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Relationship between flash guide number, capacitance and voltage. Different manufacturers have different ways of determining guide numbers (the argument is that it's only a guide), but none can get around the laws of physics. What matters ultimately is the flash energy, which can be calculated easily from the circuit electrical parameters, viz: Flash energy: E=½CVm² - ½CVx² Where Vm ("V-max.") is the full-charge HV+ voltage; and Vx ("V-ex.") is the Xenon tube extinction voltage, which can be assumed to be about 50V. The residual energy term is small, but it is incorrect to neglect it. Don't worry that the calculated energies usually come out slightly smaller than the manufacturers spec. - electrolytic capacitors usually have asymmetric tolerances, i.e., the average capacitance is slightly greater than that written on the can. Also, a few % either way is irrelevant in photographic terms. If a battery powered strobe has no voltage regulation, it will have a lower guide number if NiCd or NiMH cells are used instead of standard Zn-MnO2 (alkaline) cells. The guide number will also drop off as the battery goes flat. If you want to estimate the effect of voltage drop-off; note that the output of an unregulated inverter is not proportional to the battery voltage, but to the voltage switched across the inverter transformer, i.e., to the battery voltage minus the saturation voltage of the switching transistor(s). The transistors used in inverters typically saturate at about 1V under dynamic conditions. Consequently, a strobe running on 6V will switch about 5V across the transformer primary, but if you run it on a 4.8V NiCd or NiMH battery, it will only switch about 3.8V across the transformer. The proportional reduction in output voltage is therefore (approx.): (4.8-1) / (6-1) = 0.76 I.e., the output voltage obtained using NiCd or NiMH cells is about 76% of the output obtained using new alkaline cells. (The internal resistance of the battery affects the recycling time, but makes little difference to the final voltage, because the inverter current drops as the charging end-point is approached). Unfortunately, due to the square-law relationship between voltage and energy, the proportional reduction in energy output (in this example) will be 0.76², i.e., 0.58. The new guide number however, is simply 0.76 x the original, i.e., if the (unregulated 6V) strobe has a guide no. of 22 with a 6V battery, it will have a guide no. of about 18 with a 4.8V battery. The Guide No. "G" of a strobe is given by the relationship: G=KÖE Where K is a constant which depends on the design of the reflector and any electrical or transmission losses in the system (i.e., circuit resistance, light absorption, etc.). The reflector is the major determinant of K, since losses are usually minimal. The reflector concentrates the light in a particular direction and greatly increases the intensity in comparison to a bare tube. As the angle of coverage is increased, more energy is required to achieve a given guide no.. Capacitor values of some commercial strobes. Ikelite Batt V HV+ V Reg? C/mF E/J Angle of coverage S 6.0 330 300     M (old) 6.0 330 × 450 24   MS, MV 6.0 330 × 600 32   50 6.0 330 × 900 48 70° 100 (old) 7.2 360 1500 95   100a, Ai 6.0 4.8 340 280 × 1800 102 68 80-95° 150 7.2 360 2400 150   200 7.2 356 3000   100° 225 7.2 360 3000 191   300 7.2 360 4500 286   400 7.2 356 6000 378 110° Nikon Batt V HV+ V Reg? C/mF E/J Angle SB101 12.0 350 × 1500     SB102 9.0 330 2700 144   SB103 6.0 330 880     SB104 7.2 400 2200 173   SB105 6.0 330 1300     Sea & Sea Batt V HV+ V Reg? C/mF E/J Angle YS20 3.0 330 × 300     YS50M 6.0 330 620     YS50MS 6.0 330 × 800     YS50ttl 6.0 330 × 1100 59   YS60 6.0 330 × 1100 59   YS100 6.0   800     YS120 12.0 350 1400 84   YS200 7.2 560 1000     YS300     2400     Subatec             S200-TTL 6 - 7.5 360 1650     DWK.