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Exposure settings: ISO speed in the digital age

One of the world's first photographs required an exposure time of around eight hours. It was taken in Paris by Joseph Nicéphore Niépce in the mid-1820s. The image was captured on a pewter plate coated with bitumen of Judea (a type of asphalt). This material was not very sensitive, even to bright daylight.

However, once the technique had been proven, experiments were made with other materials and exposure times quickly dropped to a few minutes. In the early 1840s, William Henry Fox Talbot sensitised paper to light by soaking it in a solution of silver iodide. This led to the use of other silver salts (known as silver halides), which are still the basis of modern film emulsions. Today, exposure times of 1/125 second, 1/500 second, or even 1/8000 second are achievable.

The sensitivity of film emulsion to light is called its 'speed' and is given an ISO number. Low numbers, such as ISO 50 or ISO 100 indicate a relatively low sensitivity to light. Film with these speeds are called 'slow'. High numbers, such as ISO 800 or ISO 1600 indicate a high sensitivity to light. Films with these speeds are called 'fast', or 'high-speed'.

With the introduction of digital photography, it makes sense to continue with the same ISO speed system so that the sensitivities of a digital sensor can be compared to those of film emulsions.

Film contains silver (in the form of halides – iodide, chloride or, most commonly, bromide). The Latin for silver is argentum. In recent years, photography with film or film cameras is often called argentic to distinguish it from digital photography.

If you want to take pictures in a wide range of conditions, from low ambient light to bright sunlight, you need to know a little about film speeds and their digital camera equivalents. ISO 1600 is ideal for indoor shooting without flash, while ISO 400 suits daylight sports events.

ISO and all that

In the early days of photography, film manufacturers used their own methods to measure and designate the speeds of their films. This was not very convenient – it was difficult to compare films from different manufacturers.

Very soon, a number of independent film speed systems were introduced and manufacturers started to use them on their packaging. The less popular fell by the wayside until only two systems were left – ASA (America Standards Association) and DIN (Deutsche Industrie Norm – the German standards organisation).

The DIN system was based on a logarithmic scale, where an increase in 3 on the film speed scale indicated a doubling of the film sensitivity. So 24° DIN film was twice as fast as 21° DIN film. The ASA system was an arithmetic scale. Here, the ASA number doubled with the film sensitivity – a 200 ASA film was twice as fast as a 100 ASA film. Simplicity won the day and the ASA system became the most popular.

Then, in 1987, the International Organisation for Standardisation devised a standard for defining film speed. This followed the ASA standard, so ASA was changed to ISO without any change in the numbers. ISO 200 is exactly the same as 200 ASA.

ISO is often reported as being an abbreviation of the 'International Standards Organisation'. Not so. The International Organisation for Standardisation (IOS) is known as the Organisation internationale de normalisation (OIN) in France. To avoid confusion, the founders of the organisation looked to Greek. ISO is derived from the word 'isos', which means 'equal'.

Photographic standards

The ISO group is very busy. If you visit its website ( and search for 'photography', you'll find there are 138 results for standards within the photographic sector. These range from density measurements, to processing chemicals, to graphic technologies and even tripod connectors, as well as a multitude of other areas.

The one we are most interested in is:

  • ISO 12232:2006 – Photography: Digital Still Cameras: Determination of exposure index, ISO speed ratings, standard output sensitivity and recommended exposure index

However, although ISO can refer to a vast number of international standards, in photographic circles it has come to mean just one – the sensitivity of a photographic film or digital sensor.

Setting digital ISO

To set the ISO speed on most of the EOS professional cameras, hold down the 'AF' and 'Metering mode' buttons at the same time (on the EOS-1D Mark III, press the ISO button; on the EOS 5D press the Drive-ISO button). The current ISO speed will be displayed in the LCD panel on top of the camera. To alter the speed, rotate the electronic input dial in either direction. When you release the two buttons, the new ISO speed will be set.


The standard ISO range runs from 200 to 1600.
With C.Fn-03-1 you can also set ISO 100 and ISO 3200.
With C.FN-08-2, the ISO speed is displayed in the LCD on top of the camera in place of the number of remaining shots.

EOS-1D Mark II, EOS-1D Mark II N

The standard ISO range runs from 100 to 1600.
If 'ISO expansion' under the first menu tab (camera) is set to 'on', you can also set ISO 50 and ISO 3200.


The standard ISO range runs from 100 to 3200.
With C.Fn I-3 you can also set ISO 50 and ISO 6400.


The standard ISO range runs from 100 to 1250.
With C.Fn-03-1 you can also set ISO 50.
With C.FN-08-2, the ISO speed is displayed in the LCD on top of the camera in place of the number of remaining shots.

EOS-1Ds Mark II

The standard ISO range runs from 100 to 1600.
If 'ISO expansion' under the first menu tab (camera) is set to 'on', you can also set ISO 50 and ISO 3200.

EOS-1Ds Mark III

The standard ISO runs from 100-1600
If 'ISO expansion' is set to on (C.Fn 1-03) you can also set ISO 50 (L) or ISO 3200 (H)


The standard ISO range runs from 100 to 1600.
With C.Fn 08-1 you can also set ISO 50 and ISO 3200.


The standard ISO range runs from ISO 100 to ISO 1600
If 'ISO expansion' is set to on, C.Fn. 1-3, you can set ISO 3200 (H).


The standard ISO range runs from 100 to 3200.
With C.Fn 1-3 you can also set ISO 6400 (H1) and ISO 12,800 (H2).

EOS 5D Mark II

The standard ISO range runs from 100 to 6400.
With C.Fn 1-13 you can also set ISO 50 (L), ISO 12,800 (H1) and ISO 25,600 (H2).

The advantage of a digital camera over film is that the ISO speed can be changed between exposures. You can shoot one picture at ISO 100, the next at ISO 800, and the next at ISO 400, if you wish. This ease of change means that the ISO value can become an integral part of exposure adjustment.

For example, if the exposure settings are 1/60 second at f8, and you want to shoot at 1/250 second, you would normally have to adjust the aperture to f4 to maintain the same exposure. However, with a digital camera, you can keep the same aperture if you need to control depth-of-field. For example, instead of shooting at 1/60 second at f8 with ISO 100, you could shoot at 1/250 second at f8 with ISO 400.


The standard ISO range runs from ISO 100-6400 with ISO 12,800 (H) available once ISO expansion is set in CFn I-3.

EOS-1D Mark IV

The standard ISO range is from ISO 100-12,800. If CFn I-3 is enabled, the settings – 50 (L), 25,600 (H1), 51,200 (H2) and 104,200 (H3) - will be accessible.


The standard ISO range is from 100-6400.
With C.Fn 1-3 this can be expanded to 12,800 (H)


The standard ISO range is from 100-25,600.
In the second red shooting menu, the ISO speed can be expanded to 50 (L) and up to 51,200 (H1) or 102,400 (H2)


The standard ISO range is from 100-51,200.
In the second red shooting menu, the ISO speed can be expanded to 50 (L) and up to 102,400 (H1) or 204,800 (H2).