Aperture and Depth of Field

The f-number is focal length divided by entrance-pupil diameter.

An f-number is a ratio. A 50 mm lens at f/2 has an effective opening about 25 mm across; at f/4 it is about 12.5 mm. Because area changes with the square of diameter, full-stop sequences look unusual: f/1.4, 2, 2.8, 4, 5.6, 8, 11, 16.

Moving closer reduces depth of field and changes perspective.

At the same aperture, focusing closer produces a shallower zone of acceptable sharpness. Moving the subject farther from the background increases visible separation without requiring a faster lens. If you want more blur, first change the geometry: get closer to the subject and move the background away.

Equivalent framing complicates simple focal-length rules.

A longer focal length used from the same position magnifies blur and narrows the field of view. But if you step back to preserve the same framing, perspective and depth relationships change again. Compare photographs at the same framing and output size rather than reading an isolated aperture number.

Depth of field and diffraction pull in opposite directions.

Stopping down increases depth of field, but very small apertures spread point detail through diffraction. The practical optimum depends on sensor size, pixel density, output, and how much near-to-far sharpness the scene needs. Make a controlled series at f/5.6, f/8, f/11, and f/16 before treating any value as universal.

Start with the subject, not a favorite f-stop.

• Choose the focus distance and camera position that make the frame work.
• Set enough depth of field for the important planes.
• Check whether the resulting shutter speed can hold the motion.
• Use ISO to protect that shutter/aperture choice when light changes.