Q Please tell me something about "variable power" scopes
A Variable power riflescopes give the hunter obvious advantages in being adjustable to suit changing field conditions like widening the field of view for rapid acquisition of close range targets or increasing magnification for distant targets and smaller game. Apart from moderate extra cost the modern high grade variables owe nothing to the fixed power models in that they are just as accurate, hard wearing and reliable as non-variables. When bushveld is the main hunting domain choose variables with the widest field of view such as Lynx P1.25~4x26, P1.5~6x42, 1.75~5x20, P2.5~7x28 Compact and 2~7x32. When open country hunting predominates choose scopes with 9x or 10x at their top end, like Lynx 3~9x40; P3~9x42; T3~9x44 or T3.5~10x50 . August 1999
A Hunters know that variable-power riflescopes are the best choice for general hunting in a variety of environments, but other useful features of the variable scope are often overlooked. For example, the most popular variable scope (3-9x40) produces the best target visibility at 8x magnification in dawn / dusk light, while in starry-night conditions it is at it’s best at approximately 5.7x magnification; at these magnifications, the scope’s exit pupil is approximately the same size as the shooter’s eye pupil in the light conditions mentioned, 5mm and 7mm respectively, which produces optimum illumination at the retina.
The components that make all the changes in magnification, field of view and exit pupil size are two mobile lenses in the scope’s erector tube which move backward and forward as shown in the illustrations. In variable riflescopes which do not enlarge the crosshairs simultaneously with target magnification these mobile lenses must track precisely on the scope’s axis (optical centre dotted line in the illustrations) to ensure that the scope’s aimpoint stays unchanged throughout the zooming range. Lynx variable scopes are renowned for their accuracy in this respect and for their stability under long-term recoil shocks.
A If a riflescope is pointed towards the light and held at arms-length its exit pupil shows as a bright round disc in the eyepiece. If the exit pupil is 10mm or bigger in diameter the scope is suited for fast target acquisition although at close range care is needed to centre the aiming eye to avoid parallax errors. In a variable-power scope the exit pupil changes size when the power ring is turned, giving its owner the possibility of maximising its size for various purposes - one of which is to choose the one and only magnification that will give the brightest most informative target image in low light conditions; to do this it is necessary to know that optimum light transmission to the shooter’s eye occurs when the exit pupil is the same diameter as the shooter’s eye pupil (maximum transmission matters not at all in daylight but in low light may be important).
Suppose you’re hunting in the dusk, when according to convention your eye pupil will be 5mm in diameter and you want the same size exit pupil in your 3~9x40mm riflescope to maximise its light transmission. Dividing the desired 5mm into the scope’s front lens diameter gives the power to which the scope must be set - 8X. In darker conditions you’ll use 7 instead of 5 as the divisor if you’re young enough for your pupils to dilate to that size (under 40 years!).