Why are 1-4x scopes shaped differently? Do bigger scopes really gather more light?
Common sense would suggest that a larger objective lens (the lens closest to the target) will "gather more light", resulting in a brighter sight picture. Yet, many low-magnification scopes have straight bodies and tiny objective lenses, as small as 20mm. Why would a manufacturer make scopes with such small objective lenses?
The bottom scope is a common 1-4x tactical scope - a straight tube and a small objective lens.
It's a very different silhouette from the more traditional scope above. But why?
It's a very different silhouette from the more traditional scope above. But why?
The answer is fairly simple: you don't need a big objective lens to get the brightest sight picture possible. The idea that huge, hulking scopes with 50mm lenses "gather more light" is a myth. Mostly.
Outside of fiddly things like lens coatings and internal scope features, the brightness of the view through the scope is almost completely based on the size of the exit pupil - that's the disc of light that gets funneled to your eye by the eyepiece (the lens closest to you). This concept is best explained by the following three scenarios:
- If the scope's exit pupil is smaller than your eye's pupil, the image seen through the scope will appear darkened.
- If the scope's exit pupil is the same size as your eye's pupil, the image seen through the scope will be at maximum brightness.
- If the scope's exit pupil is larger than your eye's pupil (often the case with larger objective lenses or low magnifications), the image won't be any brighter because your pupil is already bringing in as much light as it can, but you can be more flexible with your head's alignment to the centerline of the scope (poor head alignment will manifest as crescent moon-shaped dark spots in the scope view, known as 'vignetting')
Before we go any further, it's helpful to know that on a sunny day, a human eye's pupil is about 3mm. In full darkness, your pupils will slowly expand to about 7mm. If you suddenly see something interesting (like a target) at any time, day or night, your pupils will contract (shrink) automatically.
So, how big is a given scope's exit pupil? It only takes a tiny bit of math to figure it out: exit pupil = objective lens diameter / magnification. Let's look at a standard hunting scope: a 40mm 3-9x.
With magnification at 3x, we've got a ~13mm exit pupil (40/3 = 13.33). That's a huge, honkin' exit pupil allowing lots of flexibility for alignment with the shooter's head and eye.
With magnification dialed all the way up to 9x, we get a ~4.5mm exit pupil (40/9 = 4.44). During the day, this will still give the brightest sight picture possible for most shooter's eyes (human pupils are about 3mm during the day), though the scope will be substantially less forgiving with respect to alignment of the shooter's head and the centerline of the scope.
Many manufacturers build their scope lineups with a careful relationship between objective lens diameter and magnification range, the goal being to keep exit pupils from ever being smaller than about 4.5mm. This prevents unwitting customers from complaining about their scope being too dark or too unforgiving of head placement. However, some long-range or varminting scopes can turn up the magnification so high that even a human eye's tiny, 3mm daytime pupil starts to overtake the exit pupil. At 25x magnification, even a hulking varmint scope with a 56mm objective lens is only yielding a 2.8mm exit pupil.
Outside of fiddly things like lens coatings and internal scope features, the brightness of the view through the scope is almost completely based on the size of the exit pupil - that's the disc of light that gets funneled to your eye by the eyepiece (the lens closest to you). This concept is best explained by the following three scenarios:
- If the scope's exit pupil is smaller than your eye's pupil, the image seen through the scope will appear darkened.
- If the scope's exit pupil is the same size as your eye's pupil, the image seen through the scope will be at maximum brightness.
- If the scope's exit pupil is larger than your eye's pupil (often the case with larger objective lenses or low magnifications), the image won't be any brighter because your pupil is already bringing in as much light as it can, but you can be more flexible with your head's alignment to the centerline of the scope (poor head alignment will manifest as crescent moon-shaped dark spots in the scope view, known as 'vignetting')
Before we go any further, it's helpful to know that on a sunny day, a human eye's pupil is about 3mm. In full darkness, your pupils will slowly expand to about 7mm. If you suddenly see something interesting (like a target) at any time, day or night, your pupils will contract (shrink) automatically.
So, how big is a given scope's exit pupil? It only takes a tiny bit of math to figure it out: exit pupil = objective lens diameter / magnification. Let's look at a standard hunting scope: a 40mm 3-9x.
With magnification at 3x, we've got a ~13mm exit pupil (40/3 = 13.33). That's a huge, honkin' exit pupil allowing lots of flexibility for alignment with the shooter's head and eye.
With magnification dialed all the way up to 9x, we get a ~4.5mm exit pupil (40/9 = 4.44). During the day, this will still give the brightest sight picture possible for most shooter's eyes (human pupils are about 3mm during the day), though the scope will be substantially less forgiving with respect to alignment of the shooter's head and the centerline of the scope.
Many manufacturers build their scope lineups with a careful relationship between objective lens diameter and magnification range, the goal being to keep exit pupils from ever being smaller than about 4.5mm. This prevents unwitting customers from complaining about their scope being too dark or too unforgiving of head placement. However, some long-range or varminting scopes can turn up the magnification so high that even a human eye's tiny, 3mm daytime pupil starts to overtake the exit pupil. At 25x magnification, even a hulking varmint scope with a 56mm objective lens is only yielding a 2.8mm exit pupil.
