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.