Frame Rates and Slow Motion - How Capture and Playback Rates Work

What You Need to Know

Frame rate is how many unique images (frames) are captured or played back per second. Slow motion happens when you capture at a higher fps than your playback (timeline/deliverable) fps, then conform the clip to the lower playback fps.

The core rule (this is the whole game)

• Capture fps = how fast the camera records frames per real second.
• Playback fps = how fast your project outputs frames per playback second.

If $f_c$ is capture fps and $f_p$ is playback fps:

• Slow-motion factor (how many times longer it plays):

$\text{SlowMo Factor} = \frac{f_c}{f_p}$

• Speed (how fast it plays relative to real time):

$\text{Speed} = \frac{f_p}{f_c}$

• Percent speed (what you type into many NLEs):

$\text{Percent Speed} = 100\times\frac{f_p}{f_c}$

Why it matters: it controls motion cadence, motion blur, flicker risk, file sizes/data rates, and whether your slow motion looks clean (true extra frames) or fake (interpolation).

Critical reminder: True slow motion comes from more captured frames, not from slowing normal footage and hoping software invents detail.

Step-by-Step Breakdown

A. Planning a slow-motion shot (the exam-style workflow)

1. Decide your delivery/timeline fps $f_p$.

   • Common: $23.976\,fps$, $24\,fps$, $25\,fps$, $29.97\,fps$, $30\,fps$.

2. Choose how slow you want it (factor or percent).

   • Want “$2\times$ slower”? You want $\text{SlowMo Factor}=2$.
   • Want “$40\%$ speed”? You want $\text{Percent Speed}=40$.

3. Compute required capture fps $f_c$.

   • From slowmo factor:

$f_c = f_p\times\text{SlowMo Factor}$

• From percent speed:

$f_c = f_p\times\frac{100}{\text{Percent Speed}}$

4. Set shutter to control motion blur (don’t ignore this).
   • Using shutter angle $\theta$:

$t_e = \frac{\theta}{360}\times\frac{1}{f_c}$

• With the common 180-degree shutter:

$t_e = \frac{1}{2f_c}$

Faster capture fps forces shorter exposure time at the same shutter angle, which can make motion look crisper/strobier unless you compensate thoughtfully.

5. Check lighting flicker constraints (especially LED/mains-powered lights).

   • In $50\,Hz$ regions, “safe-ish” exposure times are often around $\frac{1}{50}$, $\frac{1}{100}$, $\frac{1}{200}$, …
   • In $60\,Hz$ regions, often $\frac{1}{60}$, $\frac{1}{120}$, $\frac{1}{240}$, …

6. In post: conform (interpret) to playback fps for true slow motion.

   • Conform = tell the NLE the clip’s frame rate is $f_p$ (no frame creation), so duration stretches by $\frac{f_c}{f_p}$.

B. Worked micro-examples (method in action)

• You shoot $120\,fps$ and deliver $24\,fps$:

$\text{SlowMo Factor} = \frac{120}{24} = 5$

Plays $5\times$ longer (i.e., $20\%$ speed).

• You want $40\%$ speed on a $25\,fps$ timeline:

$f_c = 25\times\frac{100}{40} = 62.5\,fps$

So you’d pick the nearest supported camera rate (commonly $50\,fps$ gives $50\%$ speed; $100\,fps$ gives $25\%$ speed; $60\,fps$ on a $30\,fps$ timeline gives $50\%$ speed).

Key Formulas, Rules & Facts

A. Capture vs playback math (must-know)

B. Shutter angle and motion blur (high-yield)

Implication: If you keep $\theta=180^\circ$ while going from $24\,fps$ to $120\,fps$, exposure time drops from $\frac{1}{48}$ to $\frac{1}{240}$. That’s why high-fps often needs more light and can look sharper.

C. Common fps families you’ll actually see

Trap: $23.976$ is not “basically $24$” when you’re doing long durations, audio sync, or timecode-critical work.

D. Two different ways to “slow down” in post (know the difference)

Examples & Applications

Example 1: You shot $60\,fps$, timeline is $24\,fps$

Compute percent speed:

$\text{Percent} = 100\times\frac{24}{60} = 40\%$

Key insight: $60\,fps$ gives you a clean $2.5\times$ slow motion on a $24\,fps$ timeline.

