100BASE-FX single-mode, confusing distances

Aside from 100BaseFX being very obsolete in this day and age, the main constraint on distance with single-mode fiber is power input (.vs. fiber attenuation/loss) and receive sensitivity. But seriously, replace with 1000BaseLX when convenient and reap the rewards of 10 times more speed on the same fiber for potentially as little as $10 (used gigabit SFPs are quite inexpensive.) 10Gig will run on the same single-mode fiber but it will cost a lot more than 1 gig.

10 km single-mode is pretty simple and foolproof for moderate values of fool. If you plug two SFPs together with a 5 m patchcord, they work. If you plug them with a 10 km fiber in good shape, they work.

Longer distances can get difficult in the hands of moderate fools, as the power output by the laser may exceed the receiver's input capacity, unless it has been attenuated (by a long fiber, or by a fiber attenuator) So it becomes possible to burn the receiver if short cords are used without attenuators, say while "testing." Check the data sheets for the particular SFPs to see if this is a potential problem.