[GRASS-user] shaded relief of srtm tiles

[thorsten]

hello list,

i try to use r.shaded.relief  on srtm tiles imported with r.in.srtm.
the results look nice, but adjacent tiles have a different average 
brightness.
I used the same parameters (altitude=44 azimuth=270 units=meters) for 
both maps and tried with grass versions 6.02 and 6.20
That's how it looks like for the original srtm tiles N42W005.hgt.zip
and N42W004.hgt.zip:
http://www.flickr.com/photos/h2guide/328146671/

What am i doing wrong? (Or what am i supposed to do in order to produce 
seamless shaded reliefs?)

Thanks in advance,
Thorsten