I’ve been really enjoying film photography the last couple years.

Recently I thrifted an original Pentax Spotmatic SP. It was just the body, and the film advance lever was stuck. So I took the bottom off, watched some quick YouTube videos, and poked at the gears until things started turning.

Things looked promising so I shot a test roll. After getting it developed, I saw there were some issues with the shutter curtains.

I found pentax-manuals.com which had some useful content. But after looking more closely at the spotmatic manual I realized the it was actually for a Spotmatic SP II, and I had the SP 1(?). Eventually I got lucky with a manual on learncamerarepair.com.

Once I found where to tweak the tension of each shutter curtain, I realized I would need a new tool to time how long light was allowed to pass through the shutter curtains. I thought it would be fun to make this on my own, so I worked the Arduino Student Kit and made a super simple circuit with the phototransistor that came in the kit.

#define LIGHT_THRESHOLD 120 // picked arbirtarily while manually reading inputs
#define RECEIVER_PIN A0

void setup() {
  Serial.begin(9600);
}

void loop() {
  int light_val = analogRead(RECEIVER_PIN);     

  if (light_val > LIGHT_THRESHOLD) {
    long exposure_duration_millis = get_exposure_time_millis();
    Serial.print("exposure_time_millis:");
    Serial.println(exposure_duration_millis);
  }
}

int get_exposure_time_millis() {
  // https://www.arduino.cc/reference/en/language/functions/time/millis/
  // "This number will overflow (go back to zero), after approximately 70 minutes"
  long start_millis = millis();
  
  Serial.print("recording exposure time... ");
  while (true) { 
    if (analogRead(RECEIVER_PIN) <= LIGHT_THRESHOLD) {
      return millis() - start_millis;
    }
  }
}

After maybe ~40 rotations to the “closing curtain take-up roller worm” I was able to measure the following exposure speeds

1/n second exposure time	expected exposure time (ms)	measured exposure time (ms)
1	1000	843
2	500	400
4	250	209
8	125	102
15	67	62
30	33	36
60	16	18
125	8	8
250	4	4
500	2	2
1000	1	1

Which is a terrific success! Things might be slightly undexposed at the slowest exposure settings, but that’s something I can manually correct for using the in-body light meter! Currently I’m off shooting/developing a new roll to see if the fix worked.