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exposure triangle
The exposure triangle uses aperture, shutter speed and ISO to use a larger amount of techniques, the first of which ,ISO, is a spectrum of light sensitivity that will sacrifice the picture quality.The shutter speed is used to differentiate between a moving object and a still one, a moving object would use a high speed shutter would be used to ensure little light gets through and a slow shutter speed would be used to let in a lot more light. the last of which aperture is about the amount of light that gets in, a large aperture (f/2.0, f/2.8 etc.) lets in more light to the camera shutter for an exposure, while small aperture (f/11, f/16, f/22 etc.) has a smaller opening in the lens diaphragm to let in less light for a given exposure.
The picture on the left is a bit under exposed along with the one in the middle due to the aperture.the picture on the left is perfectly exposed however the shadow under her eye is due to the aperture.