ISO and ASA
ISO and ASA are for our purposes the same, a measure of the sensitivity of camera sensors or film. Its use is in creating the ideal Exposure, the correct amount of light recorded to produce the ideal image.
The higher the number the more sensitive it is. The scale used is convenient for photography as a doubling or halving of the number represents one stop, as Shutter Speed also creates a one stop difference by halving or doubling a direct adjustment can be made. In this case the numbers go in the same direction, so if you had a camera set to ISO400 and had an exposure of 1/100 second, increasing the ISO by one stop to 800 would double the sensitivity and therefore you need only half the time for the exposure, and can therefore also double the shutter speed to 1/200.
The third Exposure variable generally available is Aperture, the size of the hole, increasing the sensitivity means you can have a smaller hole for the same time. So an exposure at ISO200 of 1/250 at F8, can become at ISO400 1/250 at F11 as we have doubled the sensitivity (1 stop) and decreased the amount of light entering the lens by half (1 stop).
ISO and Noise
As sensitivity is increased on digital cameras beyond a point, noise also increases, later cameras having a higher point where this starts. Noise is random anomalies or odd bright pixels. Noise reduction software can reduce this but at the expense of sharpness. People with a film background accept more noise than those who have entered photography in the digital age, as film had grain that was often more destructive to the image than noise is. Noise is more noticeable on a computer monitor than when printed, as it often occurs in shadow areas. You need to experiment with your camera at different ISO values to see what level of noise you are happy with. If the choice is between some noise and blur from using two slow a Shutter speed, or the image being out of focus due to limits of Depth of Field (DOF), then you will find that the compromise with the noise present, is the best option. Noise can also be created in editing, especially pulling detail out of shadows.
As digital cameras have advanced the ISO levels have increased by several stops. Today cameras like the Nikon D300 have an ISO range from 200-3200 with High1 above being the equivalent of 6400 and Low1 below being the equivalent of 100. Where you can it's best to avoid the high and low settings and keep within the main range. There is very little noise up to an ISO of 1600. Using ISO 1600 or 3200 you can take photos inside quite low lit buildings without the need for flash or a tripod.
Historically to get fine grain film photographers had slow film, film with ASA the equivalent of ISO at 25 or 50, with perhaps 125 being the most widely used where faster film was needed. These photographers having moved to digital are often surprised to see no low ISO values, and discover that most of us use ISO 400 as standard. With older film cameras, lenses with a large aperture, for example prime lenses up to f1.4 were common while today many telephotos are around F4 at best, this is 3 stops down, so 3 stops up in ISO to compensate takes us from ISO 50 to 400, allowing us to maintain the same shutter speed. With older film cameras telephoto lenses were short, 200mm being quite a long lens, while today we will take 400 as fairly common, and many standard zooms go to 200mm. With the reduced Depth of Field (DOF) with longer lenses we need to use smaller Apertures, and therefore need the extra ISO values to allow us to make use of the power of our lenses.
In some cases where you have a bright day and want to use a time exposure (see Shutter speed ) to produce water blur, or get rid of moving items, you may feel the shortage of low ISO values is limiting. While you don't have very low ISO, you can simulate the affect of one by using a high value Neutral density filter. This can be a square filter that fits within a filter holder or a screw on filter The highest value Neutral density filter that I routinely use is an ND64, which is 6 stops. Use this in combination with an ND8 and you can get slow enough exposures to make traffic on the motorway disappear.
Some cameras have a facility within the menu that is called 'AUTO ISO', what this does is to allow the camera to automatically increase the ISO when there is not enough light to comply with inbuilt rules. It is a form of automatic exposure enhancement. While this may sound a good idea the effect is often to take control away from the photographer, and it often creates far more problems than it solves. If you are out with a photographer and they say their camera has gone wrong, in that its not taking any notice of the settings they set, then the usual cause is that Auto ISO has been set. Unset it and the problems will go away. With some cameras including most Nikons, when it is on and the camera changes the ISO setting it shows it up in the back panel amongst the image data in red, to make it obvious what has happened. I am not a fan of Auto ISO and switch it off.
The meaning of the letters, others variations and history
The sensitivity of film and camera sensors is measured in a number of scales amongst these are ISO, ASA, DIN, GOST Exposure index (IE), APEX and probably a number of others.
The letters usually relate to the different standards organisations, so ISO = International Organization for Standardization, ASA = American Standards Association, now ANSI American National Standards Institute, DIN Deutsches Institut für Normung, this translates to German Institute for Standardization. The various standards organisations have now generally come together to be a part of ISO.
Of these ISO is now the main one in use, while ASA is the same scale so can be used to mean the same thing, and GHOST, used mostly in the Soviet Union up to around 1987, is not greatly different, being about a third of a stop more sensitive for the same numeric value.
DIN is the odd one, in that it uses a scale in thirds of a stop with a value of one added for each third stop, so 3 per stop, while with the others the number doubles with a one stop increase. Din is shown with a degree symbol after it (small raised zero eg 24° which is the equivalent of ISO 200, or 27° and ISO 400.
For a full listing of equivalents and examples of some film sensitive ratings, see Comparative table of ISO, ASA, DIN and GOST.
EI or Exposure Index is the gain introduced to an electronic circuit, historically it was used to represent the use of film at a different speed, so we might have had ISO 400 film used as IE800, and we then pushed the processing so as to develop it in way that produced a good image. With a digital camera we are doing the same, processing the image so as to gain a different level of gain, however camera manufacturers simplify this by simply allowing you to set the ISO, so in this case ISO and EI is the same.
Technically there are two ISO standards, arithmetic same as ASA and logarithmic same as DIN and you may see them written fully in the form of a film rated ISO 200/24° but usually its only the arithmetic scale now used including with film.
Many older photographers with a film background will think and often mention ASA, while today we just usually see ISO, however as these are the same in practice the two scales being identical, it makes no difference.
With film larger grain film is more sensitive while fine grain film is less sensitive. Film which is over exposed looks grainier while underexposed appears to have less grain. We talk of a slow film when we mean a low sensitivity film as it required a longer exposure and a fast film as one with a higher sensitivity requiring a shorter exposure.
for details on Exposure Article Route see the Exposure page