  We report results of multiband optical monitoring of two well known blazars,
S5 0716+714 and BL Lacertae, carried out in 1996 and 2000-01 with an aim to
study optical variations on time scales from minutes to hours and longer.The
light curves were derived relative to comparison stars present on the CCD
frames. Night to night flux variations of >0.1 mag were observed in S5 0716+714
during a campaign of ~2 weeks in 1996.A good correlation between the
lightcurves in different optical bands was found for both inter-night and
intra-night observations. Two prominent events of intra-night optical
variability were detected in S5 0716+714.Each of these rapidly varying segments
of the lightcurves trace an exponential flux profile whose rate of variation is
the same in both cases. Our long-term monitoring data of S5 0716+714 showed a
distinct flare around JD 2451875 which can be identified in the BVRI bands.This
flare coincides with the brightest phase recorded during 1994-2001 in the
long-term lightcurves reported by Raiteri et al.(2003). No evidence for the
bluer when brighter trend was noticed on inter-night and intra-night time
scales. On the other hand, our nearly simultaneous multiband observations of BL
Lacertae in October 2001 showed flux variations that were not achromatic. This
blazar was found to become bluer when brighter on intra-night time scales and
there is a hint of the same trend on inter-night time scales. Based on five
nights of observations during a week, BL Lacertae showed a peak night-to-night
variability of ~0.6 mag in B. Thus, we found that the present observations of
the two blazars, reveal a contrasting behaviour in terms of the dependence of
spectral hardening with increasing brightness, at least on intra-night, and
possibly also on inter-night, time scales.
