

Resolution of the human eye - yread
http://clarkvision.com/imagedetail/eye-resolution.html

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mseebach
> The eye is not a single frame snapshot camera. It is more like a video
> stream. The eye moves rapidly in small angular amounts and continually
> updates the image in one's brain to "paint" the detail. (...) Then we would
> see (...) 576 megapixels (...) This kind of image detail requires A large
> format camera to record.

The first sentence seems to contradict the last. It's obvious that you can't
take in a 120 degree view in detail even near to 576 megapixels in any period
of time short enough to compare to take one photograph. If we instead consider
a rather modest 1.3 megapixel camera, and using the 15 fps figure that's
provided elsewhere in the article, it will take 30 seconds to "record" the
scene in full detail. Also, that assumes that the scene is intensely diverse -
it seems that the brain has a pretty efficient compression algorithm. Looking
out at the sea, you can immediately dismiss 80% of the scene as "water" and
"sky" and concentrate on the detail, arriving quickly at a very high
resolution image for the 20%, even with much less than 1,3 megapixel "per
frame" resolution.

~~~
ristretto
That's definately plausible and i think that's what the author meant by "video
stream". We only see clearly enough to read in the fovea, which is only 2
degrees of our field of view.

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emanuer
Lets imagine someone would want to build a "retina display" which's ppi value
matches the maximum ppi of the human foveal vision. (fovea is the point of the
eye with the highest density of rod & cone cells)

According to this article this is 530 ppi (pixel per inch)

Now, if my math is not entirely flawed (please check) for a 27 inch monitor
with a 16x9 ratio this would mean a resolution of

    
    
      12,472 x 7,015 
    

That is ~ 21 times higher than the best resolution available today, for such a
screen. (2560x1600)

The iPhone 4 has 326 ppi and I just took off my glasses and I really had to
focus until my eyes hurt to be able to see a single pixel. (I failed, I
couldn't see one) Also I have to get really close on to my PC screen to see
single pixels (235 ppi)

Now my question is, how could my retina have 530 ppi if I can not identify
single pixels on a 326 ppi device?

Furthermore I fail to understand the most basic assumptions & calculation in
this article:

    
    
      > 53*60/.3 = 10600
    

? Why did he multiply it times 10? Is this a calculation error, or am I just
missing something?

Assuming that this calculation is correct and he just left a part out. I have
problems understanding the first and most basic assumption:

    
    
      > Thus, one needs two pixels per line pair, and that means pixel 
      > spacing of 0.3 arc-minute! 
    
      > 0.7 arc-minute, corresponds to the resolution of a spot as non-point source
      > Again you need two pixels to say it is not a point
    
      > Again, you need an minimum of 2 pixels to define a cycle
    

If I interpret the article correctly the "line pair"/ "spot"/ "cycle" in the 3
named experiments were all the smallest size which humans were able to
identify. Why do you need 2 pixels? Wouldn't anything below 0.6 arc-minutes
already constitute as being below the smallest visible size?

If this point does not hold up, the entire following calculations are way off.
I would be thankful if someone could enlighten me.

#Edit: as ristretto stated earlier, there are ~ 100 million cells in the human
eye. There are only ~200 000 cells in the fovea. Also everything gets
compressed and send through only ~ 1 million nerve cells. Just to be
interpreted by 140 million neurons in the V1 area. Short: measuring the
megapixel of the human eye is quite a fruitless exercise. The human eye does
not work like a homogenous camera/monitor.

The maximum visible pixel per inch (ppi) on the other hand have serious
implications to technology and hardware development. Therefor I have much
greater interest in this metric.

~~~
Bud
_Now my question is, how could my retina have 530 ppi if I can not identify
single pixels on a 326 ppi device?_

Assuming your retina does indeed have 530ppi or so (there's no guarantee that
all retinas are ideal), there still remains the question of whether your eyes'
focusing systems are in good enough shape to produce that resolution, whether
any vision flaws are corrected fully by your glasses/contacts, etc.

