Boulevard of Photographers provides here a brief overview of the term ISO.

The name ISO has its roots in the Greek word " isos " meaning " equal ". The name of the organisation that sets these standards, commonly referred to ISO, is in fact the International Organization for Standardization * and not an acronym for the ISO designation. The ISO standards have replaced the previous ASA (American Standards Association) ones, which used to establish North American standards for film manufacturers.

Not surprisingly, the ISO rating given for digital cameras has its roots in film photography. As with film photography, the ISO is a measurement of the sensor's sensitivity to light. Low numbers such as 50 ISO , 64 , 100 ISO indicate a sensor that reacts less quickly when exposed to light than higher numbers such as 200 ISO , 400 ISO .

Much like film, image sensors used in digital cameras are "set" to emulate film speeds. And, again much like film, the higher the ISO speed, the more noticeable the "grain" becomes in the image. However, the grain one can see in a digital image taken at a high ISO is really interference or noise, and not film grain beads of chemicals on the emulsion that can be observed in high ISO film images.

The most important difference between image sensors and film is that the sensitivity of the sensor can be changed, while with film, the film has to be changed. In fact most digital cameras provide the user with a ISO setting that makes it possible to increase the sensitivity of the CCD, and therefore allow the use of higher shutter speeds and/or smaller apertures according to circumstances.

A look through the images below shows the influence of the ISO setting on the shutter speed.

	Sensitivity: 64 ISO Aperture: f3.6 Shutter Speed: 1/8

	Sensitivity: 100 ISO Aperture: f3.6 Shutter Speed: 1/10

 

	Sensitivity: 200 ISO Aperture: f3.6 Shutter Speed: 1/20

	Sensitivity: 400 ISO Aperture: f3.6 Shutter Speed: 1/45

The photos are taken in subdued light, and with the camera (in this case a Minolta Dimage S404) mounted to a tripod.

By increasing the ISO setting, the shutter speed goes from 1/8 of a second a difficult shot to take without a tripod to stabilize the camera to 1/45 of a second, a shot that could have been made without a tripod. Boulevard of Photographers advises you that as you increase the ISO setting, things get more challenging.

The small sections of the full size images that are included show an area susceptible to a gain in noise, which makes the image "more grainy". As can be seen, the increase noise is quite acceptable and the benefit of a higher shutter speed can be worth it in some situations.

A sensitivity of 400 ISO isn't the limit. Some cameras are able to range higher, up to 800 and 1600 ISO. The most common additional sensitivity setting is 800 ISO, and although the noise can become more pronounced, recent models have shown considerable improvements in noise reduction over what earlier models could do at 800 ISO. The two images of the same subject below show these results.

	Sensitivity: 800 ISO Aperture: f4.2 Shutter Speed: 1/85

 

	Sensitivity: 1600 ISO Aperture: f4.2 Shutter Speed: 1/170

If you are a beginner, Boulevard of Photographers advises you to test and try different shutter speeds. As can be seen, the noise increases at high ISO settings is noticeable but, once again, not unacceptable. Furthermore, a 1600 ISO sensitivity permits trading some of the shutter speed for a smaller aperture, which produces a greater depth of field, as in the example below:

	Sensitivity: 1600 ISO Aperture: f13 Shutter Speed: 1/15

The effect of the smaller aperture (f13) can be seen in the greater detail that shows in the wooden mallet, part of this Tibetan singing bowl.

Auto ISO Versus Selectable ISO

There are many different digital cameras on the market, but when it comes to sensor sensitivity, they can be grouped into 3 categories:

1.cameras that have a single, non-variable ISO sensitivity,
2.cameras that automatically vary the sensitivity of the sensor according to the ambient light and have no user setting,
3.cameras that offer both an Auto setting, and user-adjustable ISO settings.

For cameras in our category 1 above, the advantage is simplicity, and in some cases a lower purchase cost. The disadvantage is that the camera may often require the assistance of its flash to get a properly exposed image. Furthermore, it may be forced to use low shutter speeds when the weather is poor and there isn't sufficient light.

For cameras in category 2 , the advantage is a greater flexibility for taking pictures, but the disadvantage is that the image quality may lack consistency. Some cameras are quite prone to noise, and the image quality can vary considerably when the camera uses a low ISO setting and when it uses a high setting. Ideally, an Auto ISO system should have a clearly defined range that avoids the noisier settings, and in fact, many cameras have limits on the range the Auto ISO is allowed, either 50 to 100, or 100 to 200 and in some case 100 to 320 ISO.

The last of our camera categories, 3 , is the most flexible. These cameras allow the user to choose a particular setting. For example, this makes it possible to increase the sensitivity to take photos in a museum without the flash, or, outdoors light to obtain a greater depth of field through the use of a smaller aperture.

As with many other things, in our opinion, having a choice is always better than having none.