The aperture f-number is the ratio between the effective focal length and the diameter of the aperture openning.
f-number = focal length / aperture diameter.
So, the larger the aperture, the smaller the f-number.
Most of the time, the maximum aperture (smallest f-number) available for a lens is indicated in the specification of the lens. For example, 50mm f/2.8 means that the focal length is 50mm, and the maximum aperture is f/2.8.
For zoom lens, it becomes a design challenge to maintain the same f-number throughtout the entire zoom range. As you zoom without changing the setting on the aperture ring, the lens mechanism would have to change the aperture diameter to compensate for the change of focal length to maintain a constain ratio, thus constant f-number.
So, zoom lenses can be divided into 2 groups: one with constant aperture, one with variable aperture.
You lens is specified as 28-105mm, f/3.5~4.5. It means that the maximum f-number at 28mm zoom setting is f/3.5. As you zoom towards the 105mm end while keeping the aperture ring set at the f/3.5 position, the effective f-number changes gradually to f/4.5.
If it was a constant aperture zoom, then it will be marked as 28-105mm f/3.5.