(edit to add one more correction)

    
    
      > 53*60/.3 = 10600
      ? Why did he multiply it times 10? Is this a calculation error, or am I just missing something?
    

It's your calculation error; you are multiplying by .3 instead of dividing by
.3.

~~~
emanuer
Thanks a lot. Embarrassing mistake from my site.

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Sharlin
Wikipedia[1] has a nice diagram showing how rapidly our visual acuity drops as
the function of the angular distance of the fovea. This is a drastically
different situation from a camera photosensor where the distribution of pixels
is flat. Yes, our brains use saccades[2] to construct a synthetic image over a
longer integration time, but still, our non-central acuity is so poor that we
cannot even read regular-sized newspaper text except with the central couple
of degrees of our vision. I don't think any "megapixel" calculation makes much
sense given these facts.

[1] <http://en.wikipedia.org/wiki/Fovea>

[2] <http://en.wikipedia.org/wiki/Saccade>

------
Someone
More scientific, more compact list on more or less the same topic:
<http://white.stanford.edu/~brian/numbers/numbers.html>

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Symmetry
I'm not sure I agree with the methodology here. You can't take a person's
visual acuity at the center of their field of view and just extrapolate it to
the rest of what they can see like you can with a camera. Human vision varies
across where you're looking. The figures I've seen elsewhere based on more
physiological reasoning put the megapixel equivalent at close to 100
megapixels, and with only a region of about 5 megapixels actually in focus.

~~~
Dylan16807
But the point of these calculations is to say what resolution the thing you're
looking at needs to have. Which, unless it physically tracks the eyeball
somehow, needs to be as crisp as the center of vision throughout.

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yread
found this when looking for what's the closest two objects we can make out.
Apparently there are people who can distinguish 20 arc seconds which would be
two black points on white 15 micrometers apart from 10cm away. Which should be
almost enough for seeing bacteria.

~~~
jwingy
Fighter pilots? Serious question...

~~~
arethuza
Or short sighted folks - I have -6 in both eyes and at 10cm things do look
awfully sharp. Though more than 10cm away (without glasses/contacts) things
are rather fuzzy.

~~~
sukuriant
As another nearsighted individual (-4ish), I can attest to this. At short
range, without glasses or contacts, I have ridiculous vision, much tighter and
sharper than when I have my contacts in, again at very close range. Long range
is much less impressive.

~~~
ristretto
Short sightedness is a convergence problem. There's nothing wrong with your
fovea, so you still have the sharp vision when using glasses or looking close.

~~~
sukuriant
It's more than that. It's "I have sharper vision looking very close without my
glasses than with my glasses." I believe I heard it was a common issue.

~~~
ristretto
i do remember seeing very sharp closely before my lasik, but i assumed it was
because of reflections and diffusion through the glasses

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Volpe
I'm confused by the quote of number of pixels required to match the eyes
resolution. I don't see a definition of the size of a pixel in that
calculation? Though the whole thing seems way to thorough for them to have
missed that, so I'm assuming I misunderstood.

~~~
masklinn
> How many pixels are needed to match the resolution of the human eye? Each
> pixel must appear no larger than 0.3 arc-minute. Consider a 20 x 13.3-inch
> print viewed at 20 inches. The Print subtends an angle of 53 x 35.3 degrees,
> thus requiring 53 _60/.3 = 10600 x 35_ 60/.3 = 7000 pixels

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Synaesthesia
I don't quite get how he estimated the ISO equivalent of the eye - he compares
it to a camera taking 12 second exposures, whereas the eye is essentially
video, as he says, implying 1/15th sec exposures.

~~~
j-g-faustus
He also says that "at low light levels, the human eye integrates [light] up to
about 15 seconds".

