**EXPOSURE EQUATIONand LIGHT METER CALIBRATION**

Main exposure equation

(camera settings based on scene Luminance and film speed):

(N^2)/t = L*S/K

Where:

N is F-numbers

t is exposure time

L is average scene luminance

S is ISO speed

K is calibration constant

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Per Lambert's Cosine Law if we have a perfect diffuser, then:

Reflectance R is:

R = pi*L/E = pi * K/C

Where:

L is luminance

E is illuminance.

K is the calibration constant for a reflected meter and

C is calibration constant for an incident meter

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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).

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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.

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**RECOMMENDATION FOR CALIBRATING SEKONIC DUALMASTER**

FOR PRACTICAL ON-SET USE

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."