High speed video and streak camera imaging are used to measure peak pressures for explosions of spherical charges of the high explosive C-4 (92 % trimethylenetrinitramine, C3H6N6O6). The technique measures the velocity of the air shock produced by the detonation of the explosive charges, converts this velocity to a Mach number, and uses the Mach number to determine a peak shock pressure. Peak pressure measurements are reported from a few millimeters to approximately one meter from the charge surface. Optical peak pressure measurements are compared to peak pressures measured using piezoelectric pressure transducers, and to peak pressure measurements estimated using the blast computer code CONWEP. A discussion of accuracy of peak pressures determined optically is provided.