Often referred to as “true zoom,” optical zoom is performed completely within the lens. The optics within the lens physically move to magnify, or enlarge, a part of the view onto the sensor.
Standalone cameras, like DSLRs and mirrorless, refer to zoom capability in focal length.
Expressed in millimeters (mm), the higher the focal length, the more magnification the lens provides. For example, the 80 – 400 mm zoom lens has a minimum focal length of 80 mm and a maximum focal length of 400 mm. It can also focus at any length in between.
Point-and-shoot cameras express their optical zoom in x numbers, such as 2x, 4x, etc. This just means the camera’s lens can magnify the image 4x larger.
Phones are slightly different. As they need to stay very thin, they don’t have lenses that move. Instead they use magnified prime lens (fixed focal length). Even though the lens doesn’t move, phone manufacturers still call this “optical zoom,” and it is also expressed in x numbers, such as 2x, 4x, etc.
Most importantly, Optical Zoom does not impact the resolution or the quality of an image. In other words, the camera sensor is still using all of the camera’s megapixels and each pixel has real data.