Aperture is the adjustable opening inside a lens. A smaller f-number means a wider opening, more light and usually shallower depth of field. A larger f-number means a narrower opening, less light and usually greater depth. The f-number is a ratio, which is why f/2 is wider than f/8.
Depth of field is not controlled by aperture alone. It also depends on focal length, camera-to-subject distance, subject-to-background distance, sensor format and the final display size. Move closer and depth becomes shallower. Move the background farther behind the subject and blur becomes stronger. This is why changing your position can improve separation more effectively than buying a faster lens.
For portraits, focus on the nearest visible eye unless the pose gives you a better storytelling reason. At very shallow depth, use a focus point or eye-detection system you understand and shoot short controlled sequences rather than dozens of identical frames. For groups, avoid placing people on widely separated planes; arrange faces at similar distances and use a moderate aperture.
For landscapes, “focus one third into the scene” is unreliable. Identify the nearest detail that must look acceptably sharp and the farthest important detail. At f/8, focus at a distance that balances both, inspect at high magnification, and adjust. If the range is too large, consider focus stacking rather than closing to an aperture where diffraction softens the entire frame.
Aperture also changes lens rendering. Very wide settings may show lower corner sharpness, vignetting or longitudinal color fringing. Very narrow settings introduce diffraction. Test your own lens on real subjects instead of assuming a universal sweet spot.
Practice exercise: place a person or object two metres from the camera with a textured background several metres behind. Photograph at each full aperture without moving. Then keep the aperture fixed and change camera distance. Compare background structure, subject sharpness and how precisely focus had to be placed.