ROLLING
The frame is scanned
With a rolling shutter, the picture is captured by scanning across the scene rather than in a single instant. Most CMOS sensors in consumer cameras work this way.
Check a sceneA photo looks like a single moment. Often it is not.
Most consumer camera sensors scan the scene row by row instead of capturing it all at once. When things move fast, or lights flicker, the rows disagree, and propellers bend, cars lean and stripes appear.
- Rows are read in turn
- Fast motion bends
- Flicker makes stripes
- One frame, many moments
THE SHORT ANSWER
Scanned, not
snapped.
A rolling shutter records parts of the frame at different times.
A global shutter records the whole frame at once.
MOTION
Spinning blades bend
When a propeller turns at around the speed the sensor is read, each blade smears into odd shapes, an effect known as spatial aliasing.
See the numbers
LIGHT
Flicker becomes stripes
A flash or a flickering light can be on for only some rows of the frame, leaving bright and dark bands across the picture.
How to avoid it
FOUR THINGS TO KNOW
Work with the
shutter you have.
These are general explanations, not settings for any particular camera. Check your own camera's manual for what its shutter modes do.
Know there are two kinds
A rolling shutter can be mechanical or electronic; a global shutter captures the whole frame at the same instant.
Watch for fast motion
Spinning blades, quick pans and shots from moving vehicles show the effects most clearly.
Be wary of flicker
Fluorescent and other flickering lights, strobes and lightning can put bands across a sequentially read frame.
Remember flash timing
With a focal-plane shutter at very fast speeds only a slit is open, so a short flash would light only part of the frame.
TWO NUMBERS WORTH KNOWING
The grid's rhythm,
in your photos.
Why indoor light can flicker,
and how fast it beats.
50 Hz
Standard UK mains frequency
The UK declared a standard frequency of 50 Hz as early as 1904, and the National Grid, begun in 1926, pushed supplies to standardise on it.
MAINS · GREAT BRITAIN
100 Hz
Flicker of basic single-phase lighting in Europe
Basic lighting on a single-phase supply flickers at double the mains frequency. Stroboscopic effects are visible at relatively low modulation near 100 Hz.
FLICKER · LIGHTING
The same 100 Hz rhythm is why machinery spinning at multiples of 50 Hz can appear stopped under such lighting, a safety issue in factories as well as a nuisance in photos.
WILL IT DISTORT?
What are you
photographing?
Pick a scene to see how likely a rolling shutter is to show its tell-tale effects. The bar shows the risk of visible distortion.
The effect depends on how fast the sensor is read out: faster readout means milder artefacts.
THE MECHANICAL VERSION
Two curtains
and a slit.
A traditional focal-plane shutter uses two curtains. At slower speeds the first opens fully, then the second closes. At faster speeds the second starts before the first has finished, so a narrow slit travels across the frame.
Faster speeds simply need a narrower slit, because the curtains normally travel at the same speed. That is why a very short flash at a fast shutter speed would expose only a small part of the frame.
In conversation
Questions people ask
What is a rolling shutter?
A way of capturing an image by scanning across the scene rapidly instead of recording it all at a single instant. Parts of the frame are therefore recorded at slightly different times.
Why do propellers look bent in photos?
When a propeller rotates at about the speed the sensor is read out, each blade is smeared across rows recorded at different moments, so the blades look curved, thinner on one side and thicker on the other.
What is a global shutter?
A shutter that captures the entire frame at the same instant. Some CMOS sensors use one, but most consumer sensors use a rolling shutter.
Why do I get stripes under some indoor lights?
Lights can flicker, and on a single-phase supply in Europe that flicker is at 100 Hz. A sensor read row by row can catch the light bright in some rows and dim in others.
Is a rolling shutter always electronic?
No. A rolling shutter can be mechanical or electronic. An electronic one lets the sensor keep gathering light during readout, which effectively increases sensitivity.
Sources
Each figure on this page comes from the sources below.