ISO speed controls how sensitive the camera's sensor is to the light that reaches it. A higher ISO setting makes your camera sensor more sensitive to light, allowing you to capture shots in a dark place. There are other ways that ISO can affect your shots as well. (Reported by: Ryosuke Takahashi)
What is ISO speed?
ISO speed is a widely used camera terminology. ISO is the abbreviation of "International Organisation for Standardisation," a body that determines international standards. In digital photography, ISO speed is used to indicate the sensitivity of the CMOS sensor toward light. This is similar to the concept of ISO speeds such as ISO 100 and 400 on film cameras. Raising the ISO value increases the level of sensitivity toward light. For example, by raising the ISO speed in a low-light condition that generally requires the use of a flash, it is possible to capture the surrounding atmosphere without having to use a flash. We have learned in the previous lesson that we can control the depth of field (amount of bokeh) with aperture, and the movement of subjects with shutter speed. With the amount of exposure (amount of light) obtained from the combination of the aperture value and shutter speed, coupled with the ISO speed, you can enjoy more diverse photographic expression.
ISO Speed & Exposure
Example of a high ISO speed shot
Manual exposure (1/800 sec., f/4)/ ISO 6400
In this example, the movement of the performer is "frozen" with the use of a fast shutter speed. I did not fire the flash to prevent it from affecting the ambience of the resulting photo. This is a typical low-light scene that can be captured using a high ISO speed.
Example of a low ISO speed shot
Aperture-priority AE (25 sec., f/25)/ ISO 100
In this example, I chose a low ISO speed, and stopped down the aperture to achieve a slow shutter speed. Doing so blurs the light from the headlight and taillight of the cars, forming lines.
Shutter speed increases with ISO speed
Raising the ISO speed allows you to produce clear photos even in a dimly-lit conditions. Not only so, choosing a high ISO speed in bright conditions enable the image sensor to capture a large amount of light in a short period of time, thus allowing you to utilise a faster shutter speed than that in a low ISO speed setting. In sports photography, it is common to use a shutter speed as fast as ISO 400 when shooting in the day.
ISO 200, 1/50 second
ISO 800, 1/200 second
In the shot with the faster shutter speed of 1/200 second, I was able to "freeze" the instant the subject jumped. An ISO-speed stop corresponds to a shutter-speed stop.
Multi Shot Noise Reduction reduces noise at high ISO speeds
Noise occurs when there is disorder in the current while converting light received by the CMOS sensor into electrical signals, or when amplifying the signals. Generally, noise tends to occur at a high ISO speed. For the EOS series, however, the cameras are equipped with a CMOS image sensor that is capable of reducing noise efficiently, while the high-performance image processor (DIGIC) works to eliminate the noise. For example, for models that come with DIGIC 5, no noticeable noise is observed up to about ISO 6400, and even shots taken at ISO 25600 can be used for printing 3R size photo prints. Meanwhile, cameras released in 2012 or later are built in with a "Multi Shot Noise Reduction feature, which takes four consecutive shots with every press of the shutter, and merges them automatically to eliminate noise.
Photos using Multi Shot Noise Reduction
Even when the shot is taken at a high ISO speed of 12800, noise is reduced significantly with the Multi Shot Noise Reduction feature.
What is noise?
Noise refers to the spots that appear on an image captured at a high ISO speed. There is a need to amplify the electrical signals in order to raise the ISO speed, and noise signals are generated during the process. Noise is an inherent characteristic of digital cameras, and the acceptable degree varies across individuals.
Effect of Noise on ISO Speed
Born in Aichi in 1960, Takahashi started his freelance career in 1987 after working with an advertising photo studio and a publishing house. Photographing for major magazines, he has travelled to many parts of the world from his bases in Japan and China. Takahashi is a member of the Japan Professional Photographers Society (JPS).