  Imaging the fields of magnetic materials provides crucial insight into the
physical and chemical processes surrounding magnetism, and has been a key
ingredient in the spectacular development of magnetic data storage. Existing
approaches using the magneto-optic Kerr effect (MOKE), x-ray and electron
microscopy have limitations that constrain further development, and there is
increasing demand for imaging and characterisation of magnetic phenomena in
real time with high spatial resolution. In this work, we show how the
magneto-optical response of an array of negatively-charged nitrogen-vacancy
spins in diamond can be used to image and map the sub-micron stray magnetic
field patterns from thin ferromagnetic films. Using optically detected magnetic
resonance, we demonstrate wide-field magnetic imaging over 100x100 {\mu}m^2
with a diffraction-limited spatial resolution of 440 nm at video frame rates,
under ambient conditions. We demonstrate a novel all-optical spin relaxation
contrast imaging approach which can image magnetic structures in the absence of
an applied microwave field. Straightforward extensions promise imaging with
sub-{\mu}T sensitivity and sub-optical spatial and millisecond temporal
resolution. This work establishes practical diamond-based wide-field microscopy
for rapid high-sensitivity characterisation and imaging of magnetic samples,
with the capability for investigating magnetic phenomena such as domain wall
and skyrmion dynamics and the spin Hall effect in metals.
