Hi!
I need to discover the approximate FOV for a single eye, gazing straight ahead. Vertical and horizontal, in degrees.
Thanks in advance for your replies.
Barry
Hi!
I need to discover the approximate FOV for a single eye, gazing straight ahead. Vertical and horizontal, in degrees.
Thanks in advance for your replies.
Barry
95o Out
75o Down
60o In
60o Up
about 12-15o temporal and 1.5o below the horizontal is the optic nerve or blind spot which is roughly 7.5o in height and 5.5o in width.
(these figures use a 3mm white target on a perimeter of 330o if the target was increased the temporal can be pushed to about 110o)
The Ophthalmic Assistant - Stein, Slatt, Stein pg.395
Most perimetery is performed on the central 30o and a post yesterday from Darryl confirmed that 30o is used in lens design because apparently beyond this their seems to be head movement along with rotation. Also the first ANSI required off axis powers to be within a set standard for 30o of rotation.
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Thnaks, Harry!
Barry
You can deduct a little for big noses or highly inset eyes. Some from top with droopy lids. This like everything else in this racket varies with the patient.
You might even add a little for big eyed girls.
Chip
Hey, and for an anatomical perspective, notice this:
Notice that the macula is central, and fifteen visual degrees nasally is the optic n. (physiologic blind spot). Notice that the main retinal "arcades" of the superior and inferior branches of the central retinal artery and veins pretty much circumscribe 30 degrees. That's a working definition of the "posterior pole" of the eye.
Pathological changes inside the arcades are considered more serious than elsewhere, because it's functionally "the central vision" (although not "dead center" which of course is the foveal 5 degrees or parafoveal 10 degrees).
The central 30 degrees of vision is approximately the size of a paper plate held at arm's length.
Notice from Harry's post, the total horizontal field is 150 degrees and vertical field is 130 degrees (average it to 140), so the central part of our vision is thus loosely defined as about 20% and the peripheral vision is loosely defined as about 80%. (Another 80/20 rule in life.)
Hey Doc,
Is that some Drusen I see in that picture?
;)
Harry, this is IMHO a bit misleading. The +/- 30° for lens design is based on the idea of eye rotation (already), but no head movement. If you consider a fixed eye only (as I assume this thread is all about), you could significantly increase the tolerances for off-axis astigmatism at such large angles without anybody noticing, because the off axis resolution of the fixed is PRETTY poor, only our "built-in" automatic image scanning and composition "firmware" we all have creates the illusion of a large, clear field of view. (That most detailed viewing is only done at the eye optical axis also explains why the lateral color of the eye is insignificant compared to lateral color of the eyeglass, while both have a similar effective nue)
The visual acuity vs. field angle of a fixed eye is approximately a power law, I remember a nice diagram in a book that I NOW have handy, which says: radius for 0.5 visus (20/40 vision): 2 degrees, 0.1 visus (20/200): 20 degrees, 0.05 visus (20/400): 30 degrees, 0.02 visus (20/1000): 50 degrees. (This is from Warren J. Smith, Modern Optical Engineering, but the original, uncited source is likely a different one)
Sincerly
Last edited by xiaowei; 03-07-2008 at 09:51 AM. Reason: Update for acuity vs. field numbers
I think we may have misunderstood one another, the 30o is like you mentioned without head movement, from what I understood the post iluded to and I have read in a few places I can look them up at some point that beyond 30o we start to move our heads. For example if we were to try and view an object 95o in our periphery the head would more than likely rotate lest just say for example 65o and the eyes would rotate 30o of course if we were to try and focus on that object we would continue to rotate the head untill our gaze was directly upon the object with next to no rotation of the eye. I think I understand what your saying that the fovea is a small area where 20/20 or better vision occurs and as we get farther from here, larger agles; the density of cones is lacking so the visual acuity drops as well.
Man xiaowei, your seem to be on point with your responses. Was your PHD in optics or another area of physics?
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Yes, the mysterious Xiaowei is very good...
Thanks for the good math, Harry.
So, the central 30 degrees of vision (that we are trying to conceptualize) will be "serviced" by a circular section of the spectacle lens that is about 11mm in diameter from the MRP (when looking straight ahead, say, driving down the road staring at the horizon). That's less than a dime.
So, let's use the radius of 5 mm, to keep it simple. That means if a seg line, or a progressive corridor encroached on this "personal space" it would be probably rather distracting. Anyone fit segs that close to the pupil? Maybe in a TF...What about where the progression begins in a lens? Zeiss is said to have begun the progression 4 mm below the fitting cross...everyone else is 2 mm, right? I know I fudge it downwards by 1mm routinely...
What about scratches on a lens? If inside the "central dime", they would visually bother people the most, I'd say.
Another application of the very good "dime" construct: how does a dime compare to a cut out and mounted bifocal segment? Is the seg only a paltry 9 mm tall? That will not be perceived as a "full field of view". You'd need about half of a Round 22, or somewhat less than half (40%) of the D28.
What about a 7 mm tall intermediate seg? Doesn't meet the dime criterion...you'd need those 14x35's if you want to spend any serious time on the computer.
What about the rough area of the near zone of a short-corridor progressive? Is it roughly 3.1416 x 5 mm = 16 mm2 in area? (Who knows on that one...?).
One final application of the "dime"...how can IZON optimize a lens with numerous dime-sized packets of optics? Can't believe that one, easily...
30 degree's both ways is more a quarter and I think that's what were using, but I get your point.
As for the IZON, unless the lenses were always stationary (ie they moved with the eye) they can only really be optimized for one positions of gaze I would assume straight ahead, thats nto saying that this would be the only spot but any viewing off axis would enevitably be a comprimise as they would have to blend that optimization I would think.
As for the scratch my favorite pair of glasses got hit with a screw right in the center a week after I go them, I have been wearing them for 4 years now and that scratch is still there, I don't have the ability to focus close enough for it to be a problem to me.
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