Measuring shutter speed means verifying how long your camera’s shutter actually stays open, compared to what the dial or display says. This matters most for film cameras, where a shutter that’s off by even half a stop can ruin exposures. There are three practical ways to do it: a spinning-disk method, a DIY electronic tester, or a dedicated app or commercial tool.
How Shutter Speed Is Expressed
Shutter speeds are written as fractions of a second. A setting of 1/500 means the shutter is open for one five-hundredth of a second, while 1/30 means it stays open about 33 milliseconds. Most cameras offer a range from around 1/4000 at the fast end down to 1 second or longer at the slow end, with each standard step (1/1000, 1/500, 1/250, 1/125, 1/60, 1/30) roughly doubling the amount of light reaching the sensor or film.
The number on the dial is a target. Mechanical shutters, especially in older cameras, drift over time as springs weaken and lubricants thicken. A camera set to 1/125 might actually deliver 1/90 or 1/160. That’s what you’re trying to detect and quantify.
The Spinning Disk Method
This is the oldest and most accessible technique. You photograph a small bright object (like a white dot or LED) attached to a turntable spinning at a known speed, then measure the arc of blur in the resulting image. The length of that arc tells you exactly how long the shutter was open.
Here’s the setup: place a small light or reflective marker near the edge of a record turntable. Set the turntable to a known speed, such as 33⅓ or 45 RPM. Open the camera’s back (remove the film) or, for a digital camera, simply take a photo of the spinning dot. In the resulting image, the dot traces an arc instead of appearing as a sharp point. Measure the angle of that arc in degrees.
The formula is straightforward:
Exposure time = arc angle ÷ (2π × RPM ÷ 60)
So if you’re using a 45 RPM turntable and the arc spans about 27 degrees (0.471 radians), the exposure time works out to roughly 1/10 of a second. A smaller arc at the same RPM means a faster shutter speed. The main source of error is how precisely you measure the arc angle. If your angle measurement is off by 5%, your calculated shutter speed will also be off by about 5%.
For faster shutter speeds (1/500 and above), a standard turntable may not spin fast enough to produce a measurable arc. You’d need a faster motor or a larger radius for the marker to get a readable blur.
Building an Electronic Shutter Tester
A more precise approach uses a light sensor and a microcontroller to directly time how long light passes through the shutter. The concept is simple: shine a light through the open back of the camera, and have a sensor on the other side that records exactly when it sees light and when it stops.
A popular DIY version uses four components:
- An Arduino microcontroller to process the timing
- A phototransistor (such as the NJL7502L) as the light sensor
- A 10k ohm resistor to create a measurable voltage from the sensor’s output
- A small OLED display to show the result
You remove the lens from your camera and place a flashlight behind the body, pointing through the shutter toward the phototransistor on the front side. When you fire the shutter, the flashlight briefly illuminates the sensor. The Arduino reads the analog voltage from the phototransistor, detects the moment light arrives and the moment it disappears, and calculates the time difference. That duration is your actual shutter speed, displayed on the screen in fractions of a second.
This method is accurate to within a millisecond or better, depending on how fast the Arduino samples the sensor. It’s especially useful for testing multiple speeds in a row, since you just change the shutter dial and fire again. You can compare each result against the marked speed to see if your camera’s shutter is consistently fast, slow, or erratic.
Using a Smartphone App
If you don’t want to build anything, several smartphone apps measure shutter speed using the phone’s built-in microphone. The shutter in a mechanical camera makes two distinct sounds: one click when the first curtain opens and another when the second curtain closes. The app records the audio, identifies the two peaks, and calculates the gap between them.
These apps work reasonably well for speeds of 1/500 and slower. At faster speeds, the two clicks overlap so closely that the microphone may not resolve them cleanly. They’re a good quick check, but they’re less reliable than a light-based tester, especially at the extremes of the speed range.
What the Results Tell You
No mechanical shutter is perfectly accurate, and it doesn’t need to be. A deviation of up to about one-third of a stop from the marked speed is considered normal for a well-maintained camera. That means if your camera is set to 1/125, an actual speed anywhere between roughly 1/100 and 1/160 is within acceptable range.
What you’re really looking for are patterns. If every speed runs about one stop slow, the shutter tension springs may need adjustment but your exposures will still be consistent (you can compensate by setting the ISO one stop higher). If slow speeds are accurate but fast speeds are way off, or if results are erratic from shot to shot, the shutter mechanism likely needs a professional cleaning and lubrication, commonly called a CLA.
Test each marked speed at least three times to check for consistency. A shutter that delivers 1/120 every time is far more useful than one that bounces between 1/80 and 1/200, even if the average looks correct.
Measuring Electronic and Rolling Shutters
Digital cameras with electronic shutters don’t have physical curtains, so the microphone method won’t work. The spinning-disk technique still applies, though, and reveals something mechanical shutters don’t have to worry about: rolling shutter distortion.
Most digital camera sensors read out line by line from top to bottom rather than all at once. This means the top of the frame is exposed slightly earlier than the bottom. If you photograph a spinning disk with a digital camera, you may notice the arc appears slightly tilted or offset between the top and bottom of the frame. The difference in position reveals the sensor’s total readout time, which is separate from the exposure duration. For most photography this doesn’t matter, but it’s relevant for video, fast-moving subjects, or flash synchronization.
If you just want to confirm your digital camera’s exposure accuracy, the spinning-disk method works the same way as with film cameras. Measure the arc, plug in the RPM, and compare the result to what the camera’s display says.