We present the Capodimonte Deep Field (OACDF), a deep field covering an area of 0.5 square degrees in the B, V, R optical bands plus six medium-band filters in the wavelength range 773--913 nm. The field reaches the following limiting AB magnitudes: B~25.3, V~24.8 and R~25.1 and contains about 50000 extended sources in the AB magnitude range 18 < R < 25 . Hence, it is intermediate between deep pencil beam surveys and very wide but shallow surveys. The main scientific goal of the OACDF is the identification and characterization of early-type field galaxies at different look-back times in order to study different scenarios of galaxy formation. Parallel goals include the search for groups and clusters of galaxies and the search for rare and peculiar objects (gravitational lenses, QSOs, halo White Dwarfs). In this paper we describe the OACDF data reduction, the methods adopted for the extraction of the photometric catalogs, the photometric calibration and the quality assessment of the catalogs by means of galaxy number counts, spectroscopic and photometric redshifts and star colors. We also present the first results of the search for galaxy overdensities. The depth of the OACDF and its relatively large spatial coverage with respect to pencil beam surveys make it a good tool for further studies of galaxy formation and evolution in the redshift range 0--1, as well as for stellar studies.