  We present a study of the star formation rate (SFR) for a sample of 16
distant galaxies detected by ISOCAM at 15um in the CFRS0300+00 and CFRS1400+52
fields. Their high quality and intermediate resolution VLT/FORS spectra have
allowed a proper correction of the Balmer emission lines from the underlying
absorption. Extinction estimates using the Hbeta/Hgamma and the Halpha/Hbeta
Balmer decrement are in excellent agreement, providing a robust measurement of
the instantaneous SFR based on the extinction-corrected Halpha luminosity. Star
formation has also been estimated exploiting the correlations between IR
luminosity and those at MIR and radio wavelengths. Our study shows that the
relationship between the two SFR estimates follow two distinct regimes: (1) for
galaxies with SFRIR below ~ 100Msolar/yr, the SFR deduced from Halpha
measurements is a good approximation of the global SFR and (2) for galaxies
near of ULIRGs regime, corrected Halpha SFR understimated the SFR by a factor
of 1.5 to 2. Our analyses suggest that heavily extincted regions completely
hidden in optical bands (such as those found in Arp 220) contribute to less
than 20% of the global budget of star formation history up to z=1.
