Jairo
If you don't have a light meter yet ( I don't either), look into the grey cards. They're fairly inexpensive (less than $15-20)...Like I mentioned before, I have one and I've seen it suggested before but I'm bad about remembering to actually use it when I should have...fall in the 'hindsight is 20/20..." I think...
I have experimented with it at home and did notice a difference in being much closer to proper exposure. Here's a link that someone here shared to me before... hope it helps you in someway as well:
http://www.goshen.edu/~marvinpb/graycd.html
came from this thread:
http://www.texasphotoforum.com/forum...ad.php?t=27842