People often shoot at higher ISOs without realizing it. If you set your camera to Auto, for example, it will probably increase the ISO without telling you; that's a good reason to shoot in Program mode instead.
If you have a smartphone that you use for photography, you might have some control over ISO. Windows Phone and Android users, for example, can manually set ISO. The iPhone, however, has no ISO control--take a photo in low light, and the iPhone automatically increases ISO to try to compensate for those conditions.
Keep it cool
Camera sensors tend to generate more noise when they're hot. Thus, if you leave your camera on the dashboard of your car in blazing sunlight for a few hours and then try to take some photos, the sensor will be hot enough to generate significantly more noise, even at a low ISO. No matter what kind of camera you have--from an iPhone to a DSLR--it will take better photos if you keep it cool.
Likewise, you can raise a sensor's temperature by pushing its performance limits. The more you use the sensor, the hotter it gets. Specifically, your sensor can get quite toasty from taking very long exposures--a 30-second nighttime exposure is likely to be very noisy, for example--or from capturing images in burst mode, in which you shoot a dozen photos in just a few seconds. Probably the most noise-inducing thing you can do is to shoot video: If you record 5 minutes of video and then immediately start snapping still photos, those images will almost certainly suffer from a lot of noise.
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