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Medical Forum / General / Vision / July 2005Questions on human vision.
Dear members: I would greatly appreciate your help in providing me either answers or suggestions on where to find information on the following questions: 1. Resolution of human vision - I have read the the human eye is capable of resolving two points or dots as long as these are placed at a minimum distance from each other. What is this distance ? And does the distance between the observers' eyes and these two dots affects these distance or resolution ? 2. Minimum focusing distance of the human eye - What is the average minimum focusing distance of the human eye ? Does it vary from stereo vision (both eyes trying to focus on the same object) to mono vision (just one eye trying to focus) ? 3. Angle of vision of the human eye - What is the horizontal angle covered by human vision ? And what about the vertical angle of coverage ? My understanding is that 50 mm lenses are called "normal" lenses because they approach the angle of coverage of human vision ? Is this correct ? Thank you in advance for your help. Any suggestions of books and articles (web sites as well) where I can find more detailed information on these topics will be very appreciated. Best regards,  SignatureJoseph Chamberlain, D.D.S. Oral and Maxillofacial Surgery > Dear members: > [quoted text clipped - 7 lines] > between the observers' eyes and these two dots affects these distance or > resolution ? The minimum for recognizing an object is generally accepted to be be 1 minute of arc as the size of the critical detail, although many people see better than this. Since it is an angle, not a distance, the distance between the two dots varies with the distance that the eye is from the dots. Basic trig gives the answer. The colour and brightness of the dots affects it, as does the amount of contrast between the dots and the background. Pupil size can also affect it. The eye can discriminate the offset between two lines (vernier acuity) better than between two dots. And it can detect very small objects as existing or not existing-- at about 1 second of arc, though not with object recognition -- this may be equivalent to your "two dots" question. > 2. Minimum focusing distance of the human eye - What is the average > minimum > focusing distance of the human eye ? Does it vary from stereo vision (both > eyes trying to focus on the same object) to mono vision (just one eye > trying > to focus) ? Do you mean how close can one see? This varies with age, decreasing a little bit every year from birth. And the other factors listed in answer 1 also apply. Generally people see better with both eyes open than with only one eye. > 3. Angle of vision of the human eye - What is the horizontal angle covered > by human vision ? And what about the vertical angle of coverage ? My > understanding is that 50 mm lenses are called "normal" lenses because they > approach the angle of coverage of human vision ? Is this correct ? Roughly 180 degrees horizontal, a little less vertical with both eyes open and looking straight ahead. Can be limited in specific individuals by nose, brow and cheek size. I not up on photography terms. > Thank you in advance for your help. Any suggestions of books and articles > (web sites as well) where I can find more detailed information on these > topics will be very appreciated. If you are near a university library, books on human visual perception in the psych section, on physiological optics in the optometry section and on human sensory systems in the science section can help. Ask the librarian for help. What do you need the information for? Knowing why might help us give you the information you want. See below for a discussion of acuity. http://www.brad.ac.uk/acad/lifesci/optometry/resources/modules/stage1/pvp1/Acuit y.html Dr Judy > Best regards, On 7/5/05 8:39 PM, in article GuGdnXcW7dUSz1bfRVn-pQ@rogers.com, "Dr Judy" <mpace99nospam@rogers.com> wrote: >> Dear members: >> [quoted text clipped - 60 lines] > >> Best regards, Dear Dr. Judy: Thank you for your detailed reply to my post and questions. The information is very helpful and will follow your advice (do some research at the local library). Is there any specific text you would suggest that could be considered as today's "Bible" of either Optometry or Ophtalmology and where this information could be found well described in detail and with good illustrations ? An atlas-style text with color illustrations would be helpful as the illustrations may help visualize it better as well as contribute to a presentation I am currently working on. Thank you in advance for your help, Joseph SignatureJoseph Chamberlain, D.D.S. Oral and Maxillofacial Surgery major snip > Thank you for your detailed reply to my post and questions. The > information [quoted text clipped - 7 lines] > helpful as the illustrations may help visualize it better as well as > contribute to