Photographic Optics and Camera Systems
This section covers Photographic Optics and Camera Systems: lens behaviour, aperture and shutter control, focus and depth of field, camera types and the image-forming chain from subject to sensor or film.
Core rules
- Focal length sets angle of view on a given format. Wide-angle means short focal length, broader view and exaggerated near/far relationships; telephoto means long focal length, narrower view and compressed perspective.
- f-number = focal length divided by effective aperture diameter. f/1.4, f/2, f/2.8, f/4, f/5.6, f/8 and so on: each step halves light. A larger f-number means a smaller physical aperture.
- Depth of field increases with a smaller aperture, shorter focal length, greater subject distance and smaller format. Diffraction softens the image at very small apertures.
- Shutter speed controls motion: fast speeds freeze, slow speeds blur. Flash sync limits the fastest usable shutter with focal-plane shutters.
- Common aberrations include chromatic colour fringing, spherical aberration, coma, astigmatism, field curvature, and barrel or pincushion distortion.
- A crop sensor does not change focal length; it changes field of view through the crop factor.
Worked example
A lens is set from f/2 to f/8. Count stops: f/2 to 2.8 to 4 to 5.6 to 8 is four stops. Light is reduced to 1/16. If the original shutter was 1/500 s, keep exposure by using 1/30 s, four slower stops: 1/250, 1/125, 1/60, 1/30. Depth of field increases, but subject-motion risk rises. The reasoning shows that stops are multiplicative, not linear.
Traps
- Thinking f/16 lets in more light than f/4.
- Claiming a 50 mm lens becomes 75 mm on APS-C; the lens is still 50 mm, only the angle of view changes.
- Using zoom to change perspective; perspective changes only when camera position moves.
- Ignoring minimum focus distance and then blaming autofocus.