A TED talk on Intelligent and sub-optimal design.

[Deleted member 619]

Also, just a side note: Cephalopods can actually distinguish between different colours. When light passes through virtually any lens, the colours separate like how a prism does. This is called chromatic aberration. But cephalopods managed to make use of this.

Physics pedant interjection for clarity:

This isn't called chromatic aberration. The way light separates is called refraction, which varies by medium (each medium having a refractive index detailing how it separates and at what angles).

Chromatic aberration is the resultant phenomenon of light focusing at different points after having passed through a lens, i.e., having come back to focus imperfectly because it's functionally impossible to build a perfect lens. Very closely related, the one being a function of the other, but I know you value clarity as much as I. You weren't incorrect, exactly, but the nomenclature was potentially confusing.

 

[Nesslig20]

Physics pedant interjection for clarity:

This isn't called chromatic aberration. The way light separates is called refraction, which varies by medium (each medium having a refractive index detailing how it separates and at what angles).

Chromatic aberration is the resultant phenomenon of light focusing at different points after having passed through a lens, i.e., having come back to focus imperfectly because it's functionally impossible to build a perfect lens. Very closely related, the one being a function of the other, but I know you value clarity as much as I. You weren't incorrect, exactly, but the nomenclature was potentially confusing.

Oh yes, I meant to say that the way different light frequencies refract from the lens, thereby ending up at different focal points (which causes chromatic aberration).


from paper: https://www.pnas.org/content/113/29/8206 (see how different wavelenghts are focued on different places on the retina of the weirdly shaped pupils of the cephalopod eye (C).

Fig. 2 shows the mechanism that we are proposing for how chromatic aberration can be exploited to achieve spectral sensitivity. As we show below, the off-axis pupils of cephalopods combine with the wavelength dependence of the lens index of refraction to generate chromatic blur; different wavelengths come into focus at different distances from the lens. The spectral content of a structured scene can be deduced by sweeping through focus (i.e., changing the lens to retina distance) and seeing how the image blurring varies. A key element in our argument is the observation that the off-axis pupils common in cephalopods actually maximize the chromatic blurring in their visual system (Table S1). These animals would have better acuity if they had evolved a small, on-axis pupil, such as the one in the eye of the reader. Instead, they seem to have sacrificed overall acuity in favor of chromatic blurring, which we suggest here as a mechanism for spectral discrimination. This mechanism is similar to that in the recent observation (25) that vertical and horizontal pupils produce astigmatic blurring.

 

[Dragan Glas]

Greetings,

That's not really true. There are indeed mammals that use a "reflective" eye to see better in the dark, but they also detect the light that comes directly onto the photoreceptors during the day. But this extra reflection also allows for secondary reflections to occur, causing the image to be blurred at the cost of gaining good night vision. This is why diurnal species like us have the back of our eyes heavily pigmented to prevent this reflection from happening

I corrected this in my following post.

As you say, the light triggers the photo-receptors on the way in.

However, we are one of those mammals who have a tapetum.

The water/land distinction doesn't really explain the inverted retina (+ blind spot) in vertebrates, since the first vertebrates that evolved this type of eye lived underwater, and over half of all extant vertebrates still live in water. The best explanation for the differences in eye type between the otherwise superficially similar eyes of vertebrates and mollusc is evolutionary contingency. Our ancestral line just happen to have evolved the eye in this peculiar way, and we got stuck with it, but after some evolutionary jury-rigging and work around solutions, the vertebrate eye works pretty darn well, despite the blindspot and backward facing photoreceptors.

Again, as I pointed out in the above post, I indicated that this wasn't just an accident - the fact that light has to pass through the full thickness of the retina to reach the photo-receptors means the glial cells focus red-green light onto them, whilst scattering blue light.

Also, just a side note: Cephalopods can actually distinguish between different colours. When light passes through virtually any lens, the colours separate like how a prism does. This is called chromatic aberration. But cephalopods managed to make use of this.

