Breaking down ISO
When photographers talk about ISO, what are they talking about exactly? The ISO setting on your camera controls the sensor’s sensitivity to light. ISO is one of the three parts of the ‘exposure triangle’ (Aperture and Shutter Speed being the other two) and is has some major effect on your images. In the most basic terms ISO is a setting that will either brighten or darken your photo. But more specially and even more technically ISO is as follows. ISO in digital cameras (specifically ISO standard 12232:2019) is designed to resemble the ASA film speed standard that was adopted by the International Organization for Standardization in 1974.
So what does that mean for you? It means, that you can take an image in a darker setting. As along as you are more flexible and use your aperture and shutter speed settings on your camera. In saying that though the higher you go on the ISO scale the more noise the image will have. This random digital noise, creates a ‘speckled’ effect. The camera can process this out to a degree, and using software later, but this can produce a ‘smoothed over’ look that’s just as bad. Ideally you would try to adjust your other settings and keep your ISO lower to help with this noise.
Aperture: controls the area over which light can enter your camera.
Shutter speed: controls the duration of the exposure.
ISO speed: controls the sensitivity of your camera’s sensor to a given amount of light.
So to explain this a little further, an image taken at ISO 200 is twice as bright as ISO 100. That means an image that you take at ISO 400 is twice as bright as ISO 200. Which was twice as bright as ISO 100. In todays Newer DSLR cameras the base ISO is typically ISO 100. This is designed to give the user the best quality image. Adjusting the other settings and keeping the ISO las close to this will give you the best image possible.