Light is what photography is about. The capture of light on a sensitive surface, preserving that moment for effectively for ever. So how do you use light effectively and ensure a properly exposed image.
Speaking only for digital cameras it is vital that the photon cells are allowed together as much information as possible. Why? Well this ensures that the signal to noise ratio is at its optimum. However one must not drive the cells to the point that they are full as the result there is just white and nothing else, no discernible usable information.
Now I have done two shots at very high ISO on a Canon 5D M2, and you can see what I am talking about.
1) Under exposed.
HIstogram as is from camera. The histogram is more to the left indicating underexposure.
Now we adjust the exposure so that the histogram is central
And this is the result a noisy picture
Now we over expose the same scene.
Histogram of image as is from camera, note that it is close to the right edge without touching it. This ensures that we have not lost any information in our image.
Adjust so that the histogram is in the middle
this is the result, it is clear that the image has far less noise. You will also notice that there is more detail in the fabric.
Now you can experiment so that one gets to understand the importance of ensuring adequate signal to noise ratio do the following:
Take your camera into a darker environment, prevent flash from firing, set to take a RAW image if possible on your camera.
Increase the ISO up to say 2000 or greater.
If you know how to use the histogram then use that as a guide in the following steps.
Adjust other settings, easiest to do if you set the camera to manual mode, until the picture is under exposed and then take another image with the settings adjusted to overexpose. Use the histogram to ensure that you have not driven the exposure to white in the brighter areas.
Then compare the two images, the underexposed image will be noisy in the darker areas and if one tries to adjust the exposure (increase) post camera the noise will get worse.
Look at the other image and note the amount of noise and then adjust the exposure downwards until correct and assess the amount of noise. The second image (over exposed slightly) will always have less noise.
Now remember that for every exposure going forward, use the histogram to ensure that the graph is pushed to the right, without overexposing. Your images will always benefit from this technique, especially in those situations where light intensity is limited.