The high power impulse magnetron sputtering (HiPIMS) discharge brings about increased ionization of the sputtered atoms due to an increased electron density and efficient electron energization during the active period of the pulse. The ionization is effective mainly within the electron trapping zone, an ionization region (IR), defined by the magnet configuration. Here, the average extension and the volume of the IR are determined based on measuring the optical emission from an excited level of the argon working gas atoms. For particular HiPIMS conditions, argon species ionization and excitation processes are assumed to be proportional. Hence, the light emission from certain excited atoms is assumed to reflect the IR extension. The light emission was recorded above a 100 mm diameter titanium target through a 763 nm bandpass filter using a gated camera. The recorded images directly indicate the effect of the magnet configuration on the average IR size. It is observed that the shape of the IR matches the shape of the magnetic field lines rather well. The IR is found to expand from 10 and 17 mm from the target surface when the parallel magnetic field strength 11 mm above the racetrack is lowered from 24 to 12 mT at a constant peak discharge current.