Optical System Design
Optical System Design
This domain tests first-order layout, aperture definition, aberration control, image quality metrics, and tolerancing. The exam expects you to reason from stops and pupils to f-number, field, and diffraction-limited performance.
Core rules
- Aperture stop limits the axial cone; entrance pupil is its image seen from object space.
- F-number is (f/#=f/D_{\text{entrance pupil}}); numerical aperture in air is (NA\approx1/(2f/#)) for small angles.
- Field stop limits field of view; chief ray passes through the centre of the aperture stop.
- Seidel aberrations are spherical, coma, astigmatism, field curvature, and distortion; chromatic aberrations arise from dispersion.
- MTF describes contrast versus spatial frequency and combines diffraction, aberrations, sampling, and tolerances.
Worked example
A lens has focal length 200 mm and entrance pupil diameter 50 mm, so (f/#=4). At (\lambda=0.5,\mu m), the Airy disk diameter is about (2.44\lambda f/#=2.44\times0.5\times4=4.88,\mu m). A detector with 5 µm pixels is therefore close to critically sampled at this f-number. If the design is opened to f/2, the Airy diameter falls to 2.44 µm, but aberrations and tolerance sensitivity usually increase.
Traps
Do not confuse aperture stop with entrance pupil. Do not quote f-number without defining the pupil. Paraxial layouts can look perfect while real rays fail. MTF at a single frequency is not a complete image-quality statement. Tolerancing must include compensators and assembly shifts, not only component fabrication errors.
Exam focus
Separate diffraction limit, aberration limit, and detector limit before proposing a fix.