I photographed this scene in northern China near the Mongolian border. For action shots like this, I typically use manual exposure mode so I can specifically choose the shutter and the aperture. In this case, I wanted 1/1250th of a second because from experience, I knew that was fast enough to freeze the horses as well as every water drop. My f/9 lens aperture provided enough depth of field, given I was shooting with a 100-400mm telephoto zoom, to encompass the herd of horses. The ISO, then, was set to auto. This meant that it would vary depending on the light. In this situation, the ISO turned out to be 3200. Some photographers set an upper limit to the ISO in an effort to minimize noise, but when using manual mode, this is a mistake. It can result in underexposure. In other words, if the light is diminished enough and the ISO needs to go higher to produce a correct exposure, but it can't because a limit has been sent, then the pictures will be too dark.
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