In article , Alan Browne
says...
You can do a standard candle test, if you like. Shoot a grey card in
direct sunlight.

Well, the sun light strength may differ depending on the day of the year
and the geographical location.
--
Alfred Molon

Olympus E-series DSLRs and micro 4/3 forum at
http://tech.groups.yahoo.com/group/MyOlympus/
http://myolympus.org/ photo sharing site