so i have read some about film speed and film curve, but i can't know how the two relate. for example, you may have two films with the same personal speed,but still to reach the same density they may need different exposures, as their curve could be different. so, what am i setting my film speed for in terms of exposure? should it be where the meter's reading is medium gray, or where a couple stops under gives good shadows, even if then the meters reading is not middle gray becasue of the curve? or for highlights? how does it all come together?