Sensitivity

Traditionally with film based cameras, the photographer would carry different films of varying film speeds, usually measured in the old ASA (American Standards Association) and the old German DIN scale.

Today the ISO (International Standards Organisation) scale is used, as this is more familiar to most photographers, as it is based on the more widely used ASA scale.  In the ISO arithmetic scale doubling the speed of a film (that is, halving the amount of light that is necessary to expose the film) implies doubling the numeric value that designates the film speed. In the ISO logarithmic scale, which corresponds to the older DIN scale, doubling the speed of a film implies adding 3° to the numeric value that designates the film speed. For example, a film rated ISO 200/24° is twice as sensitive as a film rated ISO 100/21°.  Commonly, the logarithmic speed is omitted, and only the arithmetic speed is given (e.g., “ISO 100”).

With photographic film I have used films ranging from Kodak Ektar 25 (a very slow film, with very fine grain) for taking photographs of people and places, as well as helicopters, on bright sunny days. With the helicopters, this slow film speed has enabled me to capture the movement of the rotor blades, right up to Kodak Ektar 1000 (a very fast film, but with Kodak's “T-grain” which reduces the grain and therefore produces a photograph) for taking photographs at concerts or in other lowlight situations or where very high shutter speeds are required. I have used Fujifilm 1600, but I found that too grainy and have not used it since.

With digital cameras the sensor is fixed so the sensitivity in controlled within the camera and therefore the full range of ISO (sensitivity) ratings is available, without the photographer being limited to the speeds of the film that they carry with them. Although increasing the ISO rating allows pictures to be taken in darker situations than a lower ISO, as with all photography there is a trade off. This will be an increase in the sensor noise (which in the case of photographic film was grain), see above.

Another problem with increasing the sensitivity or ISO rating of a photographic film, is that colour temperature can become an issue. The different colour temperatures of light are shown below:

This is the reason that when you take pictures inside a room where there is normal tungsten lighting, using daylight film, at a ISO of 400 or above there is a yellow cast to the picture. This can easily be corrected using colour correction filters (eg 80a).  This problem rarely appears in digital photographs with modern cameras, but if it did it could be corrected either by changing the white balance (pre-shutter) or at the post production stage.  Although if the photograph was captured as a RAW image the white balance could still be corrected easily at the RAW workflow (pre-conversion) stage.