I should first start off with wishing everyone a happy holiday.
In the last few posts, I've tried to explain what each of the three main parts of an exposure do individually and now I'll try to put them all together.
It all comes down to controlling the amount of light entering into the camera. Say for instance it's a really bright sunny day, and you want to use a wide aperture to reduce your depth of focus. Your options would be to reduce your ISO setting number (I always shoot with the lowest number I can get away with), Or increase your shutter speed.
In the above example, in the same light and same lens (Nikkor 85mm f1.4 D IF), I shot the image on the left with the aperture wide open to reduce the depth of focus, and got away with 1/50th of a second shutter speed. The image on the right was made with the aperture closed to the ideal minimum on my camera (I start to lose image quality smaller than f16), and the shutter speed reduced to 2.5 seconds- thank goodness for tripods, I never would have been able to hold the camera steady for that long. Due to the much smaller aperture, the image on the right is far more in focus from front to back, but due to the much longer shutter speed, it would have been the epitome of blurry because of camera shake if I had tried to hand hold it.
In this example, I was aiming for a clear image that was focused evenly from front to back, so I stayed with f16 on the same lens as before. At first glance, they're both reasonably decent shots. The first one, again, would have been impossible to hand hold at 2.5 seconds. I turned the ISO all the way up to 3200 and got a more respectable 1/10 of a second for the exposure, which still would have been tricky to hold, but not as much as before. The drawback to that is that with the ISO that high, I've introduced a good deal of noise to the image. If I were to hand hold it, I likely would have compromised a bit on the aperture (widen it a bit) to let more light in so that I could use a faster shutter speed.