A small laboratory for a familiar camera
How it works
Change one setting. Watch the image. Follow the rays to understand why.
Focus is a meeting place
Light from a point on an object converges at its ideal image plane. Move the sensor to that plane and geometric defocus disappears. An object at another distance focuses at a different plane, so its light forms a disc on the sensor.
Aperture changes the cone of light
A smaller opening narrows the ray bundle. Away from perfect focus, that usually makes the blur disc smaller. Try the aperture guide with the far object selected.
Brightness and the SLR preview button
The preview always shows blur at your selected aperture. Brightness is compensated by default to make comparisons easy. Turn on Show aperture light loss to see how much darker a smaller opening would be, relative to f/2 at the same geometry.
A film SLR normally keeps its lens wide open while you view and focus. Its depth-of-field preview button stops the lens down, changing both depth of field and brightness. This lab keeps the selected aperture's blur visible in both brightness modes. The toggle changes only brightness.
Read the diagram
The diagram is a schematic cross-section. Object distances and small sensor movements use separate scales, labeled in the view. The ideal image plane is where the selected object's rays converge; the sensor plane is where you record them. The normal camera preview is turned upright; Sensor view shows the physical inversion.
The model
For focal length f and object distance u, the ideal image distance is v = fu / (u − f). Ideal-plane magnification is −v/u. At sensor distance s, the geometric blur diameter is (f/N) × |1 − s/v|, with aperture f-number N. All calculations use consistent millimetre units.
This is ideal geometric optics. Diffraction, lens aberrations, dispersion, vignetting, noise, and transmission losses are not simulated. Small apertures therefore do not acquire diffraction blur here. The light-loss option is an aperture-area comparison, not a complete shutter, ISO, or bellows-exposure model. Numerical blur has no universal sharp/not-sharp boundary.