I am not sure if there is such a lens as a lens with a fixed aperture.
A lens that says 50 mm f1.4 means that the widest aperture is 1.4, but it may close down to something like f22.
A lens that says 1.4-2.5 usually a zoom lens. At the shorter end of the zoom, the widest aperture may be f1.4. At the longer end of the zoom the widest aperture may be 2.5. This "variable" aperture in zooms are usually seen in lower end zoom lenses. The better ones have fixed aperture throughout the zoom range. The best I have seen are fixed at 2.8. Not that they cannot be made to be f2 or wider, but I think the cost will be so phenomenal that one will have to sell wife and kids to purchase these!
I have never seen a variable aperture from1.4 to 2.5 in zoom lenses. But then, the latest digital equipment may have things I am not aware of.