ISO, shutter speed, and aperture determines the exposure of the photograph you are taking. In one of my previous posts, I explained the concepts of Aperture and shutter speed in digital photography. These three concepts in photography revolve around one aspect of photography -- light. You are going to learn everything about ISO in this post Read: \tWhat is aperture? \tWhat is shutter speed? What is ISO? ISO is usually the less understood term in photography. If you ask a beginner about the ISO, he or she probably would not have an idea what ISO is, let alone how it works. Most people have an idea about the shutterspeed and aperture, but they barely know what ISO is. ISO denotes the sensitivity of the image sensor to light -- a lower ISO means the sensor is less sensitive to light, and the sensor become more sensitive to light when the ISO is high. So, if you want to let in more light to the camera, choose a higher ISO and vice versa. So, the rule of thumb is: if \u00a0you need more light, use high ISO. But, high ISO is not only used in low light situations, but also used for choosing a high shutter speed. As I have mentioned before, all these three factors are inter-related. That is, if you change the one of these, you need to change the others parameters to get the same exposure. That is why they are often called as an exposure triangle. What will happen when you change the ISO? The amount of light you get changes, and the image gets smoother or noisy. See the picture below The portion of the image in the offset is magnified to 300% to show the noise How to use the ISO? The most common ISO settings vary from 100 to 800. However, the lower and upper numbers vary depending on various camera models. When you change ISO from 100 to 200, you are capturing double the amount of light. So, low ISO (50-200) numbers are used when the available light is very bright, and when you want to avoid noise on your images. So, is it really necessary to change the ISO \u00a0depending on the light conditions, say, increase it when there is low light or decrease it when there is less light? Not necessarily. Since,\u00a0ISO; aperture; and shutterspeed work together to control the amount of light recorded by the camera (the exposure triangle which I will explain later), the purpose of changing one of these depends on the output you want to achieve. More on this later. Do it yourself You can check out how ISO works if you have a camera with manual mode. This is what you should do: set the camera in manual mode and take a picture. It doesn't matter if the picture is dark or bright. If it is dark, increase the ISO and then take a picture. The picture will now be a little brighter than the previous one. And if your first picture was bright, then choose a low ISO value and take a picture. The picture, you get now, will be darker than the previous one. If you don't use manual mode, you won't be able to see the effect of ISO since the camera will adjust either aperture or shutter speed (or both) to get the desired exposure. There is another way of getting an hands-on experience the ISO function in your camera. Use the live view mode of your camera and change the ISO settings. You can see the preview gets brighter as you increase the ISO and darker when you decrease it. I hope you got an idea about what ISO is all about. In my future posts, I will explain how ISO affects the noise in an image, and how it can be used to get the desired output in your photographs. Please feel free to ask any questions. I might have missed some points, so your questions can help me improve this article.