ISO Sensitivity
What is ISO?
ISO is a measurement of the camera's ability to capture light. The ISO number is directly proportional to the sensitivity of the camera towards light. Which mean, if we only adjust the ISO and fix the aperture and shutter speed, we can get bright or dark photos. Higher ISO number will give brighter image while lower ISO number otherwise. Raising the ISO helps to capture image in low light environment without the help of flash. However, high ISO sensitivity will add "noise" to your photograph making them seem grainy. Following examples show how ISO affects the exposure of the photo (Fix shutter speed and aperture).
Canon 600D @ 18mm, f/9, 1/25 sec, ISO-100
Canon 600D @ 18mm, f/9, 1/25 sec, ISO-400
Canon 600D @ 18mm, f/9, 1/25 sec, ISO-1600
Below are the photos that show the relationship between ISO and "noise":
Canon 600D @ 18mm, f/9, 1/20 sec, ISO-100
Canon 600D @ 18mm, f/9, 1/40 sec, ISO-200
Canon 600D @ 18mm, f/9, 1/80 sec, ISO-400
Canon 600D @ 18mm, f/9, 1/160 sec, ISO-800
Canon 600D @ 18mm, f/9, 1/320 sec, ISO-1600
Canon 600D @ 18mm, f/9, 1/640 sec, ISO-3200
Canon 600D @ 18mm, f/9, 1/1250 sec, ISO-6400
Canon 600D @ 18mm, f/9, 1/2500 sec, ISO-12800
All the photos were taken with fix aperture of f/9 and increasing ISO. To get the proper exposure, the camera automatically adjust the shutter speed. Take a closer look at the door in all the photos, the "noise" is visible in the photos of ISO-3200 to ISO-12800. All cameras have different sensitivity to light, so it is advisable to understand your camera's features and the limits of the sensor to avoid adding unnecessary grains to your photo.
Many cameras are completely useless in their high ISO range, for example my canon eos m10 is not very usable in low light unless you use a tripod, because of the high noise over ISO 1600 and the need to keep it completely steady having to decrease the shutter speed.
Ya, totally agreed, except for those high-end cameras. For me, the max ISO used is 800. Then in post-processing have to de-noise.
Yes, my other camera, a canon 70D is much better with higher ISOs but it also costs and weights much more.
Yea, the important thing is you can take great pictures with it