8/28/2023
EXPOSURE
By Mark Vargo, ASC
All cameras have a few things in common.
One second of a movie is a sequence of 24 still photographs.
Or simply stated: 24 frames per second (fps). This was standardized worldwide in 1927
with the introduction of the soundtrack.
There is little difference between calculating an acceptable exposure for film or still photography, so the following guidelines apply equally.
In order to achieve an exposure for a photograph or motion pictures, three
values are required:
-Sensitivity of the recording medium (ISO)
-Duration of exposure (shutter speed)
-Intensity of Light (measured by a meter, managed by the lens iris)
SENSITIVITY:
In the beginning, photographs were exposed on film. “Film” is celluloid coated
with a chemical emulsion. Emulsions were less light sensitive in the early days thus requiring cloudless days to attain an acceptable exposure. The first “interior” scenes were filmed outside on orientable platforms to capture sunshine as the primary lighting source.
Let’s give this first emulsion or “film stock” a numerical value of 4 to quantify its sensitivity to light. A film stock with a rating of 4 requires the sun for an acceptable exposure. In other words, this is a low sensitivity film stock. This is now known as a “slow” film stock.
For the first few years in the film industry, the only film stock available had an ISO rating of 4 and rendered a black and white image.
Before too long, Kodak introduced a new film stock with an ISO rating of 32. That’s 8x more sensitive or “faster” to light than ISO 4. This faster film changed the movie business forever. Filmmakers no longer needed bright sunlight to shoot so now, they could finally move their productions inside. And thus, a new era of movie lighting equipment and filming on stages began.
In summary, a film stock rated at 32 is considered a higher “speed” stock because it is 8 times more sensitive to light than a film stock rated at 4. A higher numerical rating equals more sensitivity. At the end of the film era the fastest film stock had a rating of ISO 800.
The transformation from emulsion-based film stocks to digital sensors is almost complete. RED pioneered this technology, and their first sensor had a rating of 160. That’s very low by todays standard but it was revolutionary technology and signaled the next big thing in the movie business. Knowing the sensitivity of your film stock or sensor is an essential factor in determining an accurate exposure.
SHUTTER SPEED:
Shutter speed describes the amount of time the reactive medium is exposed to light.
Shutter speeds vary from multiple seconds to milli-fractions of a second.
Because it is a single exposure, the still photographer has much more flexibility in changing the shutter for creative and technical purposes than the cinematographer.
Movie cameras have a rotating shutter that is interlocked with the cameras pull down claw - meaning that at 24 fps the shutter speed is fixed at 1/48th of second. If you slow the camera/shutter down by half or 12 fps then the shutter is open twice as long or 1/24th of a second. If you double the speed to 48 fps the exposure time is halved to 1/96th of a second.
(this will make total sense soon!)
In cinematography, the shutter speed is determined by the frame rate and since 24fps is the standard frame rate the shutter speed is fixed at 1/48th of a second (180-degree shutter).
Still photographers can place the shutter speed for whatever required – high shutter speed for sports and nature photography and slower shutter speeds in low light scenarios.
Changing shutter speeds affects exposure and must be compensated for by adjusting the iris or the ISO.
IRIS/APERTURE
Irises are housed within lenses and they control the amount of light that reaches the film plane.
Think of them as a “light valve”.
In a low light environment, more light is required so the iris is “opened up”. Conversely, in bright daylight, the iris is narrowed or “stopped down”. Iris’s are regulated by numerical designations that pertain to the diameter or aperture opening. These are known as F-Stops and each lens has up to nine of these: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22. The smaller the number the wider the iris or aperture. Wide open: f1.4 and closed-down: f22.
Light:
Modern light meters require two inputs before a reading can occur: shutter speed and ISO
(ISO is the sensitivity of the film or sensor, ex: ISO 32).
The third and final step to attain an exposure setting is the measurement of light on the subject. In taking that light reading, the meter computes an f stop appropriate for the shutter speed and ISO that have been preset.
If properly calibrated with accurate shutter speed and ISO values, light meters imitate the sensitivity of the chosen film stock or digital sensor.
This why the ISO on the meter will always reflect the native ISO of the sensor or film stock. Each are synchronized to one another!
You may set the iris to the f stop displayed on the meter and know that an acceptable exposure is guaranteed.
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