Aperture-stop explorer

This interactive explorer uses a JavaScript module. Open it through a local web server to change the system. Figure 1.5 remains the static reference for the default ray trace.

An on-axis object point 200 millimetres before a 100 millimetre focal-length lens sends candidate rays through the lens and a downstream aperture. The lens has an 8 millimetre clear diameter and the downstream opening has a 6 millimetre clear diameter. The lens imposes the smaller allowed initial slope and is the aperture stop.
The default case: the 8 mm lens, rather than the 6 mm downstream opening, limits the on-axis ray cone.

Interactive explorer

Explore: which aperture is the stop?

Trace rays from one on-axis object point through a thin lens and a later physical aperture. The aperture stop is the opening that permits the smallest initial-slope magnitude—not necessarily the smallest visible hole.

Predict first

In the default case, the downstream opening is physically smaller. Which opening do you expect limits the ray cone? Change a diameter or the downstream position, then compare the two slope bounds.

Fixed schematic axes: axial positions span −300 to +300 mm and transverse positions span ±25 mm. Axial and transverse scales intentionally differ.

Set the on-axis system

Compare the input-slope bounds

For xo = 0, the downstream height is xa = Bθo.

Lens bound
Downstream bound
B to downstream plane

Direct trace

Start at xo = 0 with a variable θo:

xlens = doθo

θafter = θo − xlens/f

xa = xlens + daθafter = Bθo