Leupold® Golden Ring VX-3 Rifle Scopes with 30mm Maintubes
FOV = field of view at 100 yards.
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The second number refers to the diameter of the bell (the far end of the scope). The 3.5x40mm scope has a bell diameter of 40mm. The larger the diameter of the bell the more light it can capture, meaning in a hunting situation you will be able to see better in low light (dusk and dawn). The advantage of more light is more pronounced at higher magnifications and long ranges, so with these 4 scopes I don't think you would notice too much difference.
The advantage of a 30mm tube is also more light. The larger the diameter of the tube, the more light can travel through, the better a shooter can see in low light situations. Also the 30mm tube allows for more vertical and horizontal adjustments for long range shooting. Hope this helps.
The number after the "x" is the objective lens diameter. The larger this number the better the low-light performance, but this also means the scope will be bulkier & heavier. For daylight use 32-40 mm objective lenses are adequate. At dawn & dusk 50 mm lenses are popular as they will let you shoot when you can't see the animals without your scope. In Europe where night hunting is more popular 56 mm lenses are popular. They are big & bulky but they will let you shoot by moonlight.
A 1" main tube was long the US standard but since rangefinders got cheap & more people are practicing at 500 yards (or more) the 30 mm main tube is becoming more popular. A 30 mm tube (about 1.125") is stiffer than a 1" tube and allows more range of turret adjustment, allowing for longer shots than possible from a 1" tube. In reality, very few people ever shoot at ranges where the 30 mm tube is necessary but the added stiffness should help with accuracy.