We are frequently asked about 'scopes and we will complete this very short and basic explanation as time permits. The 'scope pictured is a Nikko Stirling 'Mountmaster' (AO, IR, MD, Side IR - which all means - Adjustable Objective, Illuminated Reticle, Mill Dot,) This is a '4x32' scope and that means it has 4power magnification and a 32mm objective (front) lens.
PetoneGunshop.com - 291 Jackson Street, Petone
The 'Front' lens is called the 'objective' lens and the number '32' specifies the diameter of that lens.
A bigger diameter lens lets through more light BUT most eyes cannot notice the light transference difference between (for example) a 40mm and a 50mm lens - However, what the shooter will notice is the bigger bell (front swelling) means the scope must be mounted higher - which makes it less comfortable to carry and the extra height above the barrel means the shooters cheek is not so firmly steadying the rifle and the sight axis is further above the bore axis.
The 'scope pictured has a parallax adjustment - which, for most users, is an unnecessary option and complication . It is a requirement for accurate and extreme long range shooting - BUT with a 4x scope the parallax between 100m and 500m is about 25mm - much less than the grouping ability of a sporting rifle!
It is more important on a precision air rifle when shooting between varying ranges.