How to use it
- Set the frame rate.
- Set the four dials where you want them — ISO, aperture, shutter, ND.
- Lock the three you’ve decided on. Turn any of them and the free dial moves to hold the exposure.
- Tap the small labels to switch aperture between T-stops and f-stops, the shutter between angle and speed, and the ND between variable, hard (density) and stops.
How it’s calculated
Every term is in stops, so exposure holds whenever the changes add up to zero: one stop more ISO pays for one stop more ND. Exposure is always worked from the T-stop — the light that actually reaches the sensor. In angle mode the shutter time is angle ÷ 360 ÷ frame rate, so changing the frame rate changes the exposure and the free dial rebalances.
ND filters: density, stops and light
| Density | Stops | Factor | Fraction | Light through |
|---|---|---|---|---|
| ND 0.3 | 1 | ND2 | 1/2 | 50% |
| ND 0.6 | 2 | ND4 | 1/4 | 25% |
| ND 0.9 | 3 | ND8 | 1/8 | 12.5% |
| ND 1.2 | 4 | ND16 | 1/16 | 6.3% |
| ND 1.5 | 5 | ND32 | 1/32 | 3.1% |
| ND 1.8 | 6 | ND64 | 1/64 | 1.6% |
| ND 2.1 | 7 | ND128 | 1/128 | 0.78% |
| ND 2.4 | 8 | ND256 | 1/256 | 0.39% |
| ND 3.0 | 10 | ND1000 | 1/1024 | 0.10% |
Shutter angle to shutter speed
| Angle | 24 fps | 25 fps | 30 fps | 48 fps | 60 fps |
|---|---|---|---|---|---|
| 45° | 1/192 | 1/200 | 1/240 | 1/384 | 1/480 |
| 90° | 1/96 | 1/100 | 1/120 | 1/192 | 1/240 |
| 144° | 1/60 | 1/63 | 1/75 | 1/120 | 1/150 |
| 172.8° | 1/50 | 1/52 | 1/62 | 1/100 | 1/125 |
| 180° | 1/48 | 1/50 | 1/60 | 1/96 | 1/120 |
| 270° | 1/32 | 1/33 | 1/40 | 1/64 | 1/80 |
| 360° | 1/24 | 1/25 | 1/30 | 1/48 | 1/60 |
Questions
What is the 180° shutter rule?
A shutter open for half of each frame — 1/48 s at 24 fps, 1/50 at 25, 1/60 at 30. It gives motion the blur audiences are used to, which is why ND, not the shutter, usually brings the light down outdoors.
Is ND 0.9 the same as ND8?
Yes. Density 0.3 is one stop, so 0.9 is three stops, which lets through 1/8 of the light — ND8.
T-stop or f-stop for exposure?
T-stop. It measures the light that actually gets through the lens, so two lenses at the same T-stop expose the same. The calculator works exposure from T and can show either.
Why does changing the frame rate change the exposure?
In angle mode the shutter stays open for the same share of each frame, so more frames per second means less time per frame: 180° at 48 fps is 1/96 s, a stop darker than 1/48.