In the photograph below, I selected a aperture of F/16 for lots of sharpness front to back down the row of trees. This left my shutter speed at 1/15 second, too slow to stop the motion of the falling leaves in the slight breeze. So I raised the ISO from 100 to 400 which allowed me to speed up the shutter to 1/60 second.
|F/16, 1/60 second, ISO 400|
While Sheppard's focus is on nature photography, his suggestions will work well for portraits, still life, nearly any type of photography. If you understand the basic relationship among aperture, shutter speed and ISO, then you will find this article helpful. In fact, the approach he describes is one I've been following myself ever since I learned manual exposure photography with film in the 1990s! It works!
Simplified Exposure Choices by Rob Sheppard