Some examples of lenses and markings
28-70mm zoom lens
Max aperture f2.8 throughout.
70-300 zoom lens
Max aperture f4.5 at 70mm, 5.6 at 300mm
17-35mm zoom lens
Max aperture f2.8 at 17mm, f4 at 35mm, min aperture f32 (in brackets)
35-80mm zoom lens
Max aperture f4 at 35mm, f5.6 at 80mm, min aperture f22 (in brackets)