Aperture controls the bundle; depth of field sets a tolerance
Why can a smaller lens opening make more of the background look clear? The opening narrows the light bundles and reduces out-of-focus spots. Make it too small and diffraction spreads the spots again. We compare these two effects.
Try two small comparisons
- Keep focus fixed and compare the foreground, flower, and background with large and small openings.
- Move focus farther away. What becomes the clear subject?
Calculate the blur and the acceptable range
Look closer: observations and controls
The foreground leaves, main flower, vase and background sit at 1.3, 2, 3 and 6 meters. Focus on the main flower and its point bundles meet near the sensor. Light from other distances is intercepted before or after its own ideal image plane, leaving a patch instead of a point.
Closing the aperture narrows those bundles and reduces the defocused patches. More distances then look acceptably sharp. Only one object plane is exactly in focus. A depth-of-field boundary is a chosen tolerance, not a wall where sharpness suddenly changes.
The opening diagram represents the entrance pupil: the effective aperture seen from the lens front. It is not a mechanical model of a particular iris. The f-number divides focal length by pupil diameter, so f-two is a larger opening than f-eight.
- Focus at two meters and compare a wide and narrow aperture. Watch the flower center, nearby leaves, vase and distant lights separately.
- Keep aperture fixed and move focus toward six meters. Attention passes from the flower to the background.
- Turn brightness compensation off and change from f-two to f-four. The ideal pupil area falls to one quarter; at equal shutter time, light throughput also falls to about one quarter.
- Restore brightness compensation before judging depth alone. A darker image is not necessarily a sharper image.
A very small opening introduces another cost: its diffraction spot grows. The enlarged spot in the film shows that size trend, not a measured sharpness test of a real lens.
Use the same lens for every depth
D is entrance-pupil diameter and N is f-number. The focus setting determines sensor distance vₛ; another object distance has ideal image distance v. The blur disc follows from where its bundle intersects the sensor, rather than an arbitrary animation parameter.
The green range uses a 0.03 mm sensor-plane diameter as acceptable blur for an ideal 50 mm thin lens. Output size, viewing distance and resolution requirements can justify another tolerance and therefore another reported depth of field.
Relative light uses f-two as the reference, holding focus, transmission and shutter time equal, and varies as 1/N². Display brightness applies an approximate display response. It is not a linear irradiance meter or a complete shutter, ISO and noise simulation.
How the picture is rendered
The flowers, leaves, vase, buildings and lights are original programmatically drawn layers. Each layer receives defocus calculated from its distance. Isolated lights use geometric discs; extended artwork uses a Gaussian approximation sized from the same circle of confusion. This is not a full lens point-spread simulation.
The diffraction illustration uses central-spot diameter 2.44λN at 550 nm, excluding finite-conjugate corrections and real lens aberrations. This experiment establishes relationships rather than replacing measurements for a particular camera system.