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 anacronym
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. 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
As can be seen, the noise increase 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.
*When first published, this article contained
a factual error concerning the origin of the name ISO. This
error is now corrected thanks to Jean-Roch Lebel, one of
those responsible for the implementation of ISO Standards. – Editors.
