For example, f1.8 allows more light to pass through than f2, but not twice as much light, as f1.4 permits. Similarly, f3.5 is a half stop between f2.8 and f4, it allows more light to pass through than f4, but not twice as much. A zoom lens is a variable focal length lens. By internal shift of their elements, zooms vary their focal length. This variance is called range.
Zooms are popular, cost more, and eat up more light than a fixed lens, but they are versatile.
Because of their flexibility and convenience, zoom lenses dominate the market. Many zooms are capable of focusing at very close distances. A normal 50 mm lens attached to a 2X converter will make it a 100 mm telephoto lens. A 2X converter will require opening the lens two more stops and it will reduce the sharpness of the lens being used with it. To determine correct exposure, meter the brightness of a scene.