1 The idea
A stop is the shared language of light: +1 stop = twice the light. An f-number √2 smaller (f/4 → f/2.8) doubles the opening’s area = +1 stop. A shutter open twice as long = +1 stop. Double the ISO and the image is 1 stop brighter.
Each control has a price: a wider aperture softens the background, a longer shutter stretches motion blur, a higher ISO adds noise. Film work usually locks the shutter at 180° (half the frame time = 1/48 s at 24 fps) so motion blur looks natural, and controls brightness with the others or an ND filter.
stops = 2·log2(N₀/N) + log2(t/t₀) + log2(ISO/ISO₀) − ND/0.3 · t = angle ÷ (360 × fps)2 Hands-on
Task: in bright sun, get correct exposure (±⅓ stop) with a soft background (f/2.8 or wider) while keeping the 180° look
Opening from f/8 to f/2.8 adds 3 stops, but ISO can only take away 1. What else removes light without changing the look?
3 Deep dive
Stops: one language for all three controls
+1 stop = twice the light, −1 stop = half. Because every control speaks the same unit, they trade freely: open the aperture one stop, drop the ISO one stop, and the brightness is unchanged.
| Control | +1 stop means | Side effect |
|---|---|---|
| Aperture | f-number ÷ √2 (f/4 → f/2.8) | shallower depth of field |
| Shutter | open time ×2 (90° → 180°) | longer motion blur |
| ISO | ISO ×2 (200 → 400) | more noise |
| ND | remove ND 0.3 | none (with a good filter) |
Why f-numbers step by √2
f-number = focal ÷ opening diameter. Light gathered follows the opening’s area, ∝ 1/N². Doubling the light needs twice the area, so N shrinks by √2. The marks 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 are rounded powers of √2.
f/8 to f/2.8 is three steps = +3 stops = ×8 light.
Shutter angle and motion blur
Shutter time = angle ÷ (360 × fps). At 24 fps: 180° = 1/48 s, 90° = 1/96, 360° = 1/24. Motion blur length = how far a thing moves in one frame × (angle ÷ 360).
Plates shot at 1/50 at 24 fps are at 172.8°, nearly 180°. CG must use the plate’s shutter, or its blur is instantly longer or shorter than the plate’s.
- 1Read the shutter from the camera report
- 2Convert to angle or time
- 3Set it in the render
- 4Compare blur with the plate
ND: less light without changing the look
An ND filter cuts all colours equally. Density 0.3 is about 1 stop (10^−0.3 ≈ ½), 0.6 about 2, 0.9 about 3. In bright sun, if you want a shallow focus and 180° together, ND is the answer.
Lock the look first, then fix the brightness
4 In real work
In Nuke
- EXPTool: adjusts brightness in stops; use it to match CG exposure to the plate.
- Grade: multiply 2 = +1 stop, multiply 0.5 = −1 stop (in linear).
- ScanlineRender: set the motion blur samples and shutter to the plate’s shutter angle (shutter 0.5 = 180°).
- ZDefocus: use depth to give CG the depth of field of the plate’s lens and f-stop.
- Grain: add grain/noise to the CG at the end to match the plate; a higher ISO usually means more of it.
Common mistakes → the fix
- ✗ Rendering CG with a 360° shutter while the plate was shot at 180°✓ Use the plate’s shutter; twice the blur shows straight away.
- ✗ Pin-sharp CG in a shallow-focus plate✓ Get the focus distance and f-stop from set, then defocus by depth.
- ✗ Noise-free CG in a high-ISO plate✓ Match the plate’s grain as the final step.
- ✗ Adding instead of multiplying to add one stop✓ A stop is a ×2 multiply in linear.
Before you send the shot
- CG shutter = plate shutter
- CG depth of field follows the plate’s lens and f-stop
- CG exposure matches the plate (compare in stops)
- CG grain/noise matches the plate
5 Check yourself
You open from f/5.6 to f/4 and want the same brightness and the same motion blur. What do you change?
🔒 Answer the question above and this card opens
1. Shooting 25 fps at 180°: what’s the shutter time?
2. From f/8 you open to f/4 and add ND 0.6. How does brightness change?
Where the numbers come from: The stop numbers are exact: area ∝ 1/N² (f-numbers are rounded marks of √2 steps), shutter time = angle ÷ (360 × fps), ISO ×2 = +1 stop, ND 0.3 ≈ 1 stop (optical density 0.3). The blur sizes, depth of field and noise drawn on the image are a teaching version, not a camera simulation. The plate, robot and sparks are our own Blender renders.