I looked in the search function but did not see the answer to the question I have. So here it is. If you have two scopes all things being the same (glass, objective sizes, tube diameter, etc...) would a variable power scope, let's say a 3-15 or 18 x 50 compared to a fixed powered 7x50 perform as well in low light situations? It would seem that a fixed power to be better because light does not have as many lens to pass through. But would a variable power perform as well on 7 power or lower. I have pondered whether a variable on 3 or 4 power would be better than a fixed 7 power. If anyone knows for sure the answer to this question or has any experience with this particular question please let me know.
Rifle Scopes fixed or variable scope for low light performance
- Thread starter noliver
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Re: fixed or variable scope for low light performance
Since you say "everything else being equal", the 7x50 can't be better than the 3-15x50 since you always set the variable to 7x. In reality the 7x50 would have slightly more light transmission because the variable has more glass-air interfaces and light loss.
The answer is the variable is better because you can adjust the exit pupil diameter to match your pupil size for the darkness condition. Exit pupil size = objective diameter/magnification. Your eye pupil size is age dependent, but it does increase in darker conditions, for example from roughly 3-4mm to roughly 5-8mm. The 7x50 has a fixed exit pupil diameter of 50/7 = ~7mm.
The variable's exit pupil size is adjustable from ~18mm to ~3mm.
Since you say "everything else being equal", the 7x50 can't be better than the 3-15x50 since you always set the variable to 7x. In reality the 7x50 would have slightly more light transmission because the variable has more glass-air interfaces and light loss.
The answer is the variable is better because you can adjust the exit pupil diameter to match your pupil size for the darkness condition. Exit pupil size = objective diameter/magnification. Your eye pupil size is age dependent, but it does increase in darker conditions, for example from roughly 3-4mm to roughly 5-8mm. The 7x50 has a fixed exit pupil diameter of 50/7 = ~7mm.
The variable's exit pupil size is adjustable from ~18mm to ~3mm.
Re: fixed or variable scope for low light performance
+1 I'm interested in this as well.
+1 I'm interested in this as well.
Re: fixed or variable scope for low light performance
If you're looking for a "bright view" and most of your shooting is done at relatively close range, a fixed scope can do quite well. They're also usually lighter than their variable counterparts and you can usually get better glass for your money.
However, if being able to see fine detail in low light situations is important to you (spotting hits on targets, facial recognition, counting the points on a buck's rack, etc) variables have a huge advantage, especially if the shooting is more than 50 yds or so away.
The variable allows you to crank up the magnification to the maximum the light conditions will allow. At different distances and in different light conditions you will be able to see the most detail of the target on different powers.
The old "7mm exit pupil" rule of thumb isn't where you usually see the most detail--sometimes it is, but often it's not. As you add more magnification, yes, the view gets darker, but as long as you can still see the target resolution will keep increasing to a point. At some point the view seems to just "go dark" and you need to turn the scope back down again, but usually you can see the best detail right before this happens which is often a much higher power than you would have thought.
For example, using my March 3-24X42 (which is a pretty mediocre low light scope) to count the tines on whitetail's racks out in the open in the 300-400 yd range the last couple minutes of shooting light (1/2 hour after sunset) I find I end up in the 12X-18X range depending upon lighting conditions right at the end of legal light.
A scope with a larger objective (assuming also quality glass) in any given condition is going to let you turn the power up a little higher before it "goes dark" allowing you to see more detail. This is where the 50mm, 56mm objectives really shine. On the lower powers they're pretty much wasted but with a variable you can crank up the power and make the most of them.
It's also the reason tests by gunwriters where they set all the scopes at 6X (or whatever) and look through them in low light are meaningless to the scopes' abilities when actually used in the real world.
If you're looking for a "bright view" and most of your shooting is done at relatively close range, a fixed scope can do quite well. They're also usually lighter than their variable counterparts and you can usually get better glass for your money.
However, if being able to see fine detail in low light situations is important to you (spotting hits on targets, facial recognition, counting the points on a buck's rack, etc) variables have a huge advantage, especially if the shooting is more than 50 yds or so away.
The variable allows you to crank up the magnification to the maximum the light conditions will allow. At different distances and in different light conditions you will be able to see the most detail of the target on different powers.
