gewoodfo,

"Borrowing" from a prior post of my own:

(http://cr4.globalspec.com/comment/73269)

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From http://www.clarkvision.com/imagedetail/eye-resolution.html)

"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. We also have two eyes, and our brains combine the signals to increase the resolution further. We also typically move our eyes around the scene to gather more information. Because of these factors, the eye plus brain assembles a higher resolution image than possible with the number of photoreceptors in the retina. So the megapixel equivalent numbers below refer to the spatial detail in an image that would be required to show what the human eye could see when you view a scene.

Based on the above data for the resolution of the human eye, let's try a "small" example first. Consider a view in front of you that is 90 degrees by 90 degrees, like looking through an open window at a scene. The number of pixels would be

90 degrees * 60 arc-minutes/degree * 1/0.3 * 90 * 60 * 1/0.3 = 324,000,000 pixels (324 megapixels).

At any one moment, you actually do not perceive that many pixels, but your eye moves around the scene to see all the detail you want. But the human eye really sees a larger field of view, close to 180 degrees. Let's be conservative and use 120 degrees for the field of view. Then we would see

120 * 120 * 60 * 60 / (0.3 * 0.3) = 576 megapixels.

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Just my $0.02...

going way back to high school biology, it differs, depends on rods and cones in the eye? ones color the other is resolution. and field of vision is also taking into account not to mention and health issues such as stroke and the such.

sorry no hard data just (ir)relevant info

I believe the eye has a relatively small area of hi-res at the macular...peripheral vision and that between the two is more attuned to picking up movement aand general shape/colour.

That's why are eyes move as we scan a page....

Del

Its normal? Really? Do we all have them? How did they determine that you had blind spots??

Everybody has one blind spot in each eye, where the optic nerve exits at the back.

The is a well known optical illusion that shows this.

A spot and a cross a few inches apart drawn on a piecs of paper ( . + )

With one eye, you stare at the spot and as you bing the paper towards you the + will vanish.

Del

PS.

It only works one way round...can't remember which... e.g Use Right eye for . +

Left eye would be + . or vice versa... I'm sure you'll be able to work it out.

they position you to look straight ahead into a white sphere. then they have light blink on and off through out the sphere. and you acknowlege if you see it as they blink.

kinda low tech

The doctor checked and said it was not an uncommon condition and not a problem unless I started seeing double.

People tend to have a dominate eye, I have heard that also.

Guess I am just wondering whether (within reason) HI-RES or a greater pixel density makes any difference.

That is interesting, Del touch off with this on one of his posts (#22). I'll tell you something that I experienced with this.

I was managing the CNC shop at a shipyard. and were working on the CNC routers.

We were cutting alum. and even though we had vacuum to carry away the alum. shavings, we had quite a large amount on the floor. Area approx 10' x 10' x approx. 6" deep of alum. shavings.

when one of the routers heads went down. (even though these were one of the heaviest and largest routers on the market. 5 head, 4 drill packs at the time) cutting alum. can be hard on the routers (especially when your pushing it), and at times we had to replace an item on the head.

to get at this item, we had to remove special screws (not off the shelf type) Approx. 3/16" long.

And damned if we did drop it in the pile of shavings. It was lost,....so I thought. and it was, we looked and could not find it.

One of the router operators which was helping , told us to back away, he looked at the pile of shavings from about 5 to 10 feet away from the pile. And if he didn't walk up to this pile reach in and picked up the screw. I never would have believed it if I didn't see it.

He told me he was colored blind. but he could pick out objects very easily. He went on to tell me. that in the military people like him are good at recon. because they can pick out shapes because camo color does not hid from color blindness.

It has something to do with the rods and cones in your eyes. ones for color the other for resolution (don't know which) And when you have less of one, you tend to have more of the other.

so there is a difference, but its seems to be a compromise

Why not go to machine vision?

Hi all: Thansk for the info!

TVP$%, I think raised the question of why not use machine vision. That may be a good solution but still need to try and get a read on the human eye's resolution. The reason i believe this is important is that the standards say that when grading a coin, the grader should do it with no more than 5x maginification. Now, to my non technical brain, if I had a system that could dintinguish far greater detail (higher resolution) than the human eye, then my system would be prone to undergrading coins. That would not bea good thing for the collector. Conversely of that, if the system was less capable that the human eye, then coins would be overgraded.

Either way, I beleive that the key to this is having a good solid way of defining the human eye's resolution at a distance of 12 - 16 inches for a object not to exceed 2" in diameter in a well light environment.

Again, thanks for the input!

You will only ever get a statistical norm with huge variation. Are you just talking monochrome?

A few years ago I worked on automating the Farnsworth Munsell 100 hue test..There are boxes of coloured discs..ech differing from the next by the smallest percepible difference......My Mum (in her 80s) could do it perfectly! I, like many males, have some red/green deficiency and was hopeles at it.

Del

From Physics Today on line:

New literature

Included in Edmund Optics' e-newsletter of 18 September 2007 are three tips for machine-vision success. The first Tech Tip describes the relationship between resolution and contrast, the second looks at the effects of different illumination profiles, and the third discusses how to find the right lens. Edmund Optics Inc, 101 East Gloucester Pike, Barrington, NJ 08007-1380, http://www.edmundoptics.com

If you want to measure YOUR eyes' resolution directly, it is relatively simple to get and print out a resolution test chart on any good quality ink jet printer and test it.

