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An understanding of the term “Twilight Factor” (1 Viewer)
- Thread starter bsantini
- Start date
I've seen the metric used to calculate it. I don't think it is very useful at all!
Not useful because it would give a 50 year old pair of very cheap 8x40 binoculars the same value as a 2023 latest alpha 8x40. I'd be surprised if the older pair was as good in dull conditions.
A more useful measure, to me, would be based on measuring how many minutes after dusk, or using a light meter, you can read a license plate at x metres with a certain pair of binoculars.
But also not useful, here in UK at least, because there is very little wildlife exclusive to Twilight! Pretty much everything is better observed in daylight, or via torch in darkness. Having a pair of binoculars to exploit the minutes in between doesn't seem particularly sensible. Just switch to torch a bit earlier.
Not useful because it would give a 50 year old pair of very cheap 8x40 binoculars the same value as a 2023 latest alpha 8x40. I'd be surprised if the older pair was as good in dull conditions.
A more useful measure, to me, would be based on measuring how many minutes after dusk, or using a light meter, you can read a license plate at x metres with a certain pair of binoculars.
But also not useful, here in UK at least, because there is very little wildlife exclusive to Twilight! Pretty much everything is better observed in daylight, or via torch in darkness. Having a pair of binoculars to exploit the minutes in between doesn't seem particularly sensible. Just switch to torch a bit earlier.
I think it is very misunderstood. But theory behind it is sound.
Twilight factor attempts to correlate the potentially greater light-gathering capability of larger objective sizes with the improved known contrast detail that can be realized when more of the eye’s retinal mosaic is involved from the image being larger/magnified
Twilight factor attempts to correlate the potentially greater light-gathering capability of larger objective sizes with the improved known contrast detail that can be realized when more of the eye’s retinal mosaic is involved from the image being larger/magnified
Lots of different metrics from playing with the bino numbers, giving an idea of the applicability to different applications. Short ransmusaion, of actual measurements you won’t have exact numbers, but then conditions vary as do eyes. If you want to observe in dull and twilight conditions you want a big exit pupil (second number divided by first), 8x40=5mm, which is good, though you can go up to 7mm (though your eyes might not). More modern binoculars will have higher transmission but you’d need to go back a few decades to find noticeably poorer anti reflection coatings. I favour a lighter binocular for ease of carrying, for night time use I have others that age better suited, like I have an 8x as I like stable views and I also carry a scope.
Peter
Peter
At a guess 5 to 10 minutes.
But that is because the Sun skirts the horizon at this time.
In addition, there is moonlight.
Venus and even Jupiter cast shadows in dark places.
For about seven evenings the Moon and Venus have been seen in clear skies.
More importantly here, the light pollution is so bad I can see foxes clearly at 3 a.m.
Also photograph them.
I can see crows in the trees all night long and sometimes owls.
They are easy to see with any binocular.
So the actual number of minutes is probably not quantifiable, and I doubt the twilight factor includes all the above variables.
The noctilucent cloud season begins now and is usually seen when the Sun is 6 to 16 degrees below the horizon.
I remember spending a week in Hammerfest where the Sun was bright at midnight and the locals don't seem to sleep and just sit outside in coffee bars.
Fortunately the hotels provide thick blinds for the weak tourists.
I have my diploma of the polar bear club somewhere.
I was one of the first to receive this, as they thought up a way to monetarise people who drove 6,000 miles in three weeks to be in midnight sun land.
Some even went by bicycle.
Regards,
B.
But that is because the Sun skirts the horizon at this time.
In addition, there is moonlight.
Venus and even Jupiter cast shadows in dark places.
For about seven evenings the Moon and Venus have been seen in clear skies.
More importantly here, the light pollution is so bad I can see foxes clearly at 3 a.m.
Also photograph them.
I can see crows in the trees all night long and sometimes owls.
They are easy to see with any binocular.
So the actual number of minutes is probably not quantifiable, and I doubt the twilight factor includes all the above variables.
The noctilucent cloud season begins now and is usually seen when the Sun is 6 to 16 degrees below the horizon.
I remember spending a week in Hammerfest where the Sun was bright at midnight and the locals don't seem to sleep and just sit outside in coffee bars.
Fortunately the hotels provide thick blinds for the weak tourists.
I have my diploma of the polar bear club somewhere.
I was one of the first to receive this, as they thought up a way to monetarise people who drove 6,000 miles in three weeks to be in midnight sun land.
Some even went by bicycle.
Regards,
B.
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Yes, I can imagine it being a major factor in Lapland/Northern areas where it is possible to get thru the night with binoculars alone. Another reason why it is foolish to go to war over something on here. There will always be regions or examples where the rules are totally different!
Here in my part of Southern England, where it does get totally dark eventually, my 8x32s just about give up completely 30 mins after sunset. But we're looking for the moth trap, bat detector and torch by this time!
Here in my part of Southern England, where it does get totally dark eventually, my 8x32s just about give up completely 30 mins after sunset. But we're looking for the moth trap, bat detector and torch by this time!
Elkhornsun
Well-known member
The best measure is the ability to discrern parallel lines as being distinct. This combines light transmission with the ability of the lenses to resolve detail. A 42mm objective has 96% more surface area than a 30mm objective and lets more light pass through. Whether it is coherent affects the ability of the viewer to discern details.
All binos of the same magnification and size objectives would produce the same image quality and details if there were not the factors that affect overall image quality for the user. This is where the twillight factor is important. The lower the contrast ambient light the more the view afforded by the binocular optics will degrade. Image magnification plays a significant role and in low light I can see a subject more clearly with 10x25 binos than with 7x50 ones.
It would be very helpful if binocular manufactures provided information about resolution at varying light levels. I can compare different binoculars from one manufacturer but not between manufacturers as there is no industry standard. This is where the seat of the pants approach to evaluating binos comes into play. I cannot go to a store and try out binos from the top manufacturers but must rely on the opinions of others.
All binos of the same magnification and size objectives would produce the same image quality and details if there were not the factors that affect overall image quality for the user. This is where the twillight factor is important. The lower the contrast ambient light the more the view afforded by the binocular optics will degrade. Image magnification plays a significant role and in low light I can see a subject more clearly with 10x25 binos than with 7x50 ones.
It would be very helpful if binocular manufactures provided information about resolution at varying light levels. I can compare different binoculars from one manufacturer but not between manufacturers as there is no industry standard. This is where the seat of the pants approach to evaluating binos comes into play. I cannot go to a store and try out binos from the top manufacturers but must rely on the opinions of others.
elkcub
Silicon Valley, California
The attached paper by Daniel Vukobratovich, one of the authors of the 2011 SPIE Field Guide "Binoculars and Scopes" appeared in the 1989 book entitled: "Current Developments in Optical Engineering and Commercial Optics" - Volume 1168, Proceedings of SPIE. The derivation of the twilight factor as a theoretical concept can be found on the third page and refers specifically to "binocular efficiency*" under the limited conditions defined in the paper.
It's hard to argue theoretically, and there have been controlled laboratory studies that support it, but as a practical selection factor one can take it or leave it.
Ed
* "Binocular efficiency" refers to the visual performance efficiency of a human observer using binoculars.
It's hard to argue theoretically, and there have been controlled laboratory studies that support it, but as a practical selection factor one can take it or leave it.
Ed
* "Binocular efficiency" refers to the visual performance efficiency of a human observer using binoculars.
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