A television camera on a tripod at the edge of a floodlit grass sports pitch at dusk, panning to follow blurred players in the distance
AI-generated image, not a photograph of a real person or place.
hdproreviews

FRAME RATES, IN PLAIN ENGLISH

Sharp until
it moves.

24 · 25 · 50 frames per second

  • Detail needs time
  • Pans test a picture
  • More frames, less blur
  • Resolution is only half the picture

A picture can be pin-sharp and still smear in motion.

Film settled on 24 frames a second in the sound era, and UK television has followed the 50 Hz mains since 1937. BBC research shows why those old rates show their age on today's big, high-definition screens.

Updated

Compare frame rates

THE SHORT ANSWER

The rates are old,
the screens are new.

Frame rates were fixed decades ago.
Bigger, sharper screens make their limits easier to see.

CINEMA

Why film runs at 24

Silent film speeds were not standard. With sound-on-film in the 1920s, film settled on 24 frames a second, each frame flashed two or three times to hide flicker.

Compare the rates

TELEVISION

Why UK TV follows 50 Hz

Since 1937, UK television has used a field rate equal to the 50 Hz mains, partly so pictures would not beat against the flicker of studio lights.

See the numbers

SPORT

Fast pans lose detail

BBC engineers found that when a camera pans fast, the extra detail of HD can vanish in the direction of motion. Higher frame rates bring it back.

What to look for

A QUICK RATE CHECK

How smooth is
each frame rate?

Pick a frame rate to see where it comes from and how it handles fast motion. The bar shows how well that rate keeps moving detail sharp, based on BBC research.

Frame rate check

Choose a frame rate
24 fps, cinema filmFilm judder

The sound-era standard. Each frame is shown two or three times to avoid flicker, so a moving object the eye follows can look doubled or jumpy, known as film judder.

25 fps, film-look TVOne frame, two fields

In 50 Hz countries a film frame becomes two fields of video, and the change from 24 to 25 is generally imperceptible. BBC advice for a film look pairs 25 fps with a shutter open about half the time.

50 Hz, UK broadcastThe mains rate

The field rate UK TV has used since 1937, matched to the mains. HD kept it, but BBC tests found fast camera pans still smear or jerk on large, sharp screens.

100 fps, BBC testsReadable in a pan

In BBC tests, lettering that was legible in a still image became unreadable during a pan at rates below 100 fps. Even 100 fps was not enough for table tennis seen side-on.

120 Hz, UHD standardTop of the range

The ITU's ultra-high definition standard, BT.2020, lists frame rates from 24 Hz up to 120 Hz, all progressive scan.

Sources: BBC Research WHP 169, High Frame-Rate Television (2008); BBC R&D WHP 053, The Film Look (2002); Recommendation ITU-R BT.2020-2 (2015).

Based on BBC Research white papers WHP 169 (2008) and WHP 053 (2002). General information about picture quality, not a buying recommendation.

TWO NUMBERS WORTH KNOWING

Where the rate came from,
and where it runs out.

One figure from the power grid,
one from BBC research on HD motion.

50Hz

Great Britain's grid frequency

Britain's electricity system operator keeps the grid frequency close to 50 Hz, within a 1% range. UK television field rates have matched the mains since the 405-line service of 1937.

THE MAINS · NESO

3 pixels a frame

Speed at which HD's extra detail is lost

BBC research says motion faster than three pixels per field or frame wipes out all the extra detail gained by high definition, in the direction of motion.

HD MOTION · BBC R&D

BBC Research also notes that doubling the frame rate is not expected to double the bandwidth of a compressed signal, because frames that are closer in time differ less from one another.

FOUR THINGS TO WATCH FOR

See frame rate
for yourself.

A large flat-screen television on a living room wall showing a blurred, fast-moving football match, seen from behind a sofa
AI-generated image, not a photograph of a real person or place.

You do not need test equipment. These are the moments BBC engineers used to judge motion, and you can spot them on any screen at home.

  1. Watch a fast camera pan

    Look at the background, not the subject. On a sharp screen, detail that is crisp when still can smear as the camera turns.

  2. Follow a ball, not the pitch

    Fast, small objects such as a tennis or table-tennis ball show blur and jumps first. BBC tests found 100 fps was still not enough for table tennis seen side-on.

