So, now that you know the three factors that effect your exposure, lets see how to use them in practice.
The image above was taken at the Way of the Cross procession, in Newcastle West, on Good Friday last. It was a cloudy day and even though it seemed bright to the human eye, light levels were low. The default ISO rating for my camera is 200 but at that level I was getting shutter speeds too slow to hand hold and avoid camera shake.
I increased my ISO rating to 500. This means that at the same F number I can get shutter speeds that are 2.5 times what I would get at 200, 500 being 2.5 times 200. With experience, I have learned to hold the camera steady at relatively slow shutter speeds, for example 1/60th or 1/125th of a second. You may need to increase your ISO to 1000 or higher, depending on how steady a hand you have.
When choosing an ISO rating, remember that you should pick the lowest rating you can get away with, so as to reduce noise levels. While it can be fixed in Photoshop and most other editors, it is always better to get is as right in camera as possible.
Next I set the camera on Aperture Priority. This is usually denoted by A on your camera's selection dial. I've already selected the ISO rating and I will now select the aperture or F number.
The camera will then automatically select the shutter speed. This doesn't mean that I can forget completely forget about my shutter speed, it's just the the camera does the math for me. I need to be aware of the shutter speed selected because if it's slow, my images will suffer from camera shake or motion blur, neither of which can be fixed.
It's always good to take a test shot, to see what speed is selected and then look at the enlarged image on your screen, to check for camera shake. If you have some, increase your ISO again and take another test shot. Only go up one step at a time, don't jump from ISO 400 to 1600, for example, try use the lowest rating you can.
During the procession, I checked my shutter speed each time the light looked to have faded. This happened when they came into narrower streets or were in the shadow of a building, for example. I also checked it regularly over time, as light fades as the afternoon passes, anyway. I maintained my shutter speed by increasing my ISO. By the end of the procession, I was using ISO 1600.
My next decision was which aperture or F number to use. This varied from photo to photo but for this one I used F11. The woman in the image was part of a group of three and I wanted to be sure that if any part of the others came into the image, they would be out of focus. As you can see, there is a large crowd(number not weight) in the background and I wanted them out of focus also.
To pick her out from the crowd I used a 70 - 300mm zoom lens at 270mm.
If you look closely at the image, you will notice that she is sharp, except for her far shoulder. A shoulder and head from other people, on the left side of the image and the people in the background, on the right side, are all out of focus.
The result is that the lady herself, my subject, stands out, without competition from any other element of the image. Had I used a much smaller F number, say F4, only the point of focus and a little bit around it would have been sharp, most of the woman would be out of focus.
Had I used a much larger F number, say F22, most of the image would be in focus and she would have had to compete with the people in the background and the bits of people visible in the foreground, for attention. As a portrait, it would not have worked.
I focused on her eye. In most portraits, the person's eyes are the most important. If you have sharp eyes and other parts of the face are out of focus, the image still works. However, if the eyes are out of focus, it doesn't matter how sharp the rest of the image is, it wont work.