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Each subject is blurred by its own circle of confusion. Small bright discs are distant lights (bokeh): their disc size is the blur circle for an object at 200 m.
The science
Pinhole (camera obscura). Light from each scene point passes through one small hole of diameter d at distance v in front of the sensor and makes a spot of geometric diameter d (1 + v/z). A smaller hole sharpens the image until diffraction takes over: the Airy disc has diameter 2.44 λ v / d. Adding the two in quadrature gives a minimum blur at d = sqrt(2.44 λ v) (about 0.26 mm for v = 50 mm at 550 nm). A pinhole is also dim: exposure scales with (d/v)2.
Thin lens. 1/f = 1/z + 1/v. A lens collects light over a wide aperture D = f/N but forms a sharp image only for the plane at the focus distance s. A point at distance z focuses at vz = f z/(z - f), so on the sensor (at vs) it becomes a disc of diameter c = (f/N) |vs - vz| / vz (similar triangles on the marginal rays).
Depth of field. A point looks sharp if its blur is below the acceptable circle of confusion clim (about 0.030 mm for full frame, roughly sensor diagonal / 1500). The hyperfocal distance is H = f2/(N clim) + f; the near and far limits are s(H - f)/(H + s - 2f) and s(H - f)/(H - s). Focus at H and everything from H/2 to infinity is acceptable.
Exposure. Light reaching the sensor is proportional to aperture area, i.e. 1/N2; one stop is a factor 2 in light, a factor sqrt 2 in N. EV = log2(N2/t) (ISO 100).
Honest simplifications. Geometric thin lens: no lens aberrations, vignetting, focus breathing or lens thickness; blur is a uniform disc (real bokeh has soft or cat-eye edges); diffraction is the on-axis Airy diameter added in quadrature; the sensor view applies a Gaussian blur of equivalent size; the camera is drawn about 6 times larger than real so it is visible. The scene objects are simple stand-ins.