  3. Look for doubled edges in films

    In 24 fps film, an object the eye follows can look doubled or jumpy. That is film judder, and film-makers use slow pans and shallow focus to soften it.

  4. Notice the motion setting

    Some LCD TVs add motion-compensated frame doubling to cut blur. BBC research notes it helps, but can add artefacts when motion is hard to predict.

WHY SHARPER IS NOT ENOUGH

Still detail and
moving detail.

An old 35mm film projector in a dim projection booth, its beam of light shining through the porthole towards the cinema screen
AI-generated image, not a photograph of a real person or place.

BBC engineers separate a picture's static resolution, how much detail a still frame holds, from its dynamic resolution, how much survives when things move. HD raised the first without changing the frame rate, so motion now limits what you see.

Their 2008 conclusion was that higher capture and display frame rates bring a step change in picture quality whatever the resolution, and that frame rates should rise along with any further increase in detail.

Questions

Questions people ask

Why do films use 24 frames per second?

In the silent era speeds were not standardised. With the arrival of sound-on-film in the 1920s, film speeds settled at 24 frames per second, according to BBC Research.

Why does UK television use 25 and 50?

Since the 405-line service of 1937, UK TV has used a field rate equal to the 50 Hz mains, partly to avoid beating against flickering studio lights. A 25 fps film frame fits neatly into two fields.

Does 50 Hz mean worse picture quality than 60 Hz?

Not in itself. Each rate was chosen to match the local mains, 50 Hz in Europe and 60 Hz in the USA, and both sit just above the 40 Hz once found necessary to avoid visible flicker on early TV screens. At 50 Hz a film frame fits neatly into two fields, while 60 Hz needs a process called 3:2 pull-down. BBC research says the motion problems of both rates become more visible as screens get larger.

Is 50 fps better than HD?

They measure different things, and UK HD already uses 50. HD describes resolution, how much detail a still frame holds; 50 fps or 50 Hz describes how many frames or fields are shown each second. UK HD kept the 50 Hz rate, and BBC engineers found that fast motion can wipe out all the extra detail HD adds, in the direction of movement. Their 2008 conclusion was that higher frame rates bring a step change in picture quality whatever the resolution.

Does 4K fix blurry sport?

Not on its own. BBC research found that fast motion can remove all the extra detail of HD in the direction of movement, and the effect grows as resolution and screen size grow. Higher frame rates address it.

What frame rates does ultra-high definition allow?

ITU-R Recommendation BT.2020 lists frame frequencies of 120, 100, 60, 50, 30, 25 and 24 Hz, plus 1.001 variants, all progressive.

Does a higher frame rate need twice the data?

Not for compressed video, BBC Research suggests. Frames closer together in time change less, so doubling the frame rate is not expected to come close to doubling the bandwidth.

What should I check before buying a TV for sport?

Four checks help. Motion settings: some LCD TVs add motion-compensated frame doubling to cut blur; BBC research notes it helps but can add artefacts when motion is hard to predict, so look for a setting you can adjust. Frame rates: UK broadcasts use 50 Hz, and the ultra-high definition standard BT.2020 lists rates up to 120 Hz, so check the set handles the rates of what you watch. Size: BBC research says motion problems become more visible as screens get larger, so judge a big screen on moving pictures, not stills. A pan test: if you can, watch a fast camera pan or a ball in play before you decide, because detail that is crisp when still can smear in motion.

Sources

Where the facts come from

  1. BBC Research White Paper WHP 169: High Frame-Rate Television (2008)
  2. BBC R&D White Paper WHP 053: The Film Look, It's Not Just Jerky Motion (2002, PDF)
  3. ITU-R Recommendation BT.2020: Parameter values for UHDTV systems
  4. NESO: Frequency response services (50 Hz within a 1% range)
  5. Salmon et al. (2014), Higher Frame Rates for Television, SMPTE Motion Imaging Journal (University of Worcester record)

What changed

  • : Added FAQs on whether 50 fps is better than HD and what to check before buying a TV for sport, both from sources already cited (BBC Research WHP 169, ITU-R BT.2020).
  • : Added an FAQ on whether 50 Hz means worse picture quality than 60 Hz, from BBC Research White Paper WHP 169 (High Frame-Rate Television).

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