Most are familiar with how ISO controls a camera's sensitivity to light and susceptibility to film grain. However, ISO has other consequences with digital cameras, and its implementation often varies. This article explores how ISO is evolving and influencing camera technique in the digital era.
With film, one had to select and load a specific magazine in advance, so the ISO speed had to remain constant. If the exposure needed tweaking afterwards, the film could be "pushed" or "pulled" during development. However, doing so was only practical within a limited range of effective sensitivities; otherwise colors, contrast, grain and other qualities would suffer.
With digital cameras, ISO functionality has improved substantially. Camera hardware no longer has to be swapped out, so ISO has effectively become an in-camera exposure setting along with aperture and shutter speed. Similar to push/pull with film, high-end cameras with RAW capabilities can also have their exposure tweaked in post production. However, depending on the camera, doing so may only be practical within a limited range, and can compromise image quality more than if ISO had been set optimally in the first place.
APPROACHES TO ISO WITH DIGITAL CAMERAS
A higher ISO just means that dimmer objects become lighter in the image. The main difference between RED® and most other cameras is when this lightening occurs. With other cameras, a higher ISO typically means the signal gets amplified and filtered before being recorded, effectively baking ISO into the RAW data. This process helped ease the initial transition from film to digital and often simplified camera electronics. However, amplifying the camera signal can also add noise and reduce the dynamic range, limiting post production flexibility.
With RED, the original signal remains unamplified before being saved as a RAW file. The idea is to record as much as possible from the sensor, and to maximize flexibility for adjusting exposure in post production. As a result, the full dynamic range is captured regardless of ISO choice, and ISO can be specified after the exposure. ISO only affects how the RAW data is interpreted. However, as we'll describe, the ISO choice is still important for maximizing image quality . . .
HOW ISO WORKS WITH RED
Raw sensor measurements are just numbers, and do not reflect anything visual until they are interpreted as shades and colors. With RED, ISO is effectively a look-up table (LUT) that controls how this interpretation process works. However, since this LUT applies to raw sensor data, it has to be more sophisticated by using floating point notation, hence RED's implementation is called FLUT®.
A higher ISO setting causes FLUT to remap dimmer objects to middle gray (shown in red), and vice versa. In the diagram below, each striped bar (on the left) is a qualitative representation of the numerical light levels captured by the sensor, and each gradient bar (on the right) is the output tones as seen by our eyes:
Note: Above spacing of numerical values has been exaggerated for visibility. Current RED cameras have at least 216 or 65536 levels in each color channel, so even at a high ISO, far more levels are available than can be distinguished by our eyes. To more closely match how we see, values are depicted to increase in density closer to their maximum.
Since the total dynamic range remains unchanged, this effectively shifts the number of stops that fall above and below middle gray, and compresses or expands tones accordingly. At low ISO speeds, most of the dynamic range is below middle gray, whereas at high ISO speeds most of this dynamic range lies above middle gray.
Note: The base ISO does not necessarily mean that middle gray originates from the perceptual halfway point of sensor values. At high ISO, sensors with analog gain typically lose one stop of dynamic range for each stop increase in ISO.
A key benefit of FLUT is that it remaps tones without clipping the highlights or crushing the shadows. All tones are retained, they're just redistributed. Therefore, unlike with other ISO implementations, if an exposure does not clip the highlights at ISO 400, then the highlights will remain unclipped at ISO 800 and higher. Furthermore, 16-bit files and REDCODE® help Exposure Assist / FLUT retain smooth tonal gradations when redistributing tones.
In practice, a higher ISO setting will provide more insurance against clipped highlights, and when highlights do clip, the transition to clipping will typically appear less abrupt. A lower ISO setting will decrease the appearance of noise, particularly within the shadows. See the tutorial on exposure strategy with RED cameras for additional advice on using ISO in practice.
MEANING OF NATIVE ISO SPEED
With most other cameras, "native ISO" originally referred to the sensitivity which was able to saturate the sensor's photosites. This is often also called the "rated" or "base" ISO speed of a camera. The native ISO is often near 100 or 200 with stills cameras; anything higher and white represents only fractional photosite occupancy, and anything lower and photosites become oversaturated. However, this definition does not apply to RED because all ISO speeds use the full potential of the sensor.
With RED, the base ISO speed describes the recommended starting point for balancing the competing trade-offs of noise in the shadows and clipping in the highlights. This does not necessarily reflect anything intrinsic to a camera sensor itself or the performance of cameras by different companies. It is a reflection of the sensor design, signal processing and quality standards all in combination. Typically anywhere from 400-2000 ISO is recommended with the RED MYSTERIUM-X® sensor, depending on application and noise preferences. Later RED sensors are more flexible, particularly at higher ISO speeds.
Even though ISO can be adjusted afterwards, this does not mean it is without consequence prior to a shoot. ISO still controls how one thinks about exposure through one's choice of aperture, shutter speed and lighting, so one should still have a target ISO in mind. RED's ISO implementation also simplifies exposure technique. With most cameras, shooting in brighter light requires a lower ISO setting to protect against clipping. With RED, lowering the ISO is not necessary since one gets the full highlight range of the sensor regardless. If the lighting is too intense, stopping down the lens or using an ND filter is advisable instead.
Ultimately though, a camera sensor sees the same light regardless of ISO choice. One therefore shouldn't have to be restricted to a particular ISO in advance. In effect, RED's approach to ISO separates out what's physical (settings controlling how much light the sensor receives) from what's digital (how that sensor data is processed). Other than simplifying the workflow and improving post production flexibility, this also retains the full dynamic range of the camera sensor. Particularly at higher ISO speeds, this approach results in a higher dynamic range than any other implementation.