Unit
- Aperture: The opening in the lens that controls the amount of light reaching the sensor. Measured in f-stops (e.g., f/2.8, f/5.6, f/16). A lower f-number means a wider aperture, allowing more light and creating a shallower depth of field.
- Depth of field: The zone of acceptable sharpness in front of and behind the focus point. Shallow depth of field isolates the subject with a blurred background, useful for portraits or dramatic scenes.
- Stop of light: Each full f-stop change doubles or halves the light. Moving from f/4 to f/5.6 reduces light by one stop. Understanding stops helps maintain consistent exposure when adjusting other settings.
- Lens focal length impact: Wider lenses (e.g., 24mm) have deeper depth of field at the same aperture, while telephoto lenses (e.g., 85mm) produce shallower depth of field. This is crucial for lens selection.
- Hyperfocal distance: The focus distance that maximizes depth of field from half that distance to infinity. Used in landscape cinematography to keep everything sharp from foreground to horizon.
- Diffraction: At very small apertures (e.g., f/22), light waves spread and reduce overall sharpness. Avoid diffraction by staying in the sweet spot of your lens, typically f/4 to f/11.
- T-stop vs f-stop: T-stops measure actual light transmission, accounting for lens glass loss. Cinema lenses use T-stops for precise exposure matching between shots, while still lenses use f-stops.
- Bokeh quality: The aesthetic quality of the blurred area. Determined by lens design, blade count, and shape. Lenses with nine or more rounded blades create smoother, more pleasing bokeh.
- Focus pulling: Rack focus between subjects requires understanding depth of field. A shallow depth of field makes focus pulls more dramatic but requires precise marks and practice.
- Exposure triangle integration: Aperture works in tandem with shutter speed and ISO to achieve correct exposure. Adjusting aperture by one stop requires compensating with shutter speed or ISO by one stop to maintain the same brightness. This interdependence is fundamental for controlling motion blur and noise while managing depth of field.
- Neutral density filters: When using wide apertures in bright conditions, ND filters reduce light entering the lens without affecting color or depth of field. This allows maintaining a shallow depth of field outdoors or achieving longer shutter speeds for motion blur effects. ND filters are essential for controlling exposure beyond the camera's native capabilities.
Design Principle
Principle: Selective Focus via Aperture Control
Definition: Using a wide aperture (low f-number) to create a shallow depth of field, directing viewer attention to a specific area within the frame. The blurred background reduces visual clutter and emphasizes the subject.
Application: In a dialogue scene between two characters, shoot close-ups at f/2.8 with the background out of focus. This keeps the audience focused on facial expressions and emotional reactions. For establishing shots, use f/11 to keep the entire location sharp.
Quick Revision
- Aperture controls light and depth of field; lower f-stop means more light and shallower depth.
- Depth of field is influenced by aperture, focal length, and distance to subject.
- Use wide apertures for selective focus and narrow apertures for deep focus.
- Diffraction reduces sharpness at very small apertures; avoid f/22 and beyond.
- T-stops provide consistent exposure measurements across cinema lenses.