ND filter test,
This checks the variable ND filters and the fixed ND filters for fading,
it wouldn't work if filters got darker as they age, but they don't.
Set the meter to dimmest spot on the middle range, find an object that is
that bright, set the meter to brightest spot on the low range, the object
should match. The same is done for brightest spot on the middle range
and dimmest spot on the high range.
Mine is within 1/2 a division (1/6th stop)