Ahmadzargon. Learning photography means learning exposure as the basic rule. In digital age, Everything is measured and regulated in electrical system. ISO or International Standard Association is the general units of sensor sensitivity of light. In Analog camera before ISO units found, Photography film Sensitivity of light is measured in ASA or American Standard Association. There is no significant difference between ISO and ASA except the media, Digital for ISO and Analog for ASA.
ISO was created on 1987 by the combination of two units. Those are ASA and DIN which is made by German Standardization Institute. Today, The combination of those unit known well as ISO and valid in the world. I means human world.
ISO Speed.
High speed sensor is signed by High Value such as 1600, 3200 and more. High value performs high sensitivity of light but in low types, Choosing DSLR high ISO produces more grain the lower ISO. High ISO produce not only high grain and noisy bat also high contrast.
Low value of ISO value such as 100, 64 and less performs low speed and low sensitivity but it reduces any grain and noisy on your photo. ISO unit System also use on pocket camera and hand-pone camera. This feature sometimes is signed with Nigh mode for High ISO and Normal mode for Low ISO.
Calculating the ISO
In digital era, everything is calculated by camera processor. But sometimes we need to calculate it manually. Long exposure photography sometimes need very long exposure time, commonly, beginner photographer and genre photographer use auto exposure time with big aperture value and high speed time exposure, Long time exposure sometimes need 120 minutes or 60 minutes. the question remain is: how to calculate ISO respect to STOP unit in photography.
First step. Set your camera on AV mode, so shutter time will be calculated by camera and the camera will set the exposure time. If camera shows ISO 100 and time 1/60 second it means you only need 1/120 second exposure time on ISO 200. ISO 100 is lower 1 stop than ISO 200. This value has a series calculating in 1 stop calculation or simply means twice or a half. The series is unique, ISO 640 does not means double form 320. To calculate the ISO we just follow the ISO series.
ISO Series
1 Stop Series
100 200 400 800 1600 3200 6400 12800 25600
1/3 Stop Series
100 125 160 200 250 320 400 450 640 800 1000 1250 1600
In digital camera specially DSLR Camera there a grid which able to rotate. It rotation is in three clicks for one stop. So if you rotate it one click it means a changing in 1/3 exposure stop.
Illustration of ISO Effect on photo.
Black and White edition
Color Edition