The FSO communication systems promise fast data transmission but are significantly affected by antagonistic meteorological conditions such as fog. This study presents a comprehensive comparative analysis of three widely used photodetectors—Avalanche Photodiode (APD), Single Photon Detector (SPD) and Positive-Intrinsic-Negative (PIN) photodiode—in the context of FSO communication under fog weather conditions. The investigation evaluates system performance based on Bit Error Rate (BER), quality factor, and received optical power as output parameters. The analysis is conducted by varying the input optical power and link length, offering insights into the trade-offs and limitations associated with each detection technique under these challenging conditions. Simulation results highlight the sensitivity and performance of each photodetector, providing a deeper understanding of their suitability for FSO systems in fog-impacted environments. The SPD proves a better selection than the other two. The findings contribute to deriving the expression of attenuation A and link length L as a function of visibility.