Example 2: You need exactly $4\times$ slow motion for a $30\,fps$ deliverable

Required capture:

$f_c = 30\times 4 = 120\,fps$

Key insight: This is why $120\,fps$ is a common high-speed mode: it maps neatly to $30\,fps$ (and $24\,fps$ gives $5\times$).

Example 3: Duration planning (how long will the slowmo clip be?)

You record $3\,s$ at $240\,fps$, and deliver at $24\,fps$.

$T_{play} = 3\times\frac{240}{24} = 30\,s$

Key insight: high fps explodes duration fast; plan your action timing and storage.

Example 4: Shutter angle reality check at high fps

You shoot $120\,fps$ with $180^\circ$ shutter.

$t_e = \frac{1}{2\times 120} = \frac{1}{240}\,s$

Key insight: motion blur will be much less than at $24\,fps$ with $180^\circ$ (which is $\frac{1}{48}\,s$). If you want “dreamier” blur, you may open shutter angle (camera permitting), but watch flicker.

Common Mistakes & Traps

1. Mixing up capture fps and playback fps

   • What goes wrong: You assume “shooting $120\,fps$” automatically outputs $120\,fps$ video.
   • Why it’s wrong: Cameras can record at $120\,fps$ but you typically **deliver** at $24/25/30\,fps$.
   • Fix: Always write down $f_c$ (capture) and $f_p$ (playback) separately.

2. Using the wrong ratio (inverting the fraction)

   • What goes wrong: You compute $\frac{f_p}{f_c}$ when asked “how many times slower,” or $\frac{f_c}{f_p}$ when asked “percent speed.”
   • Fix: Memorize: slower factor = capture over playback; percent speed = playback over capture.

3. “Retime” instead of “conform” and wondering why it looks mushy

   • What goes wrong: You slow footage on a timeline (interpolation) even though it was shot high fps.
   • Why it’s wrong: You may be blending/inventing frames unnecessarily.
   • Fix: Use interpret/conform so every captured frame is shown exactly once.

4. Forgetting shutter consequences (staccato slow motion)

   • What goes wrong: You shoot high fps at $180^\circ$ and get ultra-crisp motion that feels choppy.
   • Why it happens: Exposure time shrinks with fps, reducing motion blur.
   • Fix: Decide intentionally: crisp (sports, analysis) vs smooth blur (cinematic). Adjust shutter angle if possible, but test for flicker.

5. Ignoring flicker with LEDs or mains-powered lighting

   • What goes wrong: Banding/flicker appears, especially at high fps.
   • Why it happens: Exposure samples a light output that’s pulsing.
   • Fix: Match exposure times to local power frequency (common targets near $\frac{1}{50}$ or $\frac{1}{60}$ and their multiples) and test.

6. Assuming $23.976\,fps$ equals $24\,fps$ for timing and sync

   • What goes wrong: Long clips drift or timecode math doesn’t line up.
   • Why it’s wrong: $23.976 = \frac{24000}{1001}$, not $24$.
   • Fix: Use the exact project rate; don’t “round” in calculations.

7. Shooting variable frame rate (VFR) when you need precise slow motion

   • What goes wrong: NLE interprets timing inconsistently; audio/video issues.
   • Why it happens: Some devices (especially phones) record VFR.
   • Fix: Prefer constant frame rate capture for planned slowmo workflows; transcode to constant fps if needed.

8. Expecting slow motion with audio to stay natural

   • What goes wrong: Audio becomes stretched/unnatural (or is missing in high-fps modes).
   • Why it happens: Many high-speed modes don’t record usable sync audio; slowing down audio changes pitch/time.
   • Fix: Plan to use separate audio, sound design, or time-stretch tools intentionally.

Memory Aids & Quick Tricks

Quick Review Checklist

• You can define capture fps $f_c$ vs **playback fps** $f_p$ without mixing them up.
• You can compute slow motion length with:

$T_{play} = T_{real}\times\frac{f_c}{f_p}$

• You can compute percent speed fast:

$\text{Percent} = 100\times\frac{f_p}{f_c}$

• You know true slow motion = high capture fps + conform, not interpolation.
• You can estimate shutter at $180^\circ$:

$t_e = \frac{1}{2f_c}$

• You remember high fps usually means less motion blur, more light needed, and more flicker risk.
• You treat $23.976\,fps$, $29.97\,fps$, $59.94\,fps$ as distinct from whole-number fps.

You’ve got this: write down $f_c$ and $f_p$ first, and the rest becomes plug-and-play math.