It used to be that you had to change your film roll to adjust your camera's sensitivity to light. Now it's just a setting that you can adjust from the menu called ISO. It's a numeric scale, so if you set your camera to ISO 400 it is twice as sensitive to light as when it's set to 200. Depending on the type of photograph you're trying to take, the ISO you'll want to use can vary greatly.Even though the sensitivity exhibited at a certain ISO is standardized—ISO 100 is the same no matter what camera you are using—the performance is not. Generally speaking, image sensors with lower pixel density are capable of producing photos with less image noise at a wider range of ISO settings than those that are crammed with pixels.
The first thing you have to understand is that ISO is only one third of the equation when it comes to taking a properly exposed photograph. The other two variables—aperture and shutter speed—work in conjunction with your camera's light sensitivity in order to capture a photo that is the proper brightness. Let's just say, to keep things simple, that the aperture—how much light the lens captures—is fixed. Boosting the ISO lets you shorten the amount time it takes the shutter to open and close, making it possible to freeze motion. Conversely, there are some instances when you may want to lower the ISO and use a longer shutter speed—landscape photographers sometimes use this technique to smooth the appearance of a river or waterfall.
Pixel density is pretty easy to figure out. A typical point-and-shoot camera has an image sensor that is 4.6 by 6.2 millimeters in size—that's really, really small. A typical APS-C D-SLR has a sensor that is 18 by 24 millimeters. If both cameras have the same resolution, like the 16-megapixel Sony Alpha NEX-C3 and the inexpensive Canon PowerShot A2400 IS, the pixel density of the camera with the physically larger image sensor is much lower, giving it an innate edge at higher ISOs.
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