ISO is a measure of sensitivity of the digital camera sensor to light.
Higher the ISO setting, the more sensitive the camera is to light. This means that with a high ISO setting one can shoot even in low light conditions
ISO is written as numbers that are multiples of 100; like 100, 200, 400, 800, 1600.
With each setting , the sensitivity to light either doubles or halves. 100 ISO is half as sensitive to light as 200 ISO. 1600 ISO is double as sensitive to light as 800 ISO
Increasing ISO also increases Image noise. So it is always better to shoot in lower ISO settings to get clean, noise-free images. Choose high ISO settings only when light is very low.
Tip - The best way to check if your shot has high iso noise is to zoom into the image and look at some dark areas. The dark areas show the effect of noise very clearly
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