CAMERA



Film Speed (ISO)

Introduction to the Photosensitivity of Digital Cameras.

ISO (International Organization for Standardization) used to be associated with traditional film stock. You may remember buying photographic films with different ISO speeds that would be suited to different types of photography, however ISO has taken on a different meaning in relation to digital sensor use. This article introduces the terminology and effects of ISO in digital cameras.

Background

When recording on film stock the DoP has to select in advance the desired film emulsion with the appropriate photosensitivity (ISO). The ISO value remains constant, independently of the camera settings.

Adjusting the sensitivity after the shoot is only possible within limits, by “pushing” or “pulling” the film during the development process. Side effects of this process are inevitable, such as larger grain size, change in the contrast range and colors, etc.

Digital cameras allow a wide range of changes to the ISO values. In addition to the aperture and exposure / shutter,  ISO settings are a common tool to adjust the brightness and related properties of a recording.

However, depending on the camera model and manufacturer, the extent to which these changes can be applied usefully are still limited. Inevitably, there is always a cost of adjusting the ISO values after the recording compared to choosing the best value to start with. For cameras that use analog gain, each f-stop that is added in ISO reduces the available contrast by the same amount. Especially with higher ISO values and correspondingly greater amplification, image noise and color shifts are added.

Cameras recording in RAW , on the other hand, have the option of retroactively adjusting the ISO value of the recordings, similar to the "push" or "pull" process. The tolerance to visible artefacts in this case is much higher than with traditional film. So much so that many productions now opt to record in RAW to give them more flexibility in the post production process, however this can lead to additional post costs.

The base-ISO commonly describes the native sensitivity of the camera sensor, which tends to be fixed. It is related to the light level at which each pixel cell is saturated by incoming light. Adjustments in ISO affect the processing upon read-out from the sensor. Signal processing however can lead to noise or reduced dynamic range. Since no two cameras models behave exactly the same in this situation it is recommended that the acceptable limits are determined before choosing a particular model to use.

The dual ISO system

Panasonic were the first to introduce a Dual-ISO system in their cameras.  A Dual-ISO system relies on two separate signal paths coming from the sensor. These paths are processed with specially tuned filters to optimise their respective signals. As an example the Panasonic Lumix GH5 can be manually switched between LOW-ISO at native 400 and HIGH-ISO at native 2500.  The higher base-ISO can be amplified by a significant amount without introducing too much noise, while the lower base-ISO preserves the full dynamic range in bright daylight. Sony have announced that they will bring Dual-ISO to the VENICE soon and RED have announced the Gemini with similar technology.

ISO settings when working with RED cameras

With RED, the image signal remains unamplified (at the level of the BASE ISO) until it is saved to the RAW file. The basic idea is to store as much information as possible that the sensor can map and make it usable in post-production. This means that, regardless of the set ISO value, the full dynamic range is recorded. The exposure of the recording can then be adjusted in post-production. The ISO value in the camera only affects how the RAW data is interpreted.

Raw sensor data after all are just arrays of numbers. They gain meaning only when translated into levels of brightness and colour.

The ISO setting in RED therefore is nothing more than a LUT (Look Up Table), which performs the interpretation of the data. A higher ISO setting causes darker parts of the picture to be "pushed up" in the direction of the middle grey - that is, brighter - and vice versa.

The chosen ISO value changes the number of available aperture levels above or below middle grey. At lower ISO, the greater part of the dynamic range is below mid-grey. At high ISO, the majority of the dynamic range is above mid-grey.

A key advantage of this system is that when interpreting the sensor data, highlights do not clip and the shadows don’t crash. All gradations are preserved.

In practice, this means that with RED cameras , a higher ISO setting is more resistant to clipping the highlights. A lower ISO reduces image noise, especially in dark parts of the picture. The choice of the ISO at RED shifts the image characteristics between low background noise and preservation of the highlights. The maximum dynamic range remains unchanged.