What allows a scope to gather light?

NorthForkSportsman said:

Size of the bell or coating of lense?

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Both.

The width of the objective, divided by the magnification, gives you the "exit pupil". You might have noticed that when you zoom a scope that's further away than the eye relief the dot you see gets smaller. The image is thus dimmer. Note that once the exit pupil is larger than your pupil it stops getting brighter.

Good coatings increase how much light ends up going through the lens rather than reflecting off.

So this means that a good 7x35 binoculars can be brighter than a cheap pair of 7x50s. Indeed, I have some nice Nikon 7x35 fully coated binos that work better for star gazing than my cheapy 7x50s. The coatings make a difference. Also, in my back yard, which isn't that dark, and my eyes don't dialate as much as I've aged.

I've also read in a few places that even having the same scope with same glass having a 40 verse a 50mm is very close not much differance.

Ransom said:

I've also read in a few places that even having the same scope with same glass having a 40 verse a 50mm is very close not much differance.

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No, not a lot - if it's a 10x scope, a 40mm Objective gives you an Exit Pupil of 4mm, a 50mm yields 5mm. Not a lot, but it helps. Coating and optic design can play a greater factor than 10mm difference.

Still, every bit helps.

From what little I know: coatings help different light wave frequencies to be refracted at the same angle. Light entering a prism will separate into it's component colors. Similar problem occurs in a lense since the light has to enter and be bent twice in a single lense. The more lenses there the more bending occurs, the more the light is refracted. Coatings allow the light to not refract in the multiple lenses that are in scopes. Blue light, which is the highest visible frequency will refract more than red. That's why our sky is blu: blue lights refracts from the dust in the atmosphere.

Ramone said:

No, not a lot - if it's a 10x scope, a 40mm Objective gives you an Exit Pupil of 4mm, a 50mm yields 5mm. Not a lot, but it helps. Coating and optic design can play a greater factor than 10mm difference.

Still, every bit helps.

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well to get very technical, it helps but only to a point.

Exit pupil is calculated (in millimeters) by dividing the diameter of the objective lens by the power of the scope. Your eye pupil can vary from 4mm to 7mm in low light, and the quality rifle scopes will deliver an exit pupil of 3 to 3.5mm on the high setting.

Once the exit pupil size is physically larger that your dialated pupil, there is no more light to be gained.

this is gerneally conditional mainly on age but also genetics and health. 7mm dialed is fairly extreme.