It's time to take that step into putting your camera into full manual! This is the 3rd and final step to putting your camera into manual mode and taking complete creative control of your images. If you haven't read my first 2 steps, check them out first!
Generally, you want to keep your ISO fixed at the lowest possible setting. Most entry level cameras have a minimum ISO setting of 100. When you raise your ISO, you gradually begin to introduce grain or "noise" in your photo, which is why its best to keep it as low as possible. Most of these entry level cameras can perform well at ISO values up to 800 or 1000 before the noise is really noticeable.
So when do you need to change it? Any low light situation is when you can think about changing it. Lets go over a couple scenarios.
Lets say you go out and want to shoot a landscape at sunrise and you forgot your tripod. These are low light conditions that can be challenging if you don't know how to adjust ISO. So since you now have to hand-hold your camera, your shutter speed needs to be no slower than 1/60th of a second. So since you are in full manual, (The "M" on your dial)you also adjust your aperture to f/8. You want your total picture to be in focus, since it is a landscape, so f/8 is the lowest you want to be. (F/8 to F/11 gives you the sharpest image on most lenses). If you try and take that shot, you will see that its still very dark. So now is where ISO comes in. If you raise your ISO up you will make the photo brighter! That is essentially what you are doing with any of the 3 settings; Aperture, Shutter Speed, or ISO, also known as the Exposure Triangle.
Another example is one of my favorites; Astrophotography! I love to shoot the Milky Way! Now, in order for me to do this, my aperture needs to be at its widest setting, and my shutter speed needs to be anywhere from 20-30 seconds. So I have to raise my ISO up to 2500 0r 3200 to be able to see the Milky Way properly.
So lets put it all together. In general, lets leave the ISO at its lowest setting. Now, we think about what we want to do with our shot. Are we shooting landscapes or a portrait? For a landscape, we set out aperture between f/8 and f/11. Next we look at our camera meter, usually shown something similar to this. (Reference your camera manual/guide to see how to find this on your model).
This is your camera's meter readout. If it is on 0, then your camera meter is reading that it is properly exposed. You can see this on the back of the camera's LCD, or if you look through the viewfinder you will see it also. Make sure you are pointing the camera at whatever it is you're shooting when you are looking at this. If it isn't reading o, and is at 1, -1, 2, or -2, simply adjust either the shutter speed or the aperture( depending on what effect you're wanting). In the case of the landscape image, you'll want to adjust your shutter speed. Now you're ready to take the picture! If you like it, then sweet! If you feel that its too bright or too dark, you can simply adjust your shutter speed to get the look that you want. You may find that o looks a little too bright and -1 is perfect.
Those numbers are called stops. That is how light is measured in a camera. 3 clicks of your dial(aperture or shutter) is one full stop. That means you're letting twice as much light in. So to go from a shutter speed of 1/4 of a second to 1/2 of a second, you're increasing your shutter speed by one full stop. Clicking the dial 3 times, and you will go from 1/4, to 1/3, to 1/2.5 to 1/2. It works the exact same way for aperture. 3 clicks of the dial make it twice as bright or twice as dark, depending on which way you're turning the dial.
Now we know how to adjust the brightness and darkness of our image by changing our shutter speed or aperture! Now stop reading and get outside and try it! Take a few shots outside. Now go inside and take some more shots, adjusting your settings as needed. Now you can take control of the camera and your photography and really unleash that creativity!