How does the "large eye to see large objects" thesis make any sense at all? I can see the moon just fine with my puny little orbs. I get that the large eye will have an extremely narrow "depth of field" for small objects, but how does that mean it will see larger ones any better?
It seems a whole lot more likely to me that it's "large eye to catch more photons because it's _DARK_ down there".
EDIT: maybe something to do with the fact that the light that is down there falls off quickly so a larger aperture somehow lets you correlate the scattered light better?
The simple answer is: the larger the eye, the more "pixels" (cells of the retina which respond to light energy). So larger eye == higher resolution.
The current theory regarding "why is all that resolution necessary?" is: there are tiny creatures swimming alongside the squid, which respond to a sperm whale's sonar by emitting a tiny amount of light. When the squid sees this light, it knows that a whale attack is imminent, and runs. (Well, jets.)
If the eyes were smaller then there wouldn't be enough resolution to make out the tiny dots of light emitted by those tiny creatures.
Another way of thinking about it is: the eye needed to evolve larger in proportion to how tiny the creature is. If the creatures were twice as big, then the eye would've evolved half as large as it currently is. (If the theory is true.)
Mathematically, there need to be enough retina cells such that "1 tiny creature at distance D can be detected per degree of viewing angle". E.g. if the squid has a 130deg field of view, and it needs to be able to see the creature light up within 1 foot of its eye, then it needs enough retina cells in order to scan the entire conic volume of the space between the eye point and "1 foot in front of the eye point". By the Nyquist frequency, that suggests there need to be at least two retina cells per "tiny creature body length projected from a distance of 1 foot". (The farther it is from the eye, the smaller its projection; and thus correspondingly more retina cells are required, which requires a physically larger eyeball. Note that as the eye gets larger, the field of view remains the same.)
To clarify: the squid isn't looking at the whale at all. Rather, the squid is looking at tiny sea creatures very close to its face that light up when hit by a whale's sonar. Thus they act as an early warning system for the squid.
The squid doesn't need to resolve the little sea creatures in order to detect a whale. It only needs enough resolution to resolve a huge number of the little lights into a whale-shaped blob. For that, it needs an eye with the biggest possible aperture and sensitive sensors. The increased resolution may be an additional benefit of the better light-gathering ability of a big eye but isn't necessary.
The article says the researchers calculate that the squid eyes they measured are the optimal size for seeing something far away and massive. Why isn't the biggest possible eye that can be grown the best? Is there some other constraint besides the mechanical structural issue or risk of damage, etc.?
It seems a whole lot more likely to me that it's "large eye to catch more photons because it's _DARK_ down there".
EDIT: maybe something to do with the fact that the light that is down there falls off quickly so a larger aperture somehow lets you correlate the scattered light better?