Why do propellers look bent in phone photos?

Photograph a spinning propeller or fan with a phone, and the blades can come out curved. Sometimes they look like bananas, or even float free of the hub. The blades are straight. The strange part is the photo. It was not taken all at once.

Why does the camera bend the blade?

Most consumer cameras do not capture a whole picture in one instant. They record it row by row, from one edge of the sensor to the other. Each row starts a little later than the one before. Wikipedia: Rolling shutter.

A still subject comes out fine. A propeller does not. While the rows tick past, the blade keeps turning. The top row catches it in one place. The bottom row catches it farther around. Stacked together, those rows turn a straight blade into a curve. This is called the rolling shutter effect.

The bend depends on how far the blade turns during the scan. In our model, a blade at 1,500 rpm turns 180 degrees during a 20-millisecond scan. That is half a turn between the top row and the bottom. A faster, 5-millisecond scan cuts the turn to 45 degrees, and the bend shrinks.

▶ Play with this now in Rolling Shutter Propeller: press Global and watch the bend vanish while the blur stays.

Is blur the same thing?

No. Blur runs on its own clock. Each row stays open for a short time to collect light. That time is its exposure. If the blade moves during that time, the row smears it.

In our model, a 4-millisecond exposure smears each blade across 36 degrees. In bright light, a 1/4000-second exposure cuts the smear to 2.25 degrees. In dim light, a 1/125-second exposure stretches it to 72 degrees. The bend stays the same in all three, because the scan did not change.

A global shutter starts and stops every row together. That removes the bend completely. It does not remove blur. With a long exposure, the blades come out straight but smeared. Basler: Electronic shutter types.

What does the model leave out?

Our film and Rolling Shutter Propeller use a simplified camera-timing model. The blade turns at a steady speed, and the optics are ideal. The timings are illustrative, not measurements of any camera. Real readout times vary widely by device and mode. The model leaves out:

• Mechanical shutters with moving curtains.
• Camera shake, which can make a whole video wobble.
• Flashes and flicker. A flash can light only some rows of a frame. Wikipedia: Rolling shutter.
• Everything else. Sensor noise, lens distortion and blades that flex.

The model also draws a few hundred rows. A real sensor has thousands, but the timing rule is the same. Camera makers explain both timings in more depth. Oxford Instruments Andor: FAQs on rolling and global exposure.

Can you bend a fan with your phone?

A desk or box fan with a safety guard works well. These steps are a suggestion, so adapt them to the child beside you. Keep fingers and the phone away from the blades, and leave the guard on.

Predict. Ask your child what a photo of the spinning fan will show. Will it show straight blades, bent blades or a blur? Then ask what will change if you turn the phone sideways.

Try. Set the fan to its highest speed. Take a photo straight on, from about arm's length. Turn the phone a quarter turn and take another. Then compare a bright room with a dim one.

Explain. If the blades curve, your camera scanned while they turned. The guard stays straight, because it is not moving. Turning the phone changes the scan's direction across the fan, so the bend changes too. On a fan spinning clockwise, blades on one side can look thin and on the other side fat. Wikipedia: Rolling shutter.

In dim light, a phone usually keeps each row open longer, so the blur grows. Results vary by phone and fan. A phone with a very fast scan may show almost no bend.

For the next question, explore Bartosz Ciechanowski's Cameras and Lenses and follow light from the scene to the sensor.

Rolling Shutter Propeller is an ExplainerTools Original.