  The Zeeman splitting of a localized single spin can be used to construct a
magnetometer allowing high precision measurements of magnetic fields with
almost atomic spatial resolution. While sub-{\mu}T sensitivity can in principle
be obtained using pulsed techniques and long measurement times, a fast and
easy-to-use method without laborious data post-processing is desirable for a
scanning-probe approach with high spatial resolution. In order to measure the
resonance frequency in realtime, we applied a field-frequency lock to the
continuous wave ODMR signal of a single electron spin in a nanodiamond. In our
experiment, we achieved a sampling rate of up to 100 readings per second with a
sensitivity of 6 {\mu}T/$\sqrt{Hz}$. Using this method we have imaged the
microscopic field distribution around a magnetic wire. Images with \sim 30
{\mu}T resolution and 4096 sub-micron sized pixels were acquired in 10 minutes.
By measuring the field response of multiple spins on the same object we were
able to partly reconstruct the orientation of the field.
