and LIGHT METER CALIBRATION
Main exposure equation
(camera settings based on scene Luminance and film speed):
(N^2)/t = L*S/K
N is F-numbers
t is exposure time
L is average scene luminance
S is ISO speed
K is calibration constant
Per Lambert's Cosine Law if we have a perfect diffuser, then:
Reflectance R is:
R = pi*L/E = pi * K/C
L is luminance
E is illuminance.
K is the calibration constant for a reflected meter and
C is calibration constant for an incident meter
For an 18% gray card:
R = .18
if we set E to 1.0 (for 100%) then:
L = .18 / pi
So, the reflected measurement in cd/m^2 should be
The incident lux multiplied by .18/pi (or .0573).
Sekonic's Calibration Constants are:
K = 12.5
Lumisphere C = 340
Flat Diffuser C = 250
This means that:
With sphere retracted ("flat"): pi*12.5/250 = .157
With sphere out: pi*12.5/340 = .115
So, incident and spot should agree on either a 15.7% gray card or
a 11.5% gray card (but NOT an 18% gray card!), depending on sphere position.
RECOMMENDATION FOR CALIBRATING SEKONIC DUALMASTER
FOR PRACTICAL ON-SET USE
A 15.7% gray card is only 2 tenths of a stop darker than an 18% one and I use the
exposure read-out of the spot meter (i.e. its read-out in f/stop and shutter) much more
than I use its absolute read-out (i.e. read-out in cd/m^2 or footlamberts). So the best
calibration method for my own personal usage is:
First, retract the sphere and calibrate the incident meter to spec.
Second, do NOT calibrate the spot meter to spec. Instead, calibrate it so that its
exposure read-out (f/stop and shutter) match the incident read-out for a nearly-
collimated light falling on an 18% gray card.
By so doing, I accept a 2-tenths stop miscalibration in the spot meter's absolute reading
in exchange for effectively changing its calibration constant so that spot and incident
readings agree for 18% gray instead of 15.7% gray. This is more practical for my personal
working style, since I rarely meter in cd/m^2, but I often meter in exposure read-out, and
have already conditioned myself to think of 18% gray as "neutral."