Resolution limit of the visual system

#12 01-20-2007, 05:02 PM

- quote -

> Dr Judy wrote:

> Are you trying to understand for fun or are you trying to design
> something that depends on human visual resolution

My interest is in photography and I´m trying to understand the
perception side better in order to make better use of the medium. So
it´s either not just for fun or professional but somewhere in between.

- quote -

> You can check out
> http://webvision.med.utah.edu/KallSpatial.html
> http://www.psych.ndsu.nodak.edu/mcco...%20acuity.html
> for more detail
> And if physics and monitor resolutions are interesting or
> understandable to you:
> http://www.psych.ndsu.nodak.edu/mcco...20analysis.doc
> http://www.isd.mel.nist.gov/US&R_Rob...tandards_1.pdf

Thanks a lot for the links! That´s quite interesting material for me.

Joseph Thomas

#11 01-20-2007, 04:15 AM

- quote -

> Well, it looks like if I would have to acknowledge that I´m still a
> bit off track. So I will try to get deeper into those detection and
> recognition tasks to gain a more fundamental understanding. Thanks for
> so far, Dr. Judy!

Are you trying to understand for fun or are you trying to design
something that depends on human visual resolution

You can check out

http://webvision.med.utah.edu/KallSpatial.html

http://www.psych.ndsu.nodak.edu/mcco...%20acuity.html

for more detail

And if physics and monitor resolutions are interesting or
understandable to you:

http://www.psych.ndsu.nodak.edu/mcco...20analysis.doc

http://www.isd.mel.nist.gov/US&R_Rob...tandards_1.pdf

good luck and happy reading

Dr Judy

#10 01-19-2007, 10:49 PM

- quote -

> Dr Judy wrote:

> Joseph Thomas wrote:
> > Well I think I got it by now. I was on a completely wrong track because
> > I assumed that the csf would be for an average human with 20/20 vision.
> > - I could have known better as this is nowhere stated on all the
> > websites I browsed. But sometimes you make presumptions and stick with
> > them without further thinking. That´s why all your words got lost in
> > the dark. - Thanks for your patience!
> I think the root of your confusion lies with the common assumption that
> 20/20 is "normal" or "average" vision. Many people, and most people
> without eye disease, achieve better than 20/20, 20/15 is likely closer
> to what would be "normal".
> The CSF you looked at may have been average CSF with average Snellen of
> 20/20. However, do not confuse average acuity with "average person".
> There is no "average person" but there are average results of measuring
> many persons.
> A actual person with an actual best vision of Snellen 20/20 may have a
> max CSF of 60 cycles. This is because the two tests do not measure the
> same thing, detecting a line is a different cognitive task that
> identifying a letter.

Well, it looks like if I would have to acknowledge that I´m still a
bit off track. So I will try to get deeper into those detection and
recognition tasks to gain a more fundamental understanding. Thanks for
so far, Dr. Judy!

Joseph Thomas

#9 01-18-2007, 07:01 PM

Joseph Thomas wrote:
- quote -

> Well I think I got it by now. I was on a completely wrong track because
> I assumed that the csf would be for an average human with 20/20 vision.
> - I could have known better as this is nowhere stated on all the
> websites I browsed. But sometimes you make presumptions and stick with
> them without further thinking. That´s why all your words got lost in
> the dark. - Thanks for your patience!

I think the root of your confusion lies with the common assumption that
20/20 is "normal" or "average" vision. Many people, and most people
without eye disease, achieve better than 20/20, 20/15 is likely closer
to what would be "normal".

The CSF you looked at may have been average CSF with average Snellen of
20/20. However, do not confuse average acuity with "average person".
There is no "average person" but there are average results of measuring
many persons.

A actual person with an actual best vision of Snellen 20/20 may have a
max CSF of 60 cycles. This is because the two tests do not measure the
same thing, detecting a line is a different cognitive task that
identifying a letter.

Dr Judy

- quote -

> Joseph Thomas

#8 01-17-2007, 10:07 PM

Well I think I got it by now. I was on a completely wrong track because
I assumed that the csf would be for an average human with 20/20 vision.
- I could have known better as this is nowhere stated on all the
websites I browsed. But sometimes you make presumptions and stick with
them without further thinking. That´s why all your words got lost in
the dark. - Thanks for your patience!

Joseph Thomas

#7 01-17-2007, 08:19 PM

Joseph Thomas wrote:
- quote -

> I just found a formula to convert between Snellen and spatial
> frequency:
> Snellen denominator = 600/cycles per degree
> or
> cycles per degree = 600/Snellen denominator
> so
> 600/60 cycles/degree = 10, Snellen = 20/10
> or
> 600/20 = 30 cycles per degree
> So is it safe to state that the resolution ability of an average human
> with 20/20 vision is 30 cycles/degree or 6,88 lp/mm? If it´s so I do
> still not understand where the 60 cycles/degree of the contrast
> sensitivity function come from.

