WHAT IS ISO AT DIGITAL CAMERA ?
The ISO function in a digital camera establishes the light sensitivity for
the camera's image sensor (the digital camera equivalent of film). The range
of available settings vary considerably, by camera make and model. The higher
the sensitivity, the less light is needed to make an exposure. Digital cameras
automatically select the ISO but most have a setting to change it manually.
Auto ISO generally works best for bright scenes.
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.
Shooting at a lower ISO number requires more light than shooting at a higher
number. Lower numbers result in images with the least visible noise, which is
desirable. The higher the number, the more noise. The amount and degree of noise
varies from camera to camera. Digital single reflex cameras (dSLR), because
they have larger sensors, are best for producing noise-free images.
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 ¡ª beads of chemicals on the emulsion ¡ª
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.
(circleofphotographers.com)
How importance ISO ?
As the brightness (or amount of available light) in a scene decreases, the
camera attempts to compensate by having the shutter remain open longer. For
example, rather than the shutter being open 1/250th of a second in good light,
it may need to stay open 1/60th of a second in dimmer light.
Why is this important? The longer the shutter remains open, the greater the
chance for camera shake, that will result in a blurred photo. So, by increasing
the ISO setting, we change the camera's sensitivity to light. If it's more sensitive,
the shutter speed can become faster, while still allowing the proper exposure
of the shot.
ISO and Noise
The same trade-offs between ISO speed and camera settings and image quality
apply. For example, for a given aperture, the ISO rating is an indication of
the exposure time and the higher the rating the shorter this will be. Shorter
times are good for freezing action. Similarly, for a given exposure time, a
higher rating will mean a smaller aperture and more of the picture will be in
focus because of an increased depth of field ( the range of distances from the
camera where the scene is in focus ). There is a catch though. If you are a
serious creative photographer you may want to control the depth of field so
that you can defocus part of the scene. Because the digital camera sensors are
normally smaller than a film frame, the depth of field of a digital camera is
normally "better".
The other major trade-off is between speed and image structure. Here, we need
to consider another factor, namely the amount of "noise" generated
in the image. In a CCD sensor, photo-electrons produced by exposure to the image
are collected and the charge they generate is measured to produce the image
information. The image-wise variation in the collected charge is the image "signal".
The noise is the charge which accumulates randomly whether there is exposure
or not. When the image exposure is low, the signal is small and needs to be
amplified for image processing, storage and display. The noise is also amplified
in this process and the important factor is the signal/noise ratio not the absolute
signal level.
How low the exposure of the sensor can be before a good image is captured depends
then on the noise level as well as the efficiency with which a sensor converts
light into charge. This is the main criterion which determines the ISO rating
of a camera. Some cameras offer several ISO speed settings. This allows additional
control over the exposure time and/or aperture settings but the higher the ISO
setting the lower the exposure will be and the worse the signal/noise ratio.
Image quality will of course depend a lot on the number of pixels in the sensor.
Let's say we had two image sensors with exactly the same technology giving identical
noise levels and sensitivities but one was four times the area. The larger one
would use a longer focal length lens than the smaller one to fill the sensor
with the same image at the same aperture. The smaller sensor would see the image
at four times the light intensity, thus needing a quarter of the exposure time.
This would mean that the camera would be rated with four times the ISO speed.
However, it would only have a quarter of the number of pixels so high ISO speed
is not necessarily good.
In summary, if you want to take a object at low light condition and you don't
have any external lamp or flash, it mean you need to use high ISO, but using
high ISO will make you photo become more noise then using a small ISO. It really
depen on what kind of technology from digital camera to reduce the noise.
Example illustration :
- (Low light condition), set ISO 100, aparture f8, shutter speed
30. Result dark picture (little noise)
than change the ISO to see the diference
- (Low light condition), set ISO 400, aparture f8, shutter speed
30. Result bright picture (more noise)
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