a presentation I am currently working on. To who and what topic are you presenting? Visual acuity and field of vision is an enormous topic; if you are presenting to a lay audience interested in day to day functional vision as opposed to the detail vision researchers must keep in mind, you can keep it much simpler. Atlas style eye texts with illustrations are almost all about disease and seldom go into great detail about the limits to human visual perception; field of vision texts will be mostly about disease as well. If a lay audience is your target, you might be best served to look at past issues of Scientific American, Nat Geographic, Discovery mag, Pschology today and so on. Stephen Pinkers "How the Mind Works" has some interesting bits about vision and perception. The Reference Librarian at your local library may be of great help. Dr Judy > Thank you in advance for your help, > > Joseph > The minimum for recognizing an object is generally accepted > to be be 1 minute of arc as the size of the critical detail, Agreed for medium contrast (10:1). One inch at 100 yards or 0.10 mm at 400 mm. Lower contrast is lower. > The eye can discriminate the offset between two lines > (vernier acuity) better than between two dots. And it can The stander vernier is 0.05 mm at 500 mm distance, about 1/3 minute of arc. > detect very small objects as existing or not existing-- at > about 1 second of arc, though not with object recognition -- > this may be equivalent to your "two dots" question. The classic hi-res case is power lines around 3 seconds of arc. But only when seen against a very uniform background/sky? > Do you mean how close can one see? This varies with age, > decreasing a little bit every year from birth. And the other Yep. And myopes have an advantage here :) -- Robert > 2. Minimum focusing distance of the human eye - What is the average > minimum > focusing distance of the human eye ? Does it vary from stereo vision (both > eyes trying to focus on the same object) to mono vision (just one eye > trying > to focus) ? Judy gave good answers. As long as refractive error is neutralized and the eye is healthy, amplitude of accommodation is usually about the same in each eye and binocularly. Accommodation in each eye comes from shared innervation, so monocular vs binocular makes little difference in "near point of accommodation". Sometimes the inverse of the distance is useful, called "amplitude of accommodation" in diopters -- NPA 3 cm = 33D. Illumination (pupil size) can cause measurements to vary widely. Accommodating stimulates convergence, so measuring accommodation alone is only half the story. Many people habitually overaccommodate, often to compensate for exophoria. -MT, OD With respect to the resolution question, it should be mentioned that there are both optical and neural limits to resolution. If the optics are corrected using adaptive optics, the resolution is found to improve by a factor of about 2. This corresponds to the distance between adjacent photoreceptors, as I recall, something on the order of 20/8 in Snellen terms. DrG > With respect to the resolution question, it should be mentioned that > there are both optical and neural limits to resolution. If the optics > are corrected using adaptive optics, the resolution is found to improve > by a factor of about 2. This corresponds to the distance between > adjacent photoreceptors, as I recall, something on the order of 20/8 in > Snellen terms. Ultimately, resolution is diffraction-limited, just as there is a limit to the resolution you can get with a telescope of a certain aperture. The figure given above is about what it is, a little less than 30 seconds of arc. It is interesting to note that the retinas of some people will support resolution to the diffraction limit, though others will not. SignatureRobert T. Kopp http://analytic.tripod.com/ On 7/6/05 9:31 AM, in article pan.2005.07.06.16.31.14.344000@hevanet.com, >> With respect to the resolution question, it should be mentioned that >> there are both optical and neural limits to resolution. If the optics [quoted text clipped - 8 lines] > arc. It is interesting to note that the retinas of some people will > support resolution to the diffraction limit, though others will not. Thank you Robert, Dr. G and Mike for your replies. I appreciate the information you have shared and will use it to aid in further research. Best regards, Joseph SignatureDr. Joseph Chamberlain, D.D.S. Oral and Maxillofacial Surgery For those of you looking for reference texts, look here first: www.bestwebbuys.com/Optometry-N_10023583-books.html On 7/5/05 7:59 PM, in article BEF09724.160B8%drjchamberlain@earthlink.net, > Dear members: > [quoted text clipped - 22 lines] > > Best regards, The Optical Society of America's Handbook has some such information. The MIL-HDBK-141, may be old but still very useful. It probably can be found somewhere on the Web. Bill |
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