The two measures are measuring different things. First of all, CSF
only asks the observer to detect a black and white grid, while the
Snellen requires recognition and identification of a letter, a task
that is more difficult to do and involves different areas of the brain.

Secondly, the average best corrected Snellen acuity is 20/20; however,
as an "average", it means that about 75% of people with healthy eyes
have a best acuity of between 20/15 and 20/25 and about 98% would be
between 20/10 and 20/30. So the "average" is 20/ 20 but some people
see better. The constrast sensitivity function is the maximum seen by
any observer, so it's maximum value is equivalent to about 20/10.

The two measures are not really comparable and are useful for different
reasons. If you are designing a medication bottle label and want to
make sure most people can read it, you would use a Snellen size of at
least 20/30. If you are designing something with a grid and want to
make sure most people don't "see" the lines, then you would look at CSF
and make sure the cycles/degree is greater than 60.

Dr Judy

#6 01-17-2007, 01:18 PM

Dear Joseph,

Subject: Contrast versus two points of light.

Re: Consider a star. The Airy disc of a star is somewhere around 20
arcseconds in size to the human eye. That's the same for every star,
because all stars are point sources from this distance. That's just a
third of an arcminute, yet the contrast between the diffraction disc
and surrounding sky make bright stars plainly visible.

It was ASSUMED that the AVERAGE human could resolve
two points of light separated by 1 minute of arc.

But this is a gaussian distribution of the ability of an
eye "fully corrected" to separage two points of light.

This resolution is not necessarily "contrast".

This 1 minute of arc resolution was taken by Snellen
as the AVEAGE visual acuity that a large percentage
of eyes could have with a "corrective" lens.

As Judy stated, some people have the resolution
of 1/2 minute of arc, or 20/10 vision -- but that
is exceptional. (Part of the "tail" of
a gaussian distribution.)

The Snellen letters are 5 minute-of-arc wide, with
the assumption that this is verifies 1 minute of arc
resolution.

Best,

Otis

Joseph Thomas wrote:
- quote -

> > Mike Ruskai wrote:
> > > On or about 16 Jan 2007 15:56:56 -0800 did "Joseph Thomas"
> > > <joseph.thomas[at]gmx.net> dribble thusly:
> > > how can someone with 20/20 vision resolve better than 1 arcminute? -
> > > Even more confusion! What do I overlook?
> > Contrast?
> So ist it really the case that a) the contrast sensitivity function has
> been measured with people who have average 20/20 vision and that b) the
> ability to resolve cycles per degree rises as the contrast rises but
> that 60 cycles/degree are only possible in situations where contrast is
> unreasonably high, as Marc statet further up?
> Joseph Thomas

#5 01-17-2007, 09:26 AM

- quote -

> Mike Ruskai wrote:
> > On or about 16 Jan 2007 15:56:56 -0800 did "Joseph Thomas"
> > <joseph.thomas[at]gmx.net> dribble thusly:

> > how can someone with 20/20 vision resolve better than 1 arcminute? -
> > Even more confusion! What do I overlook?

> Contrast?

So ist it really the case that a) the contrast sensitivity function has
been measured with people who have average 20/20 vision and that b) the
ability to resolve cycles per degree rises as the contrast rises but
that 60 cycles/degree are only possible in situations where contrast is
unreasonably high, as Marc statet further up?

Joseph Thomas

#4 01-17-2007, 03:16 AM

On or about 16 Jan 2007 15:56:56 -0800 did "Joseph Thomas"
<joseph.thomas[at]gmx.net> dribble thusly:

- quote -

> how can someone with 20/20 vision resolve better than 1 arcminute? -
> Even more confusion! What do I overlook?

Contrast?

Consider a star. The Airy disc of a star is somewhere around 20
arcseconds in size to the human eye. That's the same for every star,
because all stars are point sources from this distance. That's just a
third of an arcminute, yet the contrast between the diffraction disc
and surrounding sky make bright stars plainly visible.

The same effect can be seen with bright lines on a dark background and
dark lines on a bright background. As the contrast between the line
and background increases, the width of the line necessary for
detection goes down.

How much of this is the result of optics, and how much the result of
processing in the brain, is another question.
--
- Mike

Ignore the Python in me to send e-mail.

#3 01-16-2007, 11:17 PM

I just found a formula to convert between Snellen and spatial
frequency:

Snellen denominator = 600/cycles per degree
or
cycles per degree = 600/Snellen denominator
so
600/60 cycles/degree = 10, Snellen = 20/10
or
600/20 = 30 cycles per degree

So is it safe to state that the resolution ability of an average human
with 20/20 vision is 30 cycles/degree or 6,88 lp/mm? If it´s so I do
still not understand where the 60 cycles/degree of the contrast
sensitivity function come from.

Joseph Thomas

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