The Exposure Triangle for Cinema

Four controls, not three

The stills version of the exposure triangle has three corners: aperture, shutter speed and ISO. A cine camera adds a fourth that does most of the work, the ND filter, and each of the other three behaves a little differently. The iris is marked in T-stops, the shutter is an angle tied to the frame rate, and on most cinema cameras ISO is an exposure index (EI) that moves the picture's latitude rather than adding gain.

All four move exposure in the same unit, the stop, so they can be traded against one another. What differs is the side effect: depth of field for the iris, motion blur for the shutter, noise and highlight room for EI, and nothing at all for a good ND. That is why a camera team rarely uses the triangle freely. The frame rate and shutter are set by the look, the stop by the focus, and ND and lighting absorb whatever is left.

Stops: the common unit

One stop is a factor of two in light. Every control has its own scale, but each scale can be turned into stops, and that is what lets you swap one for another.

T-stop and the third-stop series

An f-number is the focal length divided by the entrance pupil. A T-stop is the same number corrected for the light the glass loses, so a set of cine primes all give the same exposure at the same marking. Use the T-stop for exposure and the f-number for depth of field; the Cinema Calculations Handbook has the conversion. Light meters read in f-numbers, and you set that number on the T-scale.

Each full stop multiplies the number by √2 (about 1.41). Cine lenses and meters work in thirds, each a factor of 2^(1/6), about 1.12. The marked values are rounded versions of those powers of two.

Full and third stops, T1 to T32

The marked numbers are conventions, not exact values: T3.5 is 3.56 worked out, and T11 is 11.31. The differences are a few hundredths of a stop, which no meter or monitor will show.

Shutter angle

The shutter angle sets the exposure time as a fraction of the frame interval: time = angle ÷ (360 × frame rate). At 24 fps a 180° shutter gives 1/48 second. Halving the angle to 90° costs one stop and makes motion crisper and more staccato; opening to 360° gains one stop and smears it. That change in motion blur is why the shutter is usually chosen for the look and left alone.

No digital camera reaches a full 360°: the ALEXA 35, for example, runs from 5.0° to 356°. Two other things limit the shutter: the frame rate, because a faster rate shortens every exposure, and the mains frequency, because some angles make lights flicker. The shutter angle guide has the conversion charts and the flicker-free angles for 50 Hz and 60 Hz.

ISO and exposure index

On a stills camera, raising ISO amplifies the signal. On most cinema cameras shooting log or raw, the ISO setting is an exposure index: a rating that tells the camera, and the light meter, how bright to make middle gray. The sensor does not become more sensitive.

ARRI documents this. On earlier ALEXAs such as the Mini and Mini LF, which record LogC3, each EI has its own Log C curve, and every curve places 18% gray at the same code value (0.391, which is 400 of 1023 in 10-bit). The ALEXA 35's LogC4 uses one curve for every EI, with 18% gray at 0.2784; the EI changes only a gain applied before the curve. On both, rating the camera one stop higher means the meter gives you one stop less light on the sensor for the same picture brightness, and the sensor then holds one stop more highlight detail above middle gray, with one stop less below it and more visible noise. Rating it lower does the reverse. In ARRIRAW the EI is stored in the file header as metadata, so it can be changed in post. The ALEXA 35 offers EI 160 to 6400 in thirds.

ND: exposure with no side effects

A neutral density filter cuts the same fraction of light at every wavelength. It is the only exposure control that changes nothing else in the picture, which is why it does most of the work: the stop holds the depth of field, the shutter holds the motion, and ND takes up the rest. Each 0.3 of density is one stop.

Many cinema cameras have ND built in. The ALEXA 35's motorized filter has four positions: clear, 0.6, 1.2 and 1.8 (two, four and six stops). Anything past that, or a one-stop step in between, goes in the matte box. The ND filter guide covers density, filter factors, stacking and the full chart.

The order to set them on set

1. Frame rate. Fixed by the project (24, 25 or 30) or by the slow-motion factor you need.
2. Shutter angle. 180° unless the look calls for something else, or the lights need a flicker-free angle.
3. EI. The camera's recommended rating, moved up or down for where you want the latitude.
4. T-stop. Chosen for depth of field and for where the lens looks best.
5. ND and lighting. Whatever the first four leave over: ND to cut light, more light (or a wider stop) to add it.

Working it out in CineMath

CineMath's Exposure calculator takes your current T-stop, shutter angle and ISO and a change in stops, and gives four separate fixes: change the iris, the shutter, the ISO, or the ND. Each one absorbs the whole change on its own, so you pick one. A positive change means more light has arrived (the sun came out) and has to be cut; a negative one means light was lost. The EV Change field takes fractions such as +1/3 or −2/3. The free exposure calculator does the same in your browser.

This describes CineMath 1.3, the next update. Until it reaches the App Store, the version there is 1.2.1, and the Exposure screen differs in places.

From T2.8, 180°, ISO 800

The T-stop is rounded to the nearest third-stop marking, with the exact value alongside when the two differ. The app warns when a fix is not practical: the −1 row's 360° shutter is outside the 1–358° it accepts, an ISO outside 50–25,600 or a stop outside T1–T32 is flagged, and removing ND assumes a filter is in.

Next steps

• Try a change in stops against your own settings in the free exposure calculator, in your browser.
• Convert shutter angles to shutter speeds and find flicker-free settings in the shutter angle guide.
• Read ND markings in density, stops or filter factor with the ND filter chart.
• Turn a stills meter's EV reading into a T-stop with the EV chart.
• Check what the stop you chose does to focus with the Depth of Field calculator.

The Exposure calculator is free in CineMath on iPhone, iPad and Mac, so the next time the light changes you can see all four fixes at once.