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ristretto
That is a much better treatment than others i have seen. Although i think the
actual megapixels of the eye should be measured as the number of rod and cone
cells which is ~80 - 150 million. However the retina is smartly organized to
give greater acuity in the center that's why the effective megapixel is so
much higher. It's also remarkable that rod cells can detect even single
photons.

~~~
bl
True, a rod or cone cell (collectively, "photoreceptors") is the smallest,
indivisible detector unit in the retina, but in _normal_ visual processing
(i.e., daylight-lit scenes), they probably never operate in isolation. The
photoreceptor signals are immediately integrated by accessory cells in the
retina (horizontal cells, bipolar cells, amacrine cells) and the major output
neurons of the retina, the ganglion cells projecting to the thalamus, exhibit
"center-surround" sensitivity. Furthermore, any one photoreceptor can be used
in many receptive fields in the downstream visual processing pathway.

Thus, the mapping of "pixels" to the number of retinal ganglion cells is
probably less tenuous than a photoreceptor-to-pixel mapping. Now that I think
about it, perhaps an even better definition of a physiological pixel would be
a _functional_ measure: the number of distinct, electro-physiologically
measured center-surround fields in the thalamus. In that case, the effective
megapixel rating of a human visual system is only indirectly related to the
sheer numbers of photoreceptors, but more closely related to the wiring
pattern. This wiring pattern is much more difficult to experimentally measure
than simply counting neurons because it would involve flashing tiny,
contrast-y dots of light in front of a fixated mammal while poking an
electrode around in the thalamus. [This is outside of my main field, so it may
have been done, but I don't know the results.]

Two side notes of interest:

1\. Photoreceptors are oriented towards the rear of the retina and are
embedded in a dark sheet of cells called the pigment epithelium. The upshot of
this is that every photon that is involved in our visual perception has
traversed a tangle of bipolar cells, ganglion cells, and their associated
axons (it is easy to overlook this fact in diagrams, such as at
<http://en.wikipedia.org/wiki/Retina> , because it is usually only mentioned
textually in the figure caption). The fovea is relatively free of these light-
scattering objects and, in addition to a higher ratio and density of cone
cells, is why primate visual acuity is highest in the fovea.

2\. In extremely low light-adapted rod cells, the absorption of single photon
can trigger photo-transduction. Thus, our visual system has the capacity to
operate at the very limit of physics. If I recall correctly (i.e., no citation
on hand), this has even been experimentally demonstrated, although the
experiment must have been pretty demanding, what with photon shot noise and
all. [This last side note #2 is what I had in mind when I referred _normal_
visual processing in the first paragraph. Even though you might think that
this validates the photoreceptor:pixel metaphor, at best a human would
probably just report a tiny inkling of a flash in some general vicinity with
very poor spatial and temporal resolution.] Ah, I now see ristretto mention
that.

~~~
ristretto
In the end, it's wrong to try to determine how many "megapixels" the eye can
see, as its not a camera and everything before and after the optic nerve
perform a number of enhancements / processing of the signal, so what matters
in the end is what is perceptible and discriminable under specific lighting
conditions.

~~~
bl
I quite agree: I don't think megapixels will get us very far, if we could even
formulate a consensus definition. I much prefer a perception/discrimination
measure, as you said, and as it seems neuroscientists in the visual field have
settled on.

Clouding the issue even further is the fact that different areas of central
vision processing handle different features. Some areas are tuned to react to
points, some to bars, some to grids, some to movement, some to rotation, etc.
Perhaps it would be simpler to make the comparison in the other direction:
"How intricate do I have to make this visual scene to fully exercise the
perceptual abilities of grating-sensitive neurons in the Lateral Geniculate
Nucleus of the thalamus." That way, we could put some sort of upper bound on
the useful specifications of a pixelated display. We would then have to
iterate over all the known feature sensitivities (bars, grids, rotations,
etc.).

Display engineers surely must know that below such-and-such pixels per inch, a
screen can present any reasonable perceptible pattern. So outside of casual
interest, ristretto and I think that eye "megapixels" is relatively
meaningless.

