While looking at small text on a computer screen today, I had these following questions :
1> For near field vision, human eyes can resolve around 1/10th to say around 1/100th of an inch objects. I believe monkeys can do the same since they pick out lice from other monkeys. What are the evolutionary advantages for humans to have this order of resolution ? (why not more or less?)
2> Also for the far field, I believe we can see things at around a mile or so - I believe this range is to spot predators and have ample time to take precautionary measures - Is this correct ? Are eagles/predatory birds the animals that have the best far field vision ?
3> Is there any animal that can see the average bacteria ? (I know there are some special bacteria that can be almost seen by the human eye - but I am asking about the average bacteria)
I suspect our resolution was pretty much set by our ancestors before we became “human”. It’s always risky to try and proclaim why something is developed or lost (I learned that much at least from reading all those essays by Stephen Jay Gould), and often the answer is that we have certain traits because they were useful to our ancestors, and there was no evolutionary advantage in losing them or further refining them. If our ancestors had resolution good enough for catching lice (or whatever other reason), we’d probably have it, too. There are other things we can use it for. And it’s not as if we don’t still have lice.
2.) You may be able to see things a mile away, but I can see a star that’s 93 million miles away. In fact, I can see stars and galaxies that are light years away.
It doesn’t really make sense to separate resolution of distant objects from that of nearby objects. We resolve things subtending a certain angle. If I can see a louse at a distance of a foot, I can see a wildebeest (or a lion) at a mile. I don’t need to develop a special capability for distance resolution - I already have it.
3.) I don’t know of any animals that can see bacteria. You might think that small eyes, being closer to the size of the bacteria, could see them, but small optics have large diffraction spots. You want one with a large aperture – giant squid eyes, maybe. But the size of the aperture won’t do the job if you don’t have a high density of cones and rods, just as you need a high density of detectors in a digital camera if you want high resolution. I don’t know what sort of resolution a giant squid has, but I suspect they don’t see bacteria, which are on the order of a micron or so. That’s smaller than the typical diffraction spot of the human eye, so we can’t see them with the unaided eye, either.
Thank you for the detailed reply. As to diffraction, isn’t that dependent on the frequency range ? I thought birds and some other animals could see in the higher frequencies and thereby may not have the same diffraction issues.
Diffraction spot size does indeed depend on wavelength* – in fact, it’s proportional to it. The diffraction-limited spot size for a perfect lens is 2.44 times the wavelength times the f/# of the lens. (For an infinitely distant object , f/# is focal length divided by aperture diameter). It’s true that some animals see slightly outside our wavelength range – insects can see further into the ultraviolet – but birds see in pretty much the same range that we do, so they get no advantage there. (Seeing into the infrared doesn’t help, by the way – the wavelengths are longer there, making the diffraction spot bigger). You can get more resolution by having a better quality lens (the human lens is definitely not perfect) or by having a larger diameter than the human pupil (hence my argument about cat’s slit pupils, which are very long vertically, and therefore give high vertical resolution, at the expense of horizontal resolution). So birds have to deal with the same wavelength/frequencies that we do, so they get no advantage there. They might have bigger pupils (I don’t know, but it seems a possibility) and better quality lenses.
says they have higher resolution of sensing cells in their retina, but that won’t buy you anything unless the rest of your visual system can give you a smaller diffraction spot.
*wavelength is inversely proportional to frequency, so, yes, it does depend on frequency. But my brain works in wavgelength space. YMMV
Just to nitpick, there are a few extraordinarily large bacteria which can (just barely) be seen by the unaided human eye. Though that’s not really what’s under discussion here.
I think these statements might be a bit broad. Birds are generally tetrachromatic, with a number of species (I can’t find a good ref on what proportion) having at least one pigment maximally sensitive to UV light.
Based on the rest of your reasoning, that would imply at least the potential for greater visual acuity too.
Just a data point, you can easily see 1-mil wire with a diameter of 1/1000th of an inch. Obviously people’s close vision is very different and the older you get the more it tends to deteriorate. My distance vision is pretty bad, but I happen to have very good close vision.
