At the beginning of the X serie, Fuji started with zoom lenses, the XF-18-55mm f/2.8-f/4 and the XF-55-200mm f/3.5-f/4.8. Now Fuji is mostly known for their fixed lenses.
The nice thing, about these variable zooms, is that they were great bargains. Small, cheap and have pretty good optical quality.
The problem with these variable zooms is that they are variable zooms.
- The 18-55 at 18mm the aperture can be an f/2.8 but by 23mm the aperture becomes f/3.2 and at 48mm it’s an f/4.
The 55-200 at 55mm the aperture can be a f/3.5, at 100mm the aperture becomes f/4, at 135mm the aperture becomes f/4.5 and at 200mm the aperture becomes f/4.8
If you choose f/2.8 at 18mm and then zoom, the f/stop will slowly set itself to a lower value according to the zoom.
- If you choose f/4.5 at 18mm and then zoom, the f/stop will stay at f/4.5 through the full zoom range.
BTW, the XF-18-55mm f/2.8-f/4 is ⅔ of a stop faster than the standard APS-C kit lenses from either Canon, Nikon or Sony.