Weird pupils let octopuses see their colorful gardens | Berkeley

news.berkeley.edu

Although they can distinguish between colours to some extent, they do this through distinguishing shades of grey with their monochrome vision - not by actually seeing colour, They are unable to distinguish colours that appear the same shade of grey.

I covered this in the following post above, as the linked article explained.

Kindest regards,

James

 

[Led Zeppelin]

I apologize guys. I am now very certain that I was wrong.

 

[Sparhafoc]

However, we are one of those mammals who have a tapetum.

?

Humans lack a tapetum lucidum; one of the reasons we have poor night vision.

 

[Dragan Glas]

Greetings,

?

Humans lack a tapetum lucidum; one of the reasons we have poor night vision.

All these years I'd thought that we had, based on flash photography resulting in reflections from people's eyes - I tend to have this in photos with flashes.

You are right, of course.

Kindest regards,

James

 

[Deleted member 619]

Aye. Red eye is a result of direct lighting to the retina, which has a lot of blood vessels in it. It's because the light is hitting the retina on the same axis as the lens is viewing from. You mitigate it by having the flash off-axis by a decent margin.

 

[Nesslig20]

Greetings,
I corrected this in my following post.

Yeah, I didn't see that correction before I made the post.

Again, as I pointed out in the above post, I indicated that this wasn't just an accident - the fact that light has to pass through the full thickness of the retina to reach the photo-receptors means the glial cells focus red-green light onto them, whilst scattering blue light.

I don't understand what you mean by this, nor why this indicates that the specific set-up of the vertebrate eye is "not just an accient" as opposed to being the product of phylogenetic constraints.

Although they can distinguish between colours to some extent, they do this through distinguishing shades of grey with their monochrome vision - not by actually seeing colour, They are unable to distinguish colours that appear the same shade of grey.

Oh, no, you misunderstand. Although as I said, this was just an interesting side note since this wasn't really part of the discussion.

Cephalopods actually CAN see colour, but they do it differently that we do.
We see different colours by having different photoreceptors with different pigments that are sensitive to different wavelenghts of light.
Cephalopods "seperate" different wavelenghts of lights when it passes through their lenses, which results in different wavelengths of light to hit the retina in different places. This "hitting the retina at a certain spot" is interpreted as a "colour". Or the wavelenghts of light are focused at different lenghts away from the lens, which means that if the cephalopd changes the depth of the eye balls, they can focus on specific colours.

 

[Dragan Glas]

Greetings,

Yeah, I didn't see that correction before I made the post.


I don't understand what you mean by this, nor why this indicates that the specific set-up of the vertebrate eye is "not just an accient" as opposed to being the product of phylogenetic constraints.

Although originally it might have been an accident, the fact that this setup has stabilised throughout the vertebrates indicates that it is evolutionally beneficial - otherwise a smorgasbord of "designs" for eyes would have evolved.

Oh, no, you misunderstand. Although as I said, this was just an interesting side note since this wasn't really part of the discussion.

Cephalopods actually CAN see colour, but they do it differently that we do.
We see different colours by having different photoreceptors with different pigments that are sensitive to different wavelenghts of light.
Cephalopods "seperate" different wavelenghts of lights when it passes through their lenses, which results in different wavelengths of light to hit the retina in different places. This "hitting the retina at a certain spot" is interpreted as a "colour". Or the wavelenghts of light are focused at different lenghts away from the lens, which means that if the cephalopd changes the depth of the eye balls, they can focus on specific colours.

The basis for my earlier posts in relation to cephalopod vision was based on a number of articles I read some years ago. As I noted earlier, most cephalopods tested are believed to be colour-blind on the basis of their only having one pigment, whereas we have three. The only cephalopod which was confirmed as having three pigments is the pygmy squid,

Having looked for something more recent, I found this.

As the authors note:

This, of course, does not constitute a scientific proof of whether or not octopuses are able to distinguish between different colors. Our hope is that after many experiments and color tests, we might provide scientists with more material they will observe and then be able to reach more conclusive results

Still, it's a interesting development.

Kindest regards,

James