The old "7mm exit pupil" rule of thumb isn't where you usually see the most detail--sometimes it is, but often it's not. As you add more magnification, yes, the view gets darker, but as long as you can still see the target resolution will keep increasing to a point. At some point the view seems to just "go dark" and you need to turn the scope back down again, but usually you can see the best detail right before this happens which is often a much higher power than you would have thought.
For example, using my March 3-24X42 (which is a pretty mediocre low light scope) to count the tines on whitetail's racks out in the open in the 300-400 yd range the last couple minutes of shooting light (1/2 hour after sunset) I find I end up in the 12X-18X range depending upon lighting conditions right at the end of legal light.
A scope with a larger objective (assuming also quality glass) in any given condition is going to let you turn the power up a little higher before it "goes dark" allowing you to see more detail. This is where the 50mm, 56mm objectives really shine. On the lower powers they're pretty much wasted but with a variable you can crank up the power and make the most of them.
It's also the reason tests by gunwriters where they set all the scopes at 6X (or whatever) and look through them in low light are meaningless to the scopes' abilities when actually used in the real world.
Re: fixed or variable scope for low light performance
Thank you for the response. We are on the sane page. I just have never used a fixed power scope. at say 7 power and have heard that they perform better in low light situation but I tend to agree with you. Does anyone have any recommendations on very good low light scopes? Please disregard money just want to know what the best would be. Besides night vision would it be possible to see a deer 100 yards away in a field on a full moon night. With the best low light performing scope money can buy?
Thank you for the response. We are on the sane page. I just have never used a fixed power scope. at say 7 power and have heard that they perform better in low light situation but I tend to agree with you. Does anyone have any recommendations on very good low light scopes? Please disregard money just want to know what the best would be. Besides night vision would it be possible to see a deer 100 yards away in a field on a full moon night. With the best low light performing scope money can buy?
Re: fixed or variable scope for low light performance
Look for a scope with a large objective that is well designed. For example, the Hensoldt 6-24x72mm will be one of the best low-light scopes you can buy and the Finnish army snipers like them. "Dusk" can last a long time in Finland, depending on the time of year. Yes, I hope we're on the "sane" page.
Dunno about deer hunting in moonlight with a "day scope", even the Hensoldt.
Look for a scope with a large objective that is well designed. For example, the Hensoldt 6-24x72mm will be one of the best low-light scopes you can buy and the Finnish army snipers like them. "Dusk" can last a long time in Finland, depending on the time of year. Yes, I hope we're on the "sane" page.
Dunno about deer hunting in moonlight with a "day scope", even the Hensoldt.
Re: fixed or variable scope for low light performance
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Nasty Shot</div><div class="ubbcode-body">Besides night vision would it be possible to see a deer 100 yards away in a field on a full moon night. With the best low light performing scope money can buy? </div></div>
Not being sarcastic when I say a resounding yes - depending a lot on the reflectiveness of the field. If it's a ripe corn or wheat field, or a good grass field, it reflects quite well and it should be easy to see a deer under full moon - in fact around here I could probably see a deer at 100 yards with my bare eyes under some of our brighter summer/fall full moons. There are other crops and maturity stages that aren't as reflective, and of course in any woods you can forget seeing much. I have played around with some of my scopes with larger exit pupils and they definitely increase what you can see under dim light like a half to full moon. The question is whether you will be able to resolve enough to really identify a target (assuming you have a lawful reason for doing so - like pig hunting or, around here, raccoon hunting).
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Nasty Shot</div><div class="ubbcode-body">Besides night vision would it be possible to see a deer 100 yards away in a field on a full moon night. With the best low light performing scope money can buy? </div></div>
Not being sarcastic when I say a resounding yes - depending a lot on the reflectiveness of the field. If it's a ripe corn or wheat field, or a good grass field, it reflects quite well and it should be easy to see a deer under full moon - in fact around here I could probably see a deer at 100 yards with my bare eyes under some of our brighter summer/fall full moons. There are other crops and maturity stages that aren't as reflective, and of course in any woods you can forget seeing much. I have played around with some of my scopes with larger exit pupils and they definitely increase what you can see under dim light like a half to full moon. The question is whether you will be able to resolve enough to really identify a target (assuming you have a lawful reason for doing so - like pig hunting or, around here, raccoon hunting).
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