So in real life there are glasses that use photochromic lenses to adapt to light intensity 
Would it be biologically possible to have a species that uses a photochromic material instead of an iris to control light intensity?
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2$\begingroup$ (1) You probably mean a photochromic material in addition to an iris. The iris serves as a diaphragm, which limits the passage of light to the center of the optical system, where distorsion and aberrations are within acceptable limits. (2) Many humans use artificial eye coverings with such photochromic materials; so that the answer is trivially yes, there is at least one species which uses this mechanism to control the intensity of light. $\endgroup$– AlexPCommented Jan 17, 2023 at 15:13
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1$\begingroup$ I think biology will have an easier time making tearducts that synthesize dark/light-obscuring microparticulates that dispense and cover the eye in response to excessive light, or dark nictitating membranes, than trying to make a photochromic lens. $\endgroup$– LemmingCommented Jan 17, 2023 at 16:11
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2$\begingroup$ Could also go the route of making the fluid inside the eye more opaque in response to light(ocular melanin production/tanning?) but that's not really darkening the lense now is it $\endgroup$– LemmingCommented Jan 17, 2023 at 16:18
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1$\begingroup$ Hello @Arzack1112, welcome to Worldbuilding. The answer to this question is trivially, "yes." However, from an evolutionary standpoint, nature tends to take the path of least resistance. It may not seem that way, but generally speaking, whatever complexity you observe, nature took the simplest path to get there. If there's a simpler solution, it's unlikely to use this solution. Nictating membranes would be the most likely solution. However, the solution meets suspension of disbelief. Do you have a problem to solve? As-is, I'm not really sure what you're asking from a worldbuilding POV. $\endgroup$– JBHCommented Jan 17, 2023 at 18:04
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$\begingroup$ note that size and shape of the iris have an effect on the optical system beyond the amount of light this is being used by several species to extract more information from the picture. $\endgroup$– bukwyrmCommented Jun 21, 2023 at 7:03
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2 Answers
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While no known organisms use photochromic materials in their eyes, certain creatures exhibit related phenomena. For instance, cuttlefish can change the color and pattern of their skin using chromatophores, which are cells that expand or contract in response to light and other stimuli. Similarly, a photochromic system in the eye could involve specialized cells that produce and regulate light-sensitive compounds.
This adaptation would involve specialized cells capable of producing and maintaining the photochromic material, offering continuous and uniform light adjustment. Although this concept is theoretical, it aligns with known biological mechanisms and evolutionary principles.
The rub is that irises evolved first and all biological creatures with standard eyes just use those or something very similar. It is unlikely for there to be evolutionary pressures to create such a radical change over time and mutants are likely to go extinct in the interim. If intelligently designed creatures are in play, far more is possible than the life that is currently present on Earth.
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$\begingroup$ The eye evolved at least seventeen different times, and not all of the variants use an iris. $\endgroup$– MarkCommented Aug 2 at 3:23
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We don't know for sure. But biochemically there should be no problem with such a mechanism. The fact that we do not see it in nature probably means that such a photochromic lens either didn't evolve first and there was no evolutionary route to it later, or more likely the existing systems we see in eyes are simply better and more efficient at doing what they do than a photochromic lens.
Given different circumstances in terms of light intensity and wavelength perhaps such a thing might have evolved.
