All lenses have a maximum aperture or lens opening, a condition in which the lens wide open so as to let light in. diameter at the most appropriate lens itself. In most zoom lenses, the maximum aperture will change as you perform zooming (changing the focal length lenses. Zoom lens which is called a zoom lens has a maximum aperture “variable”.
For example, popular kit zoom lenses: 18-55mm f/3.5-5.6 lens. This lens when used in the short (18 mil) has a maximum aperture f/3.5. However, when used longest side (55mm), maximum aperture was going to suddenly turn into f/5.6.
By changing the aperture as in the case above, we would lose a little bit of light coming into the sensor when the lens is used its longest side.
Some of the more expensive zoom lens has a maximum aperture fixed, constant and does not change when we do the zooming. An example is the 70-20mm f/2.8 lens. If it is set in the opening (aperture) f/2.8, so you want to wear on the short side or the long side 200mm 70mm, f/2.8 aperture will always be. Type of zoom lens is relatively better quality but also much more expensive than variable zoom lens as above.
Why Maximum Aperture Zoom Lens Fixed With More Expensive?
Zoom lens with a fixed aperture are more expensive because they use more sophisticated elements in it than a lens with a variable aperture. Additionally lens uses an optical element that is larger (f/2.8 than f/5.6).