The obsolete aperture number sequence: f/6.3, f/9, f/12.5, f/18, f/25 passes 1/3 stop less light than the modern conventional sequence f/5.6, f/8, f/11, f/16, f/22.

Thus

f/6.3 passes 1/3 stop less light than f/5.6

f/9 passes 1/3 stop less light than f/8

f/12.5 passes 1/3 stop less light than f/11

f/18 passes 1/3 stop less light than f/16

f/25 passes 1/3 stop less light than f/22