What Is Focus?

What Is Focus?

Focus in photography is about making a specific part of the image sharp and clear. When a lens is focused at a certain distance, light rays from that distance converge precisely on the sensor. Objects at that distance appear sharp. Objects closer or farther away are out of focus and appear blurred. The transition from sharp to blurry is gradual, which is why some parts of an image can look acceptably sharp even if they are not at the exact focus distance.

Critical focus is essential for most photography. If the subject's eyes are slightly soft in a portrait, the image looks unprofessional. If a landscape image is slightly out of focus, it lacks the sharp detail that makes landscapes compelling. The human eye is very good at detecting what is in focus and what is not, even in a quick glance. Getting focus right is often the difference between a keeper and a reject.

The plane of focus is an important concept. A lens does not focus on a single point but on a plane that is parallel to the sensor. Everything on that plane is in focus. This means if you focus on a person's eyes and then recompose so the face is at an angle, the plane of focus shifts and the eyes may no longer be sharp. This is why focus-and-recompose can introduce focus errors, especially at wide apertures with shallow depth of field.

Modern cameras offer various tools to help achieve precise focus. Focus peaking highlights the edges that are in focus with colored overlays, which is especially useful for manual focus. Magnified view lets you zoom in on a specific area to check focus at the pixel level. Phase-detection autofocus points cover most of the frame in modern cameras, allowing you to place the focus point exactly where you want it without recomposing.

Different genres have different focus requirements. In landscape photography, you often want everything sharp from foreground to infinity, achieved through aperture selection and hyperfocal distance focusing. In macro photography, depth of field is measured in millimeters, making precise focus critical and focus stacking necessary. In street photography, you might use zone focusing where you preset the focus distance and aperture to keep a range acceptably sharp, allowing you to shoot without looking through the viewfinder.