In photography, the three most critical factors you'll constantly be tweaking on your camera are Aperture, Shutter Speed, and ISO. This article focuses on ISO. The term comes from the International Organization for Standardization, which established the scale for light sensitivity. Basically, the lower the ISO value, the less sensitive your camera is to ambient light. Conversely, the higher the ISO value, the more sensitive it becomes. See the chart below:
Lower ISO settings (like ISO 100) are perfect for shooting at high noon on a sunny day. Mid-range settings work well for overcast days or sunsets (like ISO 800). For night photography, you'll need a higher ISO (like ISO 6400) because the light is dim, and increasing the camera's sensitivity helps make the background details pop.
However, using an ISO that's too high during the day will result in overexposed photos, like this:
As you can see, when the lens is overly sensitive to light, it produces an effect that looks like someone just maxed out the exposure slider. For a scene like the one in the picture above, sticking to ISO 100 would usually yield the best result.
Just remember this simple mantra to master ISO:
Bright light, keep it low; Dim light, raise the ISO.
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