Slow motion looks smooth when each second of playback contains enough unique frames to appear fluid to the human eye. The threshold for smooth motion perception is roughly 24 frames per second. Below that, the eye starts to notice individual frames as a series of still images rather than continuous motion.
When you record at 30 frames per second and slow the video to half speed (0.5x), you stretch 30 frames across two seconds of playback. That gives you only 15 unique frames per second. The result looks noticeably choppy, especially during fast movement. This is not a software problem. It is a physics problem: the camera did not capture enough temporal information to create smooth slow motion.
How real slow motion cameras work
Professional slow motion is captured at high frame rates: 120fps, 240fps, 480fps, or higher. When 240fps footage is played back at 30fps, each second of real time becomes 8 seconds of playback (240/30 = 8x slowdown), but every second of playback still contains 30 unique frames. The motion is smooth because the camera captured 8 times more temporal detail than standard footage.
Modern smartphones support high frame rate recording: - iPhone: 120fps and 240fps at 1080p, some models support 120fps at 4K - Samsung Galaxy: 240fps at 1080p, some models at 720p for 480fps super slow-mo bursts - Google Pixel: 120fps at 1080p and 4K
If you know in advance that you want slow motion, switch your camera to high frame rate mode before recording. This gives you genuine slow motion source material.
What happens when you slow down 30fps footage anyway
When you apply a 0.5x speed to 30fps video, the software has two options:
Option 1: Simply hold each frame for twice as long. This is the simplest approach and what most basic tools do. The output is 30fps with each original frame displayed twice. Motion appears jerky because you see the same image held for two frames before jumping to the next unique frame.
Option 2: Frame interpolation. Advanced software (like Topaz Video AI, DaVinci Resolve's Optical Flow, or Adobe Premiere's Frame Blending/Optical Flow) analyzes adjacent frames and generates synthetic in-between frames. A good interpolation algorithm can create a reasonably convincing 60fps output from 30fps input slowed to 0.5x. The results are impressive for slow, predictable motion (someone walking, a ball rolling) but can produce visual artifacts (ghosting, warping, tearing) around fast or complex motion like hair, fabric, or rapid hand gestures.
Practical limits for slowing down standard footage
For 30fps source footage: - 0.75x speed: Subtle slowdown. 22.5 unique frames per second. Slightly noticeable but generally acceptable. - 0.5x speed: 15 unique frames per second. Noticeably choppy for action scenes. Acceptable for slow, steady movements. - 0.25x speed: 7.5 unique frames per second. Very choppy. Only works with frame interpolation, and even then artifacts are likely.
For 60fps source footage: - 0.5x speed: 30 unique frames per second. Smooth. This is the sweet spot for casual slow motion. - 0.25x speed: 15 unique frames per second. Same choppiness as 30fps at 0.5x.
Audio during slow motion
When you slow a video down, the audio is stretched proportionally. At 0.5x speed, the audio plays at half the original pitch and half the original speed. Voices sound deep and drawn-out. Music becomes a low rumble. Most slow motion videos either mute the original audio entirely and add music, or they strip the audio and present the visual slow motion silently.
If you want to keep the original audio at normal pitch while the video plays slowly, you need a pitch-correction algorithm (time stretching without pitch shifting). This is common in professional editors but not typical in simple speed-change tools. The MediaBrew speed tool uses FFmpeg's setpts filter for video and atempo for audio, so pitch is corrected automatically. For best results with 30fps source footage, keep the slowdown to 0.75x or 0.5x to avoid severe choppiness.