil twilight? Nautical twilight? Virtual blackness? Or failing that, what sorts of situations are we talking about? That can be every bit as important as objective diameter, magnification etc.

I don't do a great deal of observing in really low light, but from what I've seen, the darker it gets, to get a real improvement over the standard 8x42 or 10x42 you need a pretty serious step up in capability ie. 10x56 or thereabouts.
I would say low light would be 1/2 hour before and after sunset or under canopy or in a dense forest or shaded canyon.
 
When you are discussing low light performance in a binocular, you are concerned with two separate parameters. Brightness and Twilight Factor.

1) Brightness First is brightness, which is a function of the size of the binocular's exit pupil and the binocular's transmission.

EP or exit pupil is the cone of light coming out of the binocular that hit's your cornea and then passes through the vitreous humor of the eye to reach the retina which has cone and rod cells that receive the incoming light signal then interpret that information and sends it to your optics nerve which transmits the signal and then sends it to your brain.

The bigger the cone of light, the brighter the image is going to seem to your brain, but the cone of light has to get through your pupils which change in size with varying levels of light and the amount they can change varies with your age. If you are younger or less than 30 years old, your pupils can dilate to as large as 7mm, but if you are over 60 years old your pupils might only be dilating to 5mm.

So a binocular with a 7mm EP may not be noticeably brighter than a binocular with a 5mm EP if you are over 60. Another factor that comes into play is the transmission of the binocular, or what % of light that enters the objective of the binocular that actually reaches your cornea.

Take two binoculars with identical EP sizes like a 8x42 but one having 95% transmission and the other with 90% transmission.

The binocular with 95% transmission will appear brighter because even thought the EP is the same in both binoculars, that 5mm cone of light hitting your cornea is brighter than the binocular with 90% transmission.

It is like two different flashlights that have the same beam size, but one flashlight has a brighter or more intense beam.

In daylight your pupils only open to about 4mm so that is about the maximum amount of light your eye can take in from the binocular so you can see a binocular with a larger EP doesn't do you a lot of good for daytime use, but one with higher transmission will because that 4mm EP is more intense or brighter.

So in the daytime, transmission is more important than EP size after you reach the limiting factor of about 4mm and when you are over 60 years of age transmission also becomes more important than EP size because you can no longer receive an EP size of maybe anything larger than 5mm because your pupils will only dilate to about 5mm.

So all these factors interplay in how bright a binocular seems to you.

2) Twilight Factor Twilight Factor is the Square Root of the Objective Lens Diameter in mm times the magnification of the binocular, and the higher the number, the more detail you can see under low light.

What it basically means is higher magnification helps you see detail under low light the same way it helps you see detail in bright light. It has nothing to do with brightness, but rather is just describing how easy it is to see detail under low light.

It can produce irrational numbers if you plug in abnormal binocular magnifications and objective sizes, but it is still meaningful with normal binocular formats like a 8x42 and a 10x42.

3) Conclusion Both these factors interplay and will determine how bright a binocular seems and how much detail you can see with any given binocular.

In the three binoculars you mention above if they all have roughly equal transmission they would all be about equal in brightness because their EP is about the same size but the 12x50 would show the most detail under low light with the 10x42 next and the 8x42 would show the least detail due to Twilight Factor.

What all this means is that if you are young, a binocular like an SLC 8x56 with a 7mm EP and 93% transmission will appear brighter to you but if you are over 60 or during the daytime a binocular like the Habicht 7x42 with a 6mm EP and 96% transmission will appear brighter to you.

So your age determines what kind of binocular you should choose for low light, and you should look for one with about a 6mm EP and the highest transmission you can get if you're over 60. So for most of us old folks, the best low light binocular is something like the Habicht 7x42.

What you might find odd is that a binocular like the Habicht 8x30 W with its 95% transmission will appear brighter in the daytime than most 8x42's even the alphas with their 92% transmission because your eye's pupil is only opening to about 4mm so the Habicht maxes it out with EP size, but it hits your cornea with a more intense or brighter cone of light because of the higher transmission.

Good explained! 👍
Just one thing: I am sure the eye pupil will shrink to much smaller than 4mm at daytime. A sunny day it will probably go down to ~2,5mm or so.

Regards, Patric
 
Good explained! 👍
Just one thing: I am sure the eye pupil will shrink to much smaller than 4mm at daytime. A sunny day it will probably go down to ~2,5mm or so.

Regards, Patric
It depends on your age and if you're older your pupils are slower to change with changing light and will not dilate as much in low light and everybody's pupils can be a little different, but you are correct in that some peoples pupils can get down to 2mm in the daytime.

"In bright light, pupils typically constrict to a diameter of 2–4 millimeters. This is to prevent too much light from entering the eye. In the dark, pupils dilate to a size of 4–8 millimeters to allow more light in."
 

Users who are viewing this thread

Back
Top