Setting Exposure — Aperture (f-stop and T-stop), Shutter Angle, ISO/EI and ND Filters

Every way of changing exposure can be counted in stops. One stop brighter is twice the light, one stop darker is half, so if you raise one setting by a stop and lower another by a stop, brightness stays the same. Each setting leaves a different mark on the image, though.

The f-number is the lens’s focal length divided by the diameter of the opening that lets light in. The bigger the number, the smaller the opening and the darker the image. The standard scale f/1.4 · 2 · 2.8 · 4 · 5.6 · 8 · 11 · 16 is one stop per step, because shrinking the diameter by about 1.4× (√2) halves the area.

The f-number is a calculated value, so it ignores light lost in the lens’s glass and coatings. A T-stop is rated by measuring the light that actually gets through the lens, so any lenses at the same T-stop give the same exposure. That’s why cine lenses are marked in T-stops, and for a given lens the T-number is slightly larger than the f-number. Depth of field follows the size of the opening, not the amount of light, so think in f-numbers for depth of field.

Shutter angle comes from the rotary shutter of film cameras. The disc spins once per frame, and light gets in through the open part of the disc. So exposure time is open angle ÷ 360 × 1 ÷ frame rate.

On a camera that shows shutter speed instead, 180 degrees is roughly 1 over twice the frame rate (1/50 s is common at 24 and 25fps). Lights running on mains power can flicker at twice the mains frequency: 120 times a second on 60Hz power, 100 times on 50Hz. If the exposure time is a multiple of one flicker cycle (1/120 s or 1/100 s), every frame captures the same brightness. So at 24fps on 60Hz power, 1/60 s (144 degrees) is less prone to flicker, and on 50Hz power, 1/50 s works the same way.

On cinema cameras, sensitivity is also called EI (exposure index). On many cameras, changing the EI doesn’t give the sensor more light; it changes how brightly the same signal is displayed. So it helps to think of sensitivity as a setting that splits your headroom between highlights and shadows.

• Native ISO — The base sensitivity where the sensor design best balances noise and dynamic range. Start here.
• Raising the EI — You give the same scene less exposure, so you gain highlight headroom and get more noise in the shadows.
• Lowering the EI — You give more exposure, so the shadows get cleaner and the highlights clip sooner.
• Dual native ISO — Some cameras have two base sensitivities, so noise rises less at the higher one. Values differ by model, so check the manual.

Outdoors at midday there’s too much light to hold a shallow depth of field or a 180-degree shutter. That’s when you put an ND filter, which cuts only the amount of light and leaves color alone, in front of the lens or inside the camera.

• Heavy ND — Very dense filters, or several stacked, can turn black clothing reddish or brown. Use NDs that also block infrared, or test first.
• Variable ND — Two polarizers rotate against each other to change density continuously. Convenient, but at the dark end they tend to show an X-shaped pattern or a color shift, and on wide lenses they can darken unevenly.

• Frame rate and shutter — Set the project frame rate and lock the shutter angle.
• Aperture — Choose it for the depth of field you want. It changes the feel of a shot more than anything else.
• Sensitivity — Start at the camera’s base ISO and move it only when you must.
• Too much or too little light — If there’s light to spare, cut it with ND; if there isn’t enough, add lighting.