ISO refers to how sensitive the camera sensor is to light. In the old film days, the combination and proportion of chemicals used could be adjusted to make the film more, or less sensitive to light (ranging from ISO 100 – 800). Many digital camera today reach beyond ISO 3200 to capture enough light for a photo even in very low light situations.
The typical ISO settings (200 – 400 – 800 – 1600 – 3200) double in number from ISO 200 (or as low as 100) going higher. This also reflects an approximate doubling of camera sensor sensitivity to light between each full setting.
ISO AND AVAILABLE LIGHT
When taking photos in a camera shooting mode other than Full Automatic, MOST cameras allow the user to select the ISO (though there exist great differences in this feature between manufacturers).
When users are allowed to adjust the ISO it is important to get in the range of selecting a correct ISO setting to optimize the quality of the image. Simply put, ISO is set to a lower sensitivity (low ISO number) when there is abundant (bright) available light, while low-light situations require higher sensitivity (high ISO). The chart below might be useful when trying to determine which ISO setting to use.
THE CAMERA CONNECTION
There is no “ISO-Priority” shooting mode on your camera such as exists with Aperture, or Shutter Speed priority. Again, most cameras allow you to set the ISO to your preference, but keep in mind that some shooting modes may auto-select the ISO. The method for setting ISO ranges greatly between manufacturers and technologies. Its important to read your manual to learn how to adjust ISO.USES AND TRADE-OFFS
ISO adjustments are beneficial for two reasons; the obvious one being able to capture an image in lower light situations. But even when enough light is available, raising the ISO can allow for faster shutter speeds to be used for action or fats-motion photography. The two photos below were taken on bright days where ISO 100 could easily have been used. But by setting the ISO to 800, extremely fast shutter speeds could be used.
The trade-off factor impacting photos from changing the ISO from a low setting (400 or below) to a high setting (800 or above) is the introduction of “noise” to the image – meaning the image does not have a “smooth” flow across it as you can see the individual pixels. The image below captures the same view in 4 different ISO settings to demonstrate the transition from a smooth image to a noisy one.
Each camera has its own threshold for when noise becomes a factor, but most current cameras tend to do well at ISO 800 or below. This tends to not be a factor unless enlargements greater than 5×7 are made. ISO noise can be remedied by image editing software, but, this requires additional investment in time and cost adjusting photos.
The following video takes a close look at digital image noise as a result of ISO adjustments.
[youtube https://www.youtube.com/watch?v=jj7rhzMR6gw&w=420&h=315]