NB: This series of posts will convey my understanding of photographic exposures. While I hope it serves a useful purpose, I by no means describe this series as a be-all, end-all resource on exposure. As ever, if you feel I have made a factual error, please correct me.
Exposure is a combination of how long and how much light an imager (digital sensor or film) is exposed to by the lens and shutter. ISO/ASA (and less commonly nowadays DIN) is the standardised measure of an imager’s light sensitivity (i.e. sensitivity dictates how much light and how long for). In Part Three of this series, I shall focus on “sensitivity” (ISO/ASA).
ISO (ASA, though this seems to be on the wane) is a standardised measure of how sensitive an imager (be it film or digital) is to light. There also exists a DIN scale, but that’s rare enough not to have to worry about. In fact, in the past 14 years or so, the only time I’ve encountered DIN scales is in tandem with ASA scales on flash guns.
The ISO setting on a digital camera (or rating of a film) dictates the aperture and shutter speed to be used for a given amount of light. For instance, if I were to make the camera twice as sensitive to light, I would need to expose it for half as long or to half as much. One way to think of it is paper cups: a smaller (more sensitive) paper cup takes less time to fill than a larger (less sensitive) paper cup.
ISO sensitivities are easy to fathom: a high ISO is highly sensitive to light, and a lower ISO is less sensitive to light. Illustrative of this is the fact that ISO 800 is more sensitive to light than ISO 100. Every doubling of ISO number equals a doubling in sensitivity to light, i.e. ISO 400 is twice as sensitive as ISO 200. Equally, every halving of ISO number equals a halving in light sensitivity. Doubling or halving the sensitivity is equivalent to increasing or decreasing the aperture by one f/stop (see My Take on Exposure: Part One here).
Impaired dynamic range is a downside of high ISOs. Dynamic range is the range of highlight and shadow detail which can be recorded. As you increase the ISO, you typically decrease the dynamic range, resulting in photos with narrower tonal ranges. Often this is seen as dark tones favoured over light tones or light tones favoured over darker tones. Sunrise or sunset photos, where the only discernible detail is the colour in the sky, are examples of light tones being favoured over dark tones with a limited dynamic range. This is visible comparing Figures One and Two.
The other major downside of high ISOs is noise (or film grain). Noise in a digital photo manifests itself as pixels or spurious colours or incorrect illumination. In lay man’s terms, you see a lot of coloured or bright spots over the image. Comparing Figures One and Two will highlight this phenomena. Noise is usually generated by heat on the imager during exposure, and is also a problem with longer shutter speeds (generally well over a minute in length, where the sensor accumulates heat from operation). Cameras and software are able to remove noise by smoothing affected images, but this can cause a severe loss of detail. The setting is variable, so you may choose what you feel is the best noise/detail compromise.
Film suffers from graininess rather than noise. Grain in film stems from the silver halide crystals in the emulsion: larger, more visible crystals are more light sensitive than smaller, less visible crystals. This is somewhat negligible with the latest film emulsions, where advances in technology have allowed finer grain with higher sensitivity. Certainly, the ISO 400 Kodak films (BW400CN and T-Max400 black and white, Portra 400 colour) I have shot in the past five years result in photos with no visible grain, even compared with Kodak’s own ISO 100 Ektar colour film. On the other hand, graininess varies between manufacturer: I once shot a roll of Agfa 100 ASA colour film, which was gravel pit-like grainy compared to the buttery smoothness of Kodak’s Portra 400. That’s even disregarding my (blatantly obvious to most people) fondness for all Kodak film emulsions.
To summarise: ISO is the standard measure of imager light sensitivity. Every doubling or halving of ISO number doubles or halves the sensitivity, equivalent to increasing or decreasing aperture by one f/stop. Noise increases with ISO and may be corrected in-camera or by software. Dynamic range decreases as ISO increases. The same generally holds true for film, except noise is replaced by grain. Graininess may vary by manufacturer.
High ISO = high sensitivity, high noise
Low ISO = low sensitivity, low noise