Twilight factor vs exit pupil

[BenHankins]

Trying to wrap my mind around the Swarovski NL 12x42s... the exit pupil is short compared to a 8x42 but the twilight factor is quite a bit higher than the 8s. Which one is more important? Are the 12s going to still be really bright? Field of view asides do you all think the EL 12x50s would be brighter in low light? Do you all think the new NL 12s are the new 10s of the hunting industry with the stabilization bar?


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[Q child]

Both are functions of the magnification and objective lens diameter and nothing else.
Twilight Factor = sqrt(magnification x objective diameter)
Exit pupil = objective diameter / magnification
The 50s will have higher values in both, so according to these metrics they will be brighter in low light.
I waffle on which I think more accurately quantifies low light performance. Have seen merit in both metrics at one time or another.
I think the 12s will be bright. The light transmission is high. The exit pupil is not that much smaller than a pair of 15x56s, so by that metric they should perform similarly.

 

[BluMtn]

I can tell you this much, I have the 12x50EL's and I have been able to see animals with my binos way past shooting light.

 

[binoman]

Both are functions of the magnification and objective lens diameter and nothing else.
Twilight Factor = sqrt(magnification x objective diameter)
Exit pupil = objective diameter / magnification
The 50s will have higher values in both, so according to these metrics they will be brighter in low light.
I waffle on which I think more accurately quantifies low light performance. Have seen merit in both metrics at one time or another.
I think the 12s will be bright. The light transmission is high. The exit pupil is not that much smaller than a pair of 15x56s, so by that metric they should perform similarly.

Agree with your comments.
If you compare two identical binoculars (model and manufacturer) with similar exit pupils eg 10x42 and 12x50...the 12x50 will have a bigger twilight factor based on the formula. In practice you will likely notice the 12x50 will be better at resolving details in low light as well, due to its higher magnification and bigger objective lens.
If you compared two similarly manufactured binoculars with different sizes ie 12x42 vs 12x60 and all else being equal, undoubtedly the 12x60 will outperform in low light.
I recently compared a 6.5x32 kowa with a 12x50 vortex viper hd after sunset and I was able to see more with the 12x50, even though its exit pupil was smaller (4.2 vs 4.9).

Low light performance also has a lot to do with lens coatings and light transmission. I have read some reviews where people have stated the NL seems brighter than the ELs. If the coatings and technology are better it may perform better in low light than the EL but it is disadvantaged by the smaller objective lens. Would be interesting to see a direct comparison of the two under a variety of scenarios. I'm sure it will happen soon..

 

[ultraedge]

I dont agree that larger objectives always have better low light performance.When Whitetail hunting in low light scenarios my X5i 3.5 x18x50 scope is better than my X5i 5x25x56 or my Z8i 2.3x18x56.

 

[Q child]

Hey Ultraedge,
Jeez, some heavy hitters in the scope department. Rifle scopes internally mechanically control the aperture. I don't know how this is done mechanically, or why it is done physically. This results in exit pupil sizes that are not as straightforward as the formula laid out above. Swarovski's stated exit pupils for the scopes that you mentioned though are as follows:
X5i 3.5-18x50: 9.5-2.8 mm
X5i 5.0-25x56: 9.5-2.3 mm
Z8i 2.3-18x56: 8.1-3.1 mm
Assuming that exit pupil determines brightness, then the X5i 50 would be tied with the X5i 56 at the low end magnification, but brighter at the high end. And the X5i 50 would be brighter than the Z8i at the low end magnification, and darker at the high end.
All of this is pretty academic though. It can be helpful for comparisons, but I trust your testimonial more than any of it.