Shutter Speed Tester
This is an independent personal project
Background
In photography, shutter speed is one of three fundamental aspects for controlling the exposure of the film/sensor to light. Shutter speed is measured in time, usually fractions of a second. Common shutter speeds for film cameras range from 1s to 1/2000s. Variations in this speed, especially at higher shutter speeds, can result in drastically different results in the final image. It becomes paramount to time and adjust the shutter speed on a film camera to ensure even and proper exposure.
The shutter speed is controlled by two shutter curtains: the first and second curtains. These two curtains are actuated by coil springs and rewound by the film advance mechanism. Depending on the shutter speed, the behavior of the curtains will change to achieve the desired effect. The behavior can be distinguished as low-speed and high-speed shutter curtain behavior; the transition between these two behaviors varies from camera-to-camera but usually switches around 1/60s.
Photographer's Exposure Triangle; note the effect of shutter speed shown on the left side
Slow Motion Footage of a Focal Plane Shutter at 1/60s Shutter Speed
Low Speed
At low speeds, the first curtain opens fully and remains open for the duration of the shutter exposure. After the elapsed time, the second shutter closes and ends the exposure.
High Speed
At high speeds, the first curtain begins to open. As it is opening, the second curtain begins to move after the shutter speed time has passed. The two curtains move together until both close one after the other. This is observed as a "window" that travels across the frame in which the width of the window varies with shutter speed.
Slow Motion Footage of a Focal Plane Shutter at 1/1000s Shutter Speed
Problem Statement:
Slight variations in shutter speed can lead to dramatic effects to film photography. Mechanical film cameras can adjust the timing for the shutter but cannot provide feedback without other equipment or shooting a roll of film through the camera. This project seeks to create a device to test the shutter speed on a mechanical film camera at a low cost.
Process:
Using phototransistors, time the pulse length of the shutter action in a film camera
Software Difficulties
Phototransistors act as a photoreactive switch and amplifier in which a luminosity translates into a current. Using an analog pin and measuring the state change of of the phototransistor, an Arduino or similar microcontroller can time the duration of a light pulse.
With only a single phototransistor, shutter speeds can only be measured at a single point representing the whole frame. While this would be helpful in assessing the shutter speed, it doesn't give a full enough of a picture. Because the timing is critical, the speeds of the two curtains need to be matched or will result in "shutter capping" in which one part of the frame is darker/lighter than another.
Arduino provides the pulseIn()function that measures the duration of an analog pulse in microseconds. At first glace, this function fits the bill perfectly for this application. However, researching the behaviors of this function, found that the function to be a blocking function. Because this is a blocking function, two light pulses cannot be measured simultaneously. While this works for high shutter speeds, low shutter speeds will require a different method of timing. As such, I will have to write my own version of the pulseIn() function that will suit my needs.
Example Image of a Photo with Shutter Capping; Note that the right half of the image is too dark
Front of Device
Back of Device; microcontroller and phototransistors shown
Hardware Implementation
Two phototransistors are placed at opposing corners. Shutter curtains typically travel right to left or top to bottom. By placing the phototransistors in the corners, it ensures that one phototransistor will always be the phototransistor that fires first.
3D CAD was designed in Onshape.
Design Features:
Recessed front edge allows for adhesive light-sealing foam to be added to prevent stray light signals and improve reliability
Nested cap mitigates like leaks to the backside of photoresistors
Dual phototransistors can measure the specific timing of each shutter curtain
Design Limitations:
Not a stand-alone device; requires power and serial connection to display data
Light leaks are still possible from around the USB-C connector
The device has no method to attach to the camera
Results
Outcome:
Partial success:
Spacing between phototransistors is too great, partially obstructed by body of camera and/or lens vignette. Cannot register two pulses, regardless of software limitations
As a result of no. 1, software was not written to time two pulses
Able to measure a single pulse by offsetting device slightly, moving "active" phototransistor inwards
Future Expansion:
Move phototransistors inwards to allow for both to have clear sight of frame
Expand functionality for both low and high shutter speeds