Hi @OverCranked !
So to explain my setup:
My Sprocket Reading cell is plugged in a Analog Input (A0). Not Digital Input, as I need something else than 0 and 1.
When I scan a new film, I run first this small program that just run the Super8 reel slowly and print the reading of the cell (reading the laser through the side of the film) on the Serial Plotter from the Arduino IDE.
Then it give me a value telling me the difference between when there is the side of the film and when there is the sprocket hole. I keep this value (so, it not SOOO precise, in the example from the video you see that there is a big difference between hole and film as it’s a b&w positive… with negatives the difference is really smaller. ), and use it in my programm to tell the arduino: when the reading reach this value, you stop, you shoot and wait 2s and then you go back to work 
an explanation, sorry for my early morning frenchman english
The program for the reading:
// Define connections
#define LED 2
#define DIR 8
#define STEP 9
const int analOpto = A0;
int optoRead = analogRead(A0);
boolean setdir = LOW;
void setup() {
Serial.begin(230400);
pinMode(DIR, OUTPUT);
pinMode(STEP, OUTPUT);
pinMode(LED, OUTPUT);
}
void loop()
{
{
delay (20);
for(int x = 0; x < 1; x++) {
digitalWrite(STEP,HIGH);
delayMicroseconds(30);
digitalWrite(STEP,LOW);
delayMicroseconds(300);}
optoRead = analogRead(A0);
Serial.println(optoRead);
delayMicroseconds(500);
}}
Cheers
Gregory