  By digitizing astronomical photographic plates one may extract full
information stored on them, something that could not be practically achieved
with classical analogue methods. We are developing algorithms for variable
objects search using digitized photographic images and apply them to 30cm
(10x10 deg. field of view) plates obtained with the 40cm astrograph in 1940-90s
and digitized with a flatbed scanner. Having more than 100 such plates per
field, we conduct a census of high-amplitude (>0.3m) variable stars changing
their brightness in the range 13<m<17 on timescales from hours to years in
selected sky regions. This effort led to discovery of ~1000 new variable stars.
We estimate that 1.2 +/- 0.1% of all stars show easily-detectable light
variations; 0.7 +/- 0.1% of the stars are eclipsing binaries (64 +/- 4% of them
are EA type, 22 +/- 2% are EW type and 14 +/- 2% are EB type); 0.3 +/- 0.1% of
the stars are red variable giants and supergiants of M, SR and L types.
