We present wide field photometry of the Galactic Globular Cluster M 22 in the B , V and I passbands for more than 186,000 stars .
The study is complemented by the photometry in two narrowband filters centered on H _ { \alpha } and the adjacent continuum , and by infrared J , H and K magnitudes derived from the 2 MASS survey for \sim 2000 stars .
Profiting from this huge database , we completely characterized the evolved stellar sequences of the cluster by determining a variety of photometric parameters , including new photometric estimates of the mean metallicity , reddening and distance to the cluster .
In particular , from our multi-wavelength analysis , we re-examined the long-standing metallicity spread problem in M 22 .
According to our dataset , we conclude that most of the observed width of the red giant branch must be due to differential reddening , which amounts to a maximum of \Delta E ( B - V ) \simeq 0.06 , although the presence of a small metallicity spread can not be completely ruled out .
More specifically , the maximum metallicity spread allowed by our data is of the order of \Delta [ Fe/H ] \simeq 0.1 \div 0.2 dex , i.e. , not much more than what allowed by the photometric errors .
Finally , we identified most of the known variable stars and peculiar objects in our field of view .
In particular , we find additional evidence supporting previous optical identifications of the central star of the Planetary Nebula IRAS 18333-2357 , which is associated with M 22 .