Optics
What are optics and where to look for them
Optics are used by your crew to look at enemy. For you, they are used whenever you use sniper (gunner) scope.
You can always see them when you use X-ray in vehicle preview, though they often come without proper description and don't say their magnification levels. Generally, only one of them tells zoom levels.
Optics and viewports are also sometimes a weakspot of super heavy tanks, such as IS-7, though they are extremely hard to target, as they are very small.
Just damaging the optics does nothing to the tank or player's effectiveness in game so far.
Zoom levels in the game
Commander's binoculars always have about X6 zoom.
Here are the examples of zoom levels for different confirmed vehicles. Screenshots are made in test drive, always from a certain position (seen on minimap) The target is at a distance of exactly 1km on these screenshots, as should be seen by indicators. (Click on first image and browse them with arrow keys/buttons on screenshot sides to make it easier.)