Context :

Aims : We address the topic of the Intra-Night Optical Variability of the BL Lac object S5 0716+714 .

Methods : To this purpose a long term observational campaign was performed , from 1996 to 2003 , which allowed the collection of a very large data set , containing 10,675 photometric measurements obtained in 102 nights .

Results : The source brightness varied in a range of about 2 mag , although the majority of observations were performed when it was in the range 13.0 < R < 13.75 .
Variability time scales were estimated from the rates of magnitude variation , which were found to have a distribution function well fitted by an exponential law with a mean value of 0.027 mag/h , corresponding to an e-folding time scale of the flux \tau _ { F } = 37.6 h. The highest rates of magnitude variation were around 0.10–0.12 mag/h and lasted less than 2 h. These rates were observed only when the source had an R magnitude < 13.4 , but this finding can not be considered significant because of the low statistical occurrence .
The distribution of \tau _ { F } has a well defined modal value at 19 h. Assuming the recent estimate of the beaming factor \delta \sim 20 , we derived a typical size of the emitting region of about 5 \times 10 ^ { 16 } / ( 1 + z ) cm .
The possibility to search for a possible correlation between the mean magnitude variation rate and the long term changes of the velocity of superluminal components in the jet is discussed .

Conclusions :