The ISO setting on the camera stands for International Standards Organization. Back in the days of film, it was known as ASA. What it really indicates is how fast the camera’s sensor can record light. The lower the ISO number is, the less light it allows in to get a regular exposure.
For instance if the ISO is set at 200 it will have to let in twice as much light as when it’s set at 100. The higher the ISO number is, the more light it lets in. But to let in more light, the shutter speed has to be increased or the aperture has to be opened up.
The problem with higher ISO speeds is that the photos create more noise, which basically means the images appear to have more grain in them. This means they aren’t as sharp and clear. Actually high ISO also means a reduction in sharpness and contrast ratio as well.
However, different cameras will produce different amounts of noise. The best thing to do is to test your model by cranking the ISO up as high as it goes and seeing what the noise level is like. Some cameras may start to show high levels at ISO 400 while others might not be noticeable until about 1600.
Take a photo of the same subject at different ISO levels to test it. You probably won’t notice much of a difference in daylight. It’s most noticeable indoors in low-light conditions.
If the shutter speed is fast at night there won’t be much light let into the camera, so you can try opening the aperture as far as possible and/or the ISO. It’s always best to test it if you have the time to make sure your photos aren’t too grainy.