How to equalize airphoto's contrast and other params?

[Ania Oldak]

Dear Netters,

   I have several scanned airphotos which I'm trying to join together.
They are of quite good resolution (214 dpi), but their overall bright-
ness and contrast differ rather a lot. My problem is as follow. While
the rectification went without any snags, I'm still unable to find a way
to adjust their various contrast scales and to give a common one to all.
I went with a pickcomb through the Grass manuals and even through Martijn
van Leusen's excellent beginner's guide, but with no success.
I was trying to use r.mapcalc, but only managed to 'discover' something
which I find rather strange. From the overlapping area of two adjacant,
rectified photos, I picked all pixels with a given value, say 63, of one
photo. I was expecting that the corresponding pixels from the second
photo will have their values spread in a relatively narrow range. How-
ever, just the opposite was true. E.g. the spread was as broad as from 40
to 210 in some cases. It might be relevant here to mention that the resi-
duals rms were never bigger than about 3 during rectification. The reason
for this eludes me, though maybe it has something to do with rectification
errors after all. Any suggestions are welcome.

I'd very much appreciate hints and leads how to proceed,

regards
ania