what does 1:4-5.6 means on the lens. Does it means that the focal length is from f4 to f5.6 only? Or does it means that it has a range from f1 to f5.6?
It means it's a zoom lens that can open as wide as f/4 when zoomed out to the widest angle of view. When zoomed in to the longest focal length it will only be able to open up to f/5.6. For some lenses minimum aperture will be something like f/22 for all focal lengths, for other lenses it might be around f/22 on the wide end and f/32 on the long end.
Remember that apertures are expressed as ratios or fractions, so f/4 means 1:4 (the aperture is 1/4 of the focal length) and f/5.6 means 1:5.6 (the aperture is 1/5.6 of the focal length).
So 1:4-5.6 could also be expressed as 1:4-1:5.6, f/4-f/5.6.
It means that the ratio of the focal-length to aperture is f/4 at the wide end of the zoom and f/5.6 at the telephoto end. This is commonly done to saved weight and costs compared to a constant aperture zoom lens which simply has one ratio, say 1:4.
These numbers say nothing about the focal-length, it is just a ratio which is how aperture is often expressed as a dimensionless f-number rather than the physical diameter of the entrance pupil. So if it is a 50-200mm lens for example, the maximum aperture at 50mm is f/4 which means the entrance pupil is 12.5mm wide. At 200mm, the maximum aperture is f/5.6 which makes it about 36mm across.
When we talk about lens, two key values stand out.
Focal Length. This is a measurement, usually in millimeter units, that tells us about the size of the image the lens produces and the angle of view presented. A short focal length lens delivers a “wide-angle view”. A medium focal length yields what is termed a “normal” view. A long focal length magnifies, we call this view “telephoto”.
Focal Ratio. We divide the focal length by the working diameter of the aperture. The result is a ratio that we shorten to “f-number”. Since a ratio is void of dimension, we can use this focal ratio to compare any lens with any other lens, regardless of focal lengths. The lower the focal ratio, the more light the lens is capable of gathering. In other words, a f/4 lens gathers more light than a f/5.6 lens.
We all desire to mount lenses what work well in feeble light. Thus we prefer “fast” lenses. By fast we mean lenses with f-number closer to 1. Now the f-number and the focal length are intertwined. As we zoom a lens to higher magnifications (longer focal length), the ability of the less to pass light is reduced. At elevate magnification, your lens functions at f/5.6.When you zoom into the range of “normal” and “wide-angle”, your lens gathers more light and functions at f/4. In other words, the light gathering ability is not constant throughout the entire range of the zoom. A constant aperture is a feature of a more expensive zoom lens. Don’t anguish, your lens is quite good, it is a “general purpose” zoom some call a “kit lens”.
My 2 cents.
f number, f stop, relative aperture, or N is a relationship between the focal length and the diaphragm.
f = Focal lenght
D = Diameter of entrance pupil.
It is relative. It varies if one of the two numbers change.
If the focal length gets smaller, N gets smaller, and if gets larger N also increases.
Remember if the N number gets larger, the image gets darker.
So 1:4-5.6 says that the aperture wide open on that zoom lens will change between thoose values acording to the focal length.