F Stops

I wrote a LV calculator here:

pentaxforums.com/forums/photography-articles/88197-excel-2003-lv-light-value-calculator.html

Ideally you should use T-Stops, rather than F-stops, but F-stops will give you ballpark figures.

APEX (Additive System of Photographic Exposure) is covered in the 2nd last message in the link below, and using the APEX info included in image Exif data from v2.2 onwards to work out LV is covered in the last message of that thread:

pentaxforums.com/forums/photography-articles/126203-stops-shutter-speed-aperture-iso-sensitivity.html

Dan.