A 100 mm lens, a sensor and a tabletop scene. Every ray is traced through all six glass surfaces with Snell's law.
Shown upright. Blur per depth from the exact blur-circle size at this focus and f-number.
A 100 mm lens, a sensor, and a tabletop scene 45 to 100 cm away.
Three glass elements and an iris. Two positive crown lenses bend light inward; the negative flint lens between them cancels colour fringes.
Light from one point on the cabin fills the lens, bends at six surfaces, and meets at one point on the sensor. That point is in focus.
Light from the pine would meet behind the sensor, from the summit in front of it. They land as blur discs.
Focusing moves the lens. A nearer subject focuses farther behind the lens, so the plane of focus slides through the scene.
Close the iris and every cone of light narrows: smaller blur discs, a deeper sharp zone. f/4 to f/16 costs 4 stops of light.
Focus decides where sharpness sits. Aperture decides how deep it reaches.