We wanted to write a blog that would briefly explain frame rates for video, touching on aspects such as why speeds vary and why so many frame rates exist. The term “frame rate” – also known as frame frequency – refers to the frequency (rate) at which an imaging device displays consecutive images known as frames. While the term applies equally to film and video cameras, computer graphics and motion capture systems – and all frame rate data is expressed in frames per second (FPS) – we’re going to concentrate on the video aspect of frame rates.
To begin with, it should be understood that in the world of video there are three main frame rate standards in the TV and digital cinema business: 24p, 25p and 30p. However, there are many, many variations on these as well as newer emerging standards that are changing the playing field. Let’s take a brief look at these three:
• 24p refers to a “progressive” format…
(as opposed to “interlaced;” in layman’s terms, a smoother picture is achieved with a progressive signal) and is now widely adopted by those planning on transferring a video signal to film. Additionally, film and video creators use 24p even if they are not planning on transferring their productions to film simply because of the on-screen “effect” of the (low) frame rate, which matches native film.
• 25p is also a progressive format…
(hence the “p”) and runs 25 progressive frames per second – this frame rate is derived from the PAL television standard of 50i (or 50 “interlaced fields per second”) and film and television entities use this rate in 50 Hz regions for direct compatibility with television field and frame rates.
• 30p is another progressive format…
that produces, as its namesake suggests, video at 30 frames per second, and shooting video in 30p mode provides no interlacing artifacts though can introduce what’s known as “judder” on image movement and on some camera pans. As an interesting side note, the widescreen film process known as Todd-AO (used on such classic Hollywood productions as Universal Studios’ Airport) took advantage of this movie frame rate when it was adopted by filmmakers in 1955.
Why Frame Rate Matters
Did you know that the concept we flock into movie theaters to drool over, the motion picture, is a fabrication? Well, let’s just put it like this: That movement up on the screen is really just a series of still images that seem more like believable, realistic, lifelike motion the faster they “flicker” along. Indeed, faster is better, and that 48 frame-per-second version of The Hobbit was only the beginning…
The human eye is a wonderful thing, beyond just being there for us to take in those hyper-exciting colors of the latest Avengers Blu-ray release…it is capable of differentiating between 10 and 12 still images per second before it begins just interpreting them as motion. That is – at an FPS of 12 or less, our brains can tell that it’s just a bunch of still images in rapid succession, not seamless animation. Once that frame rate jumps to around 18 to 26 FPS, the motion effect actually takes hold and the brain is tricked into thinking that these individual images are actually a moving scene.
Okay, so what can we garner from this? Put succinctly, if a frame rate is too slow, motion appears “jagged” and if it’s too fast, problems arise as well. Live-action motion pictures we have come to know and love filmed at 48 FPS tend to have what we call in the industry the “soap opera effect” – something film purists happen to despise, and something many audiences hated about the aforementioned Hobbit.
Interestingly, many modern LCD HDTV sets are coming equipped with “interpolation” controls meant to smooth out video irregularities during fast motion, but the downside to using these controls is that film-based material – such as motion pictures on Blu-ray Disc – comes off exhibiting that soap opera effect wherein film tends to look like it was shot on surreal video. To the relief of film buffs everywhere using LCD TVs in their home theaters, these controls can be reduced or turned off in the television’s menu system.
In short: Higher frame rates look more real to certain viewers, but they can also make things that are not real appear even LESS real.
Jax Video Tidbit: Motion pictures on film are almost exclusively projected at a movie frame rate of 24 FPS, but television does not have an internationally-accepted TV frame rate. In fact, in Europe and many other continents around the world, PAL and SECAM use 25 FPS whereas NTSC video in North America and Japan uses 29.97 FPS. Other common frame rates are usually multiples of these.
Now, why is all this important in the realm of video? Converting video formats from one frame rate to another is technically challenging to say the least, and there are often unwanted side effects (as we touched on a bit above) – this is especially true when the frame rates do not divide evenly. For example: Converting 30 FPS to 60 FPS is fairly painless to accomplish, but converting 29.97 FPS to 25 FPS is another story altogether. What’s worse, ensuring audio stays in sync throughout the whole conversion process presents another challenge.
Some digital video formats support several frame rates within a single format, enabling variable frame rate video recording and film (24 FPS) compatibility.
The Shutter Angle and Shutter Speed Factor
The higher the frame rate, the higher the “shutter speed” is by necessity, and this yields a sharper overall image. In film, this is known as “shutter angle,” but regardless, some film and video exhibit a super-sharp image wherein this shutter speed is way faster than the standard shutter speed for that particular frame rate. Each frame rate boasts a default shutter speed which is approximately half the frame duration; at 24 FPS, the frame duration is 1/24th of a second, so the default shutter speed becomes 1/48th of a second. At 24 FPS and 1/200th – or even higher shutter speed – the image will appear ridiculously sharp, but the motion will exhibit much less smooth characteristics.
Another interesting observation here is that film is rarely projected at 24 FPS – in perfect conditions, flicker fades from perception at around 18 FPS, which happens to be the average speed of most silent films (remember those?). Even in a very dark theater environment, taking into consideration that gigantic screen, 24 FPS projection is going to flicker…which is why each frame is projected twice (or even three times) for an effective “flicker rate” of 48 or 72 flickers per second, thus making it much less perceptible.
What it Comes Down To
We here at Jax Videography believe the choice comes down to motion and effects. Indeed, higher frame rates will yield cleaner motion with the ability to create more realistic effects when engaging in chroma-keying or rotoscoping, because edges appear sharper and contain much less motion blur. From our experience, it is far easier to add motion blur in post-production than to remove it.
This is precisely why we recommend, with all projects, you always verify what your deliverables need to be before you begin shooting. Trust us when we say things will go much smoother if you shoot at the correct frame rate as opposed to converting to it after all shooting is complete.