The word "resolution" is misused by digital camera and sensor manufacturer's. A camera's resolution is ALMOST ALWAYS less than that predicted by simply counting the number of pixels per inch or per mm. The pixels only indicate the theoretical maximum possible resolution. Ususally the lens is the limiting factor.

A good source on the practical testing of camera resolution is at http://www.imatest.com . I suggest going to that site and entering "resolution" in the search box. You will find much on the subject, including how to print out test charts. You can use these charts to test your eye's resolution as well as your camera's.

I also recommend the Imatest software. You can download a free trial from the site.

Resolution is measured in line pairs. It can be defined in linear units such as line pairs per millimeter or line pairs per inch, or it can be defined in angular terms such as line pairs per degree or line pairs per radian (sometimes you'll see the term cycles used instead of line pairs.

The principle is that the limiting resolution of an optical system (in this case the eye) can be determined by attempting to resolve a series of parallel lines, alternating dark and light, of a specific size. The smallest pair of lines that can be resolved indicates the limiting resolution. For the human eye with 'normal' vision (20/20 or a visual accuity of 1.0), this is about 30 to 60 linepairs per degree.

What you need to do is establish the distance from the eye to your 2" object, so that you can determine the object's angular size. For convenience let's say it is several feet away so that it has an angular size of 1 degree. Therefore a human with normal vision should be able to resolve at least 30 line pairs. Since it takes two column of pixels to create a line pair (one column all bright and the other all dark), this would mean you could resolve 60 pixels in a 2" area, or 30 pixels in a 1" area. Obviously these would be very large pixels, but this is the general principle.

To summarize: (1) limit of the human eye is ~30 line pairs/degree (2) it takes two pixels to create a line pair.

Remember however that this is somewhat simplified. The reason that resolution is limited is that as the line pairs (pixels) get smaller, the contrast decreases. This is because the bright lines and dark lines blend together. The human eye can respond to fairly low contrast levels. Assuming that the brigh lines start out at 100% 'bright" and the dark lines ar 0% 'bright', the maximum limit of the human eye will occur when the bright lines are about 53% bright, and the dark lines are about 47% bright, a contrast difference of 6%. In the early 50's the Navy did studies that indicate humans can resolve contrasts as low as 3%, but these studies were done in a darkened theater, where the subjects had an hour or more to adjust to low light levels.

The important factor here is that external lighting and many other factors (did the viewer eat his/her carrots?) will have an impact on limiting resolution, and make an absolute judgement on this question extremely complicated. I believe however that the 30 linepairs per degree is conservative enough to work for your purposes.

Easy enough to measure your resolution...stick a pin against a contrasting background and see how far away you can go and still see it. Take the measurements and do the arithmetic.

Del

Hmm - just for once I think you have strayed too far out-of-field.

The ability to see an object against a contrasting background is affected by resolution, but is certainly not a way to measure it.

To give an example of where this is the case: if the background is large and black and the object is pale, the amount of light that the object is reflecting will dominate whether you can see it or not. The reason for this is that once the object is smaller than your ability to resolve it, the light from the object will (at any time) be falling within the minimum resolution capability of your eye, at which point its size is unimportant.

As stated by jonfotl, the least unreliable method for determining resolution is to check when a grid with varying spacing becomes visible; even then, you will need to control the contrast and the conditions of measurement if you are to obtain reasonably consistent results (which will also very between individuals, of course).

Regards

Fyz

To give an example of where this is the case: if the background is large and black and the object is pale, the amount of light that the object is reflecting will dominate whether you can see it or not.

Right so that's how you can see objects you can't see....
Actually I was more assuming a pin silhouetted against a light background...

This seems to cover it...

You seem to have willfully assumed I was suggesting stupid conditions under which to try and measure resolution....(dunno why )...basically one is trying to measure an angle under some sort of representative and controlled conditions...there are many variables, as the article explains.

Del

Yes, that's a pretty good article.

It even covers the case of your single pin with the following phrase:
"Again you need two pixels to say it is not a point".

That is to say, resolution is the ability to distinguish the presence of two objects, rather than the ability to detect whether something is present in the first place.

No, I wasn't "wilfully assuming stupid conditions", just finding an obvious example to illustrate why this wouldn't be a sensible way to measure resolution. I think you have an electronics background, so I'd say it was like trying to measure the frequency response of an oscilloscope by finding the minimum pulse width you can detect - you can easily see that this has as much or more to do with pulse energy and noise levels as with the width of the pulse.

Returning to your "light background". Your ability to see the pin would be determined by the level of contrast you can detect in a single pixel. If the pinhead was (say) black, you would be able to detect its presence when the contrast had dropped to a relatively small fraction of the 100% - if it was (say) 1.5%, the pinhead would at this point subtend in the order of 1/8 of the resolvable separation. Yes, for a given observer and lighting level and... you would be able to assess a relationship between the resolution and the point at which you lose the ability to detect the presence of the pin - but it's still the pulse height that is important rather than the size of the pin.

For corrected vision (i.e. correctly prescribed spectacles if required, cataract-free, etc) 100% contrast lines gives reasonably repeatable results under a sensible range of lighting conditions. That would scarcely apply to the use of a single pinhead (or worse a linearly oriented pin), even if perfectly black against a well-lit background.

BTW, even when using a grid for this measurement the light must not be so bright that your pupil closes down below about 0.8-mm diameter.