= Aperture
{wiki}
An <aperture> is an opening that admits <light> into or through an <optical instrument>. Its shape and size select the transmitted <light rays>; an <aperture> alone does not bend them as an <optical lens> does. The <aperture stop> is the opening that most restricts the bundle from an on-axis object point. The <entrance pupil> is the image of that <aperture stop> formed by the preceding <optical lens> elements, viewed from the object side. Consequently the effective collecting <diameter> $D$ is the <entrance pupil> <diameter>, which can differ from the physical hole's <diameter>. For example, preceding <optical lens> elements that enlarge a $10\,\mathrm{mm}$ <aperture stop> by a factor of two give $D=20\,\mathrm{mm}$; measuring the physical hole alone would misidentify the <focal ratio>. https://www.rp-photonics.com/entrance_and_exit_pupil.html[Rüdiger Paschotta, RP Photonics Encyclopedia, Entrance and Exit Pupil]{ref}.
For an unobscured circular <entrance pupil> under a uniform beam normal to its plane, the collecting <area> and admitted <radiant flux> are
$$
A=\frac{\pi D^2}{4},\qquad \Phi_{\rm admitted}=E_{\rm pupil}A,
$$
where $E_{\rm pupil}$ is incident <irradiance>. Doubling $D$ quadruples admitted <radiant flux>. Obstructions, nonuniform illumination and losses after the <entrance pupil> require the actual illuminated <area> and transmission to be included. The same $D$ also sets an angular <diffraction-limited resolution> scale $\lambda/D$ at <wavelength> $\lambda$. Collection and <diffraction> therefore answer different questions from <field of view>: an <aperture> selects the bundle from each object point, while the receiving plane's extent helps select which object directions appear in the image.
NASA/JPL's published pinhole demonstration uses two white cards, foil taped over a window in one card, and a small hole pierced in the foil. Sunlight passes through the hole onto the second card; the projected image is viewed on that card. The activity describes a larger image when the card separation increases, improved visibility when the receiving card is shaded, and a separate image from each of several holes. These are qualitative observations of <image formation>, rather than calibrated measurements of <radiant flux> or <diffraction>. https://www.jpl.nasa.gov/edu/resources/project/how-to-make-a-pinhole-camera/[NASA/JPL Education, How to Make a Pinhole Camera, apparatus and projection observations]{ref}.
A proposed quantitative extension replaces sunlight by a steadily illuminated asymmetric target and puts the foil and a translucent receiving screen in a dark box. Record target distance $u$, hole-to-screen distance $b$, hole <diameter> $d$, image height, orientation and edge blur. Measure screen brightness with a fixed <photodetector>, or record the screen with an <optical camera> whose integration time and gain stay fixed. Exchange centred foils of measured $d$ at fixed $u,b$; then vary $b$ with the same foil. <Geometrical optics> predicts an inverted image with <transverse magnification> $m=-b/u$ and a point's geometric blur <diameter> $d(1+b/u)$. Thus a larger hole admits more <radiant flux> without changing the central image scale, whereas increasing $b$ enlarges the image. These are predictions to test, not reported data. Keep target illumination and detector position fixed, subtract a hole-covered dark reading, repeat the original foil to check drift, and avoid clipping the detector response. Foil thickness and ragged edges can block oblique <light rays>; at sufficiently small $d$, <diffraction> invalidates the claim that ever smaller holes always sharpen the image. A lensless <optical camera> has no <optical lens> <focal length>: $b$ is a propagation distance, not that <focal length>.
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