What Is Rolling Shutter?
Most phone, webcam and mirrorless sensors do not capture the whole image at once. They read it out row by row, top to bottom. The time that takes is the readout time. Anything that moves or changes during readout is recorded at different moments in different rows: fast pans lean, propellers bend and flashes light only part of the frame. A global shutter reads every row at the same instant and has none of this.
Why a Screen Shows Bands, Not a Bent Needle
A monitor does not move continuously: it shows one still image per refresh and redraws it from top to bottom. When a rolling shutter films it, rows read before a redraw see one frame and rows read after it see the next. The result is a stack of horizontal bands. On the dial, the needle appears broken into offset segments, one per display frame. Measuring the height of those bands tells you the readout time.
Because the screen scans in the same direction as the sensor, a photo taken with the camera upright measures the difference between camera readout and screen scan. Turning the camera upside-down reverses its scan direction and measures their sum, which is why the method uses two shots.
How to Measure Readout Time
- Enter fullscreen. For the clearest bands switch to the Flicker pattern at half your refresh rate (one frame bright, one dark); the dial works too.
- Set a fast shutter (shorter than one display frame, e.g. 1/500 s) and use the electronic shutter, which is the one with rolling readout.
- Take one photo with the camera upside-down (rotated 180°), filling the frame with the screen. Optionally take one upright as well.
- Measure the height of one band as a percentage of the screen height and enter it in the calculator below the pattern.
Formula: readout + screen scan = display frame time ÷ band height (upside-down). Most displays scan over almost their entire refresh period, so readout ≈ that sum minus one frame time. Example at 60 Hz (16.7 ms): bands 40 % of the screen tall → 16.7 ÷ 0.40 = 41.7 ms → readout ≈ 41.7 − 16.7 = 25 ms. Adding the upright shot lets the calculator solve for both readout and scan time without that assumption.
Typical Readout Times
| Sensor type | Readout (rough guide) | What you’ll see |
|---|---|---|
| Global shutter | 0 ms | One unbroken needle in both shots |
| Stacked / fast-readout sensors | A few ms | Very tall bands |
| Typical phone, webcam and consumer sensors | Roughly 10–30 ms | Bands a third to two-thirds of the screen tall |
These ranges are only for orientation; readout varies with resolution, frame rate and bit depth on the same camera, so test the mode you actually use.
Reading Exposure and Dropped Frames from the Dial
Inside one band, several faint ghost needles mean the exposure spanned that many display frames. The 10-cell frame strip lights one cell per refresh, and the clock in the centre prints the exact time of each frame (page time, local time or UTC), so consecutive video frames reveal the camera’s real frame interval. For dedicated exposure measurements use the shutter speed test.
Rolling Shutter FAQ
Why is the needle broken instead of curved?
The screen shows one still image per refresh, so the needle jumps between discrete positions. A rolling shutter records different display frames in different rows, which appears as offset segments separated by horizontal breaks. Their spacing, not their angle, encodes the readout time.
Why do I need to turn the camera upside-down?
Screens redraw from top to bottom, the same direction most sensors read out. Upright, the two scans partly cancel and you measure their difference; rotated 180°, they add up. The upside-down shot is therefore the more sensitive measurement.
Does a higher refresh rate help?
Yes. Each band represents one display frame, so a 120 Hz or 240 Hz screen produces thinner, more numerous bands that can be measured more precisely than at 60 Hz.
How do I know if my camera has a global shutter?
A global shutter captures every row at the same moment, so the needle stays in one piece and the flicker pattern shows a uniform frame — bright or dark, never banded — whichever way the camera is held.