Sara

In photography, there is such a thing as Depth of Field, or DOF for short. It is controlled by your aperture or f/stop. If you have a large opening or low numbered f/stop (kind of confusing, I know), you'll have a more narrow DOF. A smaller opening in the iris of the lens (aperture) will yield a larger DOF. So, after having explained that, what does this mean to me?
It means that if you're not careful, to enable the amount of light to reach the sensor in your camera, you may open your iris (aperture) so much that your DOF is too small, making your subject "soft" or not sharp in focus, as I've done in the image above. In portraits, you want the eyes, as well as all of the face to be in focus. The aperture I've used made the DOF so small not all of the subject's face is in focus, but the image is properly exposed. What I should have done was to use either a slower shutter speed, or a higher ISO so that I could have used a smaller iris opening on my lens, creating a larger DOF, while maintaining a proper exposure.