ISO is a standard measurement of the light sensitivity of the camera’s sensor. If you are on a fully automatic mode your camera will adjust the ISO for a properly exposed picture. However like everything on your camera, automatic isn’t always the best mode. For the best quality picture with the least amount of noise and the greatest amount of sharpness, you will want to manually control your ISO.
You should try to use the lowest ISO possible, which will allow you to expose the picture in the given lighting condition accurately. Therefore, you would use a low ISO like 100 or 200 in bright conditions. You can use a higher ISO like 400 or 800, for indoors if you need too and for mixed lighting like on a cloudy day you can use something like 160 or 200. Most of the time, you should shoot at ISO 100 to 400.
For the best quality, try using a tripod in low light conditions which will allow you to lower your ISO for better quality. This works well unless your subject is moving and then you should try to increase the ISO so you can use a shutter speed that will allow you to freeze the motion.
Each camera may have different ISO setting capabilities, use the numbers from this article that are as close to your cameras as you can. Also note that newer cameras have better noise handling and the amount of noise that is perceptible at a given ISO may differ from camera/senor model to camera/sensor model.
ISO (sensor light sensitivity) is one of three factors that impact your image exposure. The other two being shutter speed (how long the shutter is open letting light onto the senor) and aperture (how wide the area is opened to let light into the sensor). Your camera’s meter adjusts to the different ISO settings. As your ISO goes up, you can drop your shutter speed or decrease the size of your aperture (increase the f-stop), but remember the increased ISO degrades your image quality so keep it as low as you can. This decrease in image quality is especially seen when printing enlarged images.
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