200 mm Prime Lens Tutorial (Part B): Ray Anatomy and Vignetting Checks

Introduction

In this part we build a practical “pre-metric” workflow: you will read the lens by eye using only the 3D ray paths and the detector image. The goal is to answer three questions quickly: (i) where is the stop and what rays does it accept, (ii) which rays behave like paraxial vs marginal rays, and (iii) are you seeing clipping/vignetting that will damage off-axis performance.

1. Find the stop and confirm what it limits

In the 3D view, locate the stop/aperture object (typically a plate with a circular opening). Rotate the scene so you can see rays approaching and passing through the stop (??). If your scene includes an adjustable diameter parameter (e.g. D0), note where it is edited (e.g. via Mesh editor).

2. Compare paraxial and marginal ray bundles

Create two runs that differ only in which part of the pupil is illuminated: an on-axis “central” case and an “edge” case. Use the same detector position and the same wavelength setup.

• Central (paraxial) bundle: position the source so the beam passes through the lens centre.
• Edge (marginal) bundle: shift the source so the beam enters near the edge of the first element, without changing the beam direction.

After each run, open detector0/RAY_image.csv and compare the footprints. The central bundle should typically look more compact and symmetric; the edge bundle is where asymmetry, smear, and clipping tend to appear first.

3. Diagnose clipping and vignetting by eye

Now introduce a small field angle (tilt the beam) and repeat the same comparison. In OghmaNano this is often done using a rotation parameter such as Rotate Phi in the source editor. Use a modest angle (e.g. 5–10°).

Run two cases at the same field angle: (i) stop wide open, (ii) stop stopped down. Compare both the 3D ray paths and the detector footprints. You are looking for the characteristic signatures of vignetting: rays being blocked before reaching the detector and a reduced/warped footprint.