We first saw optical options with multiple zoom levels in the first Black Ops, with three magnification level option available for the sniper rifles. Such an attachment made a return in Modern Warfare 3, and once again in Black Ops 2.
In real-life, rifle scopes can indeed offer multiple magnification levels within a single optical unit: such variable power scopes a boon when hunting or otherwise shooting ate non-fixed distance. The different zoom levels are implemented through mechanical movement or superposition of different optical elements: operating in a similar fashion to a camera lens. Such options tend to have greater internal complexity than a fixed-power scope, and as such tend to be more expensive - but they do offer a greater degree of flexibility.
In Black Ops 2, Variable Zoom is available for the LMGs, Sniper Rifles, and the Crossbow. It is only incompatible with other optical options, with the exception of the Dual Band scope, which when combined with the variable zoom gives you a flexible thermal option. The Vary Zoom's primary effect is to grant you a magnification level switchable between two distinct settings. In the case of the sniper rifles, you'll see a 2.0 times magnification with the low option, and 4.8 times with the high option.
LMGs see a slightly higher level of magnification, 2.4 times with the low option, and 5.8 times when zoomed in. These two option give you the ability to dynamically adjust to your firing position, giving youth perfect compromise between view on target and peripheral vision. Beyond this selectable magnification level, there are some other fringe benefits too: your weapon's idle sway will be reduced by 5 percent, granting a slight advantage at longer ranges.
In addition, for the LMGs the gun kick factor of recoil is removed, making such weapons easier to control in some respects. To compensate for this, there is one downside to the Variable Zoom for such weapons: Lugsail see a 10 percent center speed penalty, increasing the effects of view kick in lieu of the absent gunkick. This works out as a shift in recoil characteristics rather than a straightforward reduction - but with good fire control, LMGs can remain quite effective with this attachment.
The Variable Zoom optic is a great way to enhance your weapon's flexibility over mid-to-extreme ranges, with the adjustable magnification allowing you to alter your view for maximum comfort. It's great with those weapons that can comfortably reach out to an extreme range - the sniper rifles and low-recoil LMGs the prime candidates. The lower zoom option can work well with a moderate dose of aggression with the sniper rifles - the wider field of view allowing for more effective use over shorter ranges. It’s perhaps less useful for the higher recoil LMGs (such as the QBB LSW and HAMR), as you’ll struggle to reliably hit targets at the ranges the higher zoom is designed for. It might not be ideal for every sniper build, either - if you're content to stick to a static long-range role, a single purpose scope might suit you just fine - and any scope option would be entirely useless for an aggressive quick scoping build.
Still, the attachment has no real downsides and will slightly extend your weapon's versatility, so it's definitely worth considering. If flexibility isn't your forte, set your sights on this optic......and add some variety to your loadout.