feat: Create PhotographyFormulas.md

PhotographyFormulas.md

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+# Photography Formulas 
+
+## Calculation of Values
+
+The simple calculations that lead to a reading are based on the Wikipedia entry on [exposure value](https://en.wikipedia.org/wiki/Exposure_value).
+
+The [illuminance](https://en.wikipedia.org/wiki/Illuminance) returned by the Pimoroni BH1745 ( $L$ ) is converted to an exposure value ( $E_v$ ) for ISO 100 using
+
+$$E_v=\log _2  {{L} \over {C}},$$
+
+where $C$ is the light meter [calibration constant](https://en.wikipedia.org/wiki/Light_meter#Calibration_constants).
+
+This exposure value is then adjusted to an Exposure value for the chosen ISO ( $E_{ISO}$ ) using
+
+$$E_{ISO}=E_v + \log_2 {{ISO}\over{100}}.$$
+
+Then, depending on the priority on the light meter, the remaining value is calculated using
+
+$$t = {{N^2} \over {2^{E_{ISO}}}}$$  
+
+or
+
+$$N = \sqrt{t 2^{ E_{ISO}}}$$
+
+where $t$ is shutter speed and $N$ is f-stop. The value is then rounded to the nearest nominal value and displayed on the screen.
+