The /f/ sound is spelled with <f> or <ff>, and <ph> in lymph. The /z/ sound is spelled <s> or <z>. Many pairs arise from contrasting a noun ending in /f/ with a plural noun. This includes a number of alphabet letter names, B's, L's, O's and Y's.
The contrast is between a voiceless labio-dental fricative and a voiced palato-alveolar fricative. Most languages operate with similar sounds as phonemes, so the contrast is not usually a problem.
Interesting pairs include:
The mean density value is surprisingly low at 0.6%. The list makes 54 semantic distinctions, a loading of 71%.
baffle Basil bailiff baileys beef B's biff biz brief breeze briefing breezing briefed breezed buff buzz buffing buzzing buffed buzzed buffer buzzer buffers buzzers calf cars chief cheese deaf Des define design defined designed defines designs defining designing delf dells elf L's far Tsar Fed Z fee zee fen Zen feoff fees gulf gulls hoof whose if is leaf leas life lies loaf lows loofah loser loofahs losers lymph limbs muffle muzzle muffled muzzled muffles muzzles muffling muzzling oaf O's off oz phone zone phoned zoned phones zones phoning zoning quiff quiz raffle razzle redefine redesign redefined redesigned redefines redesigns redefining redesigning ref res refine resign refined resigned refines resigns refining resigning rife rise roof rues roof ruse scarf scars self cells serf sirs shelf shells sheriff sherries staff stars sylph sills tariff tarries undefined undesigned waif ways wharf wars whiff whiz wife Y's wife wise wifely wisely wolf wools woof woos
John Higgins, Shaftesbury, October 2010.