# @stem_antics on Instagram

- **Type:** Video
- **Original URL:** https://www.instagram.com/p/DeH_Ri6SCJz
- **Gondola URL:** https://gondola.cc/posts/71427943-stem-antics-instagram
- **Thumbnail:** https://img.gondola.cc/tr:w-,h-,fo-auto/postThumbnails/e85de41964.jpg
- **Posted:** 2026-10-05T19:44:49.000+00:00
- **Account Owner:** Stem Antics (@stem_antics) — https://gondola.cc/stem_antics

## Caption

What you are seeing up close is not a camera. It is a living optical instrument assembled from collagen, crystalline protein, and pigment, refined over roughly 500 million years since the first light-sensitive patches appeared in Cambrian animals.

The iris is the star of any macro shot. Its colour is not produced by blue or green pigment at all. Every human iris contains only melanin, and the visible hue is set by how much melanin sits in the anterior stroma and how that stroma scatters light:

- Brown irises carry dense melanin that absorbs most incoming light
- Blue irises carry very little stromal melanin, so short wavelengths are preferentially scattered back by the collagen fibres, the same Tyndall-type scattering that gives skim milk a bluish cast
- Green and hazel sit in between, mixing modest melanin with scattering
- The deep posterior pigment epithelium is dark in nearly every eye regardless of surface colour

The radial ridges and crypts visible in close-up are the sphincter and dilator architecture beneath. The sphincter pupillae, driven by the parasympathetic oculomotor pathway, constricts the pupil to around 2 mm in bright light. The dilator pupillae, under sympathetic control, opens it to roughly 8 mm in darkness, a change in aperture area of about sixteen times.

That texture is also why iris recognition works. John Daugman at the University of Cambridge published the foundational algorithm in 1993, encoding iris patterns with 2D Gabor wavelets into a 2,048-bit IrisCode. The pattern forms through chaotic morphogenesis during gestation, so even identical twins and the left and right eyes of one person carry statistically independent irises.

Every detail in a macro image is a mechanism, not a decoration.

#stemantics #eyeanatomy #optics #macrophotography #biology

FC

## Stats

- **Views:** 10,573
- **Likes:** 149
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- **Comments:** 4

## Tags

optics, macrophotography, biology, eyeanatomy, stemantics

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