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There seems to me to be two main effects that should be considered in answering this question.

The first is that the iris dilates in low-light settings, which causes an increase in the numerical aperture of the eyeball and therefore an increase in the amount of light that gets focused onto the retina. However, there is also a smaller amount of ambient light that will hit the eyeball under these low-light settings than in normal daylight.

The second factor to consider is that the eyeball itself is not a perfect lens. Rather, it is subject to quite large spherical aberrations, which distort the perceived image. In low-light scenarios, when the iris dilates, more of the eye (and thus more of its spherical aberrations) are exposed to the incoming light, thus altering the produced image even more than in daylight.

My feeling is that there has to be an ambient light level at which the iris is as dilated as it can be to allow the most light to hit the retina, while simultaneously not being too dilated as to introduce too much spherical aberration to the image. What is this level?

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Vision in the daylight and vision during night are two different processes performed, in the eye, by two different kind of cells.

The vision under well-lit conditions is called photopic vision, while vision in low levels of light is called scotopic vision. Photopic vision is performed by cone cells, that are mainly concentrated in the fovea, that is, roughly speaking, the point in which the image you are looking at is focused in your eyes. On the other hand, scotopic vision is performed by rod cells, that are distributed on the whole retina (with a varying density) with the exception of the fovea, where they are less concentrated. The first consequence is that, while in photopic vision it is possible to directly stare a certain point focusing it, in scotopic vision (for example, star gazing with naked eye) often it happens that objects that are visible in the peripheral field of view suddenly are no longer visible when we focus on them. (Actually, I am not a biologist, but this seems reasonable from an evolutionary point of view. In the dark, you have to control the whole environment for predators, therefore peripheral vision is extremely important, while in the light predators are more visible and the main goal is thus to focus things).

A second, important difference is that in photopic vision the cone cells are sensitive to certain wavelengths and they can distinguish between them, thus allowing color vision, while in scotopic vision different colors cannot be distinguished and the sensitivity as a function of the wavelength is different.

As a side note, it is remarkable to notice that in scotopic vision we can see (in this case, I mean perceive with a signal in our nerves) just a few photons (less than ten in the right spectral region), while in photopic vision this number is bigger. Notice that the active substance in rod cells, that is called rhodopsin, is actually sensitive to the single photon (in certain spectral regions), but a conscious response is obtained only when a few more photons comes to the retina, otherwise the noise in our vision process would be too high.

To get back to your question, I think that from the point of view of a physicist it is actually quite difficult to answer. The process of vision is extremely complicated and it is not only related to the sensitivity of our eye to light. In fact, it involves also the response of the human brain to the optical stimulus and all the ways in which it interprets it.

Last, in this Wikipedia article, it states that:

Night vision is of a much poorer quality than day vision because it is limited by a reduced resolution and therefore provides the ability to only discriminate between shades of black and white.

The reference for this sentence (and probably references therein) could be a good starting point for understanding the factors that are playing a role in this difference of resolution.

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I'm no biologist but i'd bet all my money that evolution has taken care of this and that our eyes are perfectly adapted to the properties of light on earth. Such as in the same way that the visual spectrum our eyes allow is placed where the suns blackbody plot peaks.

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