Note that the above numbers are rounded estimates and are for demonstration only - the exact figures can vary in the real world, and a scope's actual magnification is often slightly different from the numbers stated in the brochure. Any adjustable magnification scope claiming to offer 1x magnification is particularly suspect, as they are typically closer to 1.1x or 1.3x, whereas dot sights and other 1x sight systems usually offer a true 1x view.
This is why many 1-4x scopes have straight bodies and small objective lenses - bigger lenses just aren't needed, and bigger lenses are heavier, bulkier, and more expensive. At the highest magnification (4x), even a 20mm objective still gives a 5mm exit pupil (assuming the scope truly is 1x and not 1.1x or 1.3x), and many of these scopes are built with 30mm tubes and 24mm objective lenses, yielding a generous 6mm exit pupil at the highest magnification.
Yet, the notion persists that if you want a scope that gives you a bright sight picture even in dim light, go big (with objective lens) or go home. There is a tiny bit of truth to that, but it has nothing to do with "gathering light".
Take a 32mm objective scope up to 7x magnification, and you get a ~4.5mm exit pupil. During the day, this is fine and dandy because your pupils are shrunk down to about 3mm. As light fades, however, your pupils widen - you might find yourself well into a dusky evening with pupils around 6mm. At this point, your scope's ~4.5mm exit pupil is smaller than your eye's pupil, so the image through the scope will not be as bright as it could be.
However, slam a beefy 50mm objective scope on that rifle and crank it to 7x, the same magnification, and you'll get about ~7mm exit pupil, more than enough to give your light-sensitive eyes the brightest picture possible. In this case, the bigger objective will indeed provide a brighter picture. If you want to get the same image brightness from the smaller scope, you would need to dial back the magnification until the scope's exit pupil matches your eye's pupil.
So, all a bigger objective lens lets you do is crank the magnification up higher before the image starts to darken, which is NOT the same as transferring or gathering more light for the shooter's eye. For most shooters, this is only a benefit for dim light shooting - if the lighting already isn't great, being able to zoom in farther on your target could be quite helpful. Long-range shooters and/or varminters occupy the other small niche that can benefit from big objective lenses, due to their scope's extremely high magnification range, which can yield exit pupils well under 3mm.
Whether or not this benefit is worth the added weight and bulk of a bigger objective is something to decide for yourself, since that benefit is ONLY present when shooting in dim lighting or at extremely high magnification. If you do all your shooting during the day, or at close range, the only benefit to a big objective lens is more flexibility with respect to head placement due to the larger exit pupil, and the drawbacks (weight, bulk, expense, and making your rifle feel top-heavy) can be substantial.
So, hopefully that's two birds with one stone: that's why 1-4x scopes are shaped the way they are, and that's why you should slap any scope salesperson who uses the phrase "gathers more light."
Happy shooting.
Yet, the notion persists that if you want a scope that gives you a bright sight picture even in dim light, go big (with objective lens) or go home. There is a tiny bit of truth to that, but it has nothing to do with "gathering light".
Take a 32mm objective scope up to 7x magnification, and you get a ~4.5mm exit pupil. During the day, this is fine and dandy because your pupils are shrunk down to about 3mm. As light fades, however, your pupils widen - you might find yourself well into a dusky evening with pupils around 6mm. At this point, your scope's ~4.5mm exit pupil is smaller than your eye's pupil, so the image through the scope will not be as bright as it could be.
However, slam a beefy 50mm objective scope on that rifle and crank it to 7x, the same magnification, and you'll get about ~7mm exit pupil, more than enough to give your light-sensitive eyes the brightest picture possible. In this case, the bigger objective will indeed provide a brighter picture. If you want to get the same image brightness from the smaller scope, you would need to dial back the magnification until the scope's exit pupil matches your eye's pupil.
So, all a bigger objective lens lets you do is crank the magnification up higher before the image starts to darken, which is NOT the same as transferring or gathering more light for the shooter's eye. For most shooters, this is only a benefit for dim light shooting - if the lighting already isn't great, being able to zoom in farther on your target could be quite helpful. Long-range shooters and/or varminters occupy the other small niche that can benefit from big objective lenses, due to their scope's extremely high magnification range, which can yield exit pupils well under 3mm.
Whether or not this benefit is worth the added weight and bulk of a bigger objective is something to decide for yourself, since that benefit is ONLY present when shooting in dim lighting or at extremely high magnification. If you do all your shooting during the day, or at close range, the only benefit to a big objective lens is more flexibility with respect to head placement due to the larger exit pupil, and the drawbacks (weight, bulk, expense, and making your rifle feel top-heavy) can be substantial.
So, hopefully that's two birds with one stone: that's why 1-4x scopes are shaped the way they are, and that's why you should slap any scope salesperson who uses the phrase "gathers more light."
Happy shooting.
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