This article seems to think that at best the human eye can resolve two lines with a gap of .026mm (.001 inch).
It appears that primates had good vision compared to most mammals because if you are living in trees, vision is more important that smell and so the olfactory bulb shrank and the visual cortex, along with the vision to support it, grew. So our poor smell and good vision (compared to most terrestrial mammals) are the heritage of arboreal ancestory. Incidentally, much the same is true of birds.
<side comment>Actually, it might just as likely be to spot prey. Humans are more often predators themselves rather than prey for another predator.
After all, there are only a few predators that see humans as prey: the big cats, larger bears, the larger crocodiles/alligators, some sharks, the largest squid, etc. Whereas humans see as prey and eligible to be eaten nearly every other animal, insect, and most plants.
I think (s)he’s nitpicking the use of the word “almost”. That is to say, the biggest bacteria are not almost big enough to be seen be the unaided human eye, they are big enough to be seen in the right conditions.
Some birds (along with insects) can see further into the ultraviolet, as this plot shows ( Tetrachromacy - Wikipedia ). But I can see 330 nm light, too. It’s not that the bird can see further into the UV as much as that it has greater sensitivity where it can see. Atmospheric absorption and the absorption of the tissues making up the eye itself are going to keep birds from seeing wavelengths much beyond where I can see.
So I can see the same wavelengths the bird can, at the same resolution. It’s just that the finer features will be brighter for the bird.
The reason humans cannot see UV is because the lens of the eye is opaque to UV. If you remove the lens, we can see UV, but we have no custom pigment for those wavelengths, so it just appears as a bluish-white (UV light activates all cones of the eye, slightly more affecting the blue cones). However we of course cannot say whether detecting those wavelengths allows greater visual acuity since those people by definition don’t have a lens in that eye.
Birds and insects however have a pigment that in many cases has all, or almost all, of its activation wavelengths within the UV range. If their eyes blocked these wavelengths, it would be curious indeed that this pigment would remain so prevalent in nature. Also note that many flowers have patterns in UV that insects and birds do appear to use (e.g. concentric circles guiding insects in like a target).
Finally, atmospheric absorption of UV …obviously this is not enough to block out all UV and make it useless for vision.
So, indeed, birds can see significantly further in the UV . mea culpa
But that’s a matter of response of the cones, not of the material in the eye blocking it. I don’t believe there’s a significant difference between the materials in bird’s eyes and human eyes. You seem to think that I’m saying the material will block long wave UV. It doesn’t. But it does cut off eventually in the UV.
As for atmosphere blocking UV, be thankful it does, or your skin cancer rate would go up.
It seems birds flying through brush have some better sight/reaction time/something else going for them.
Could it in part be because the actual physical distance from the eyes to the part of brain needed be used for recognition, decision, reaction of muscles ( faster ‘twitch’ than human have? ) to enable this?
Do some smaller animals have this, or is it just the faster muscle movement that is better?
What would be the animal that has the farthest distance between the eye and the brain vs birds and how could we test it?
Time of information getting to the brain?
Better brain operation in this area?
Signal to and response to the call to action of the muscles?
Mongoose, average dog, average human, average elephant, average whale???
Yep. And note that your original point was that birds can see no shorter wavelengths than humans can.
I’m not sure about the material, however I’m sure that it would not be the case that so many species would have a pigment like this, with activation range almost entirely within the UV, without UV being able to make it to the retina.
This is a goalpost shift. Of course birds cannot see all of the UV spectrum, which is much, much wider than the spectrum of frequencies we call visible light.
Your point was that birds cannot see any further into UV than we can, and it’s incorrect.
Obviously some UV gets blocked. You were suggesting however that birds do not see more UV than we do for that reason.
But plenty of UV light reaches the earth’s surface. This is why you still need sunblock. And why it can be utilized by birds, insects and other kinds of animal.
…obviously this is not enough to block out all UV and make it useless for vision.