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Qosmius
Up and Coming
October 25, 2013

Want help from an optometristudent? ask me in this thread

hi, i see alot of different threads here regarding eye sight and such..if anyone is interested and have any questions about it. pls tell them to me and i will try to answer them as accurately as i can and try to help out...thou im just a second year student i think i will be able to help most of you guys and girls



Mvh Qos

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    QosmiusAuthor
    Up and Coming
    October 28, 2013
    "Fredz" wrote:
    "Qosmius" wrote:
    you seem to answer yourself in another question..if you already have a model that uses 1 nodalpoint just use that one :P
    As I've already said in my previous messages, I want to find a model that uses the real size of the human eye, not a reduced model with a non-realistic size. I don't know if such a model does exist though, I guess not considering your answers and what I've read so far.

    you will never be able to find a real sized eye by looking at those standard eyes...every eye in every human will have different values...they are just a sample of the perfect eye, if all the paramterers would be perfect that are those numbers you would get...in real life the world is not perfect. even if you find the size of your own eye...every other eye will vary in some sort of way

    "Qosmius" wrote:
    i do not know if there are more reduced eye models, atleast we havent learnt anymore than 3 standard models that is widely known, there is no point in knowing more than them
    Yes, I can understand that it would be a waste of time for an optometrist. I did find several references to other schematic and reduced eye models but they're in research papers and behind paywalls unfortunately.

    "Qosmius" wrote:
    if you want to know more about reduced eye models maybe you should talk to someone who is taking a master or doctor since those are mostly used for science i believe, they have not much practical use
    Yes that would probably be the best route I guess, your help has been useful though, thanks.

    "Qosmius" wrote:
    what are you going to use those numbers for anyways?
    At first it was only to be able to calculate the FOV more exactly in DIY HMDs and to design algorithms for image correction, because I thought using the center of the eye or the pupil position wasn't correct (which I now know to be true). But then it evolved into a need to know more about the subject because I don't like it when I can't answer a simple question. :)

    the eye is not a simple question my dear :P there are still scientists today that uncover new discoveries from time to time that affects what we know about the eye :)

    I guess for FOV calculation the reduced eye models would be enough, but I need to read more papers to know why the position of their nodal point differs. If the models were all representative of a human eye the values should be identical or at least quite close.


    but why do you need to calculate the Field Of View? just set it to a standard and let the user adjust it by needs...IPD is important for this if you want to look through lences.. but other than that i think normal values are approximately a bit over 190 degres..the values are not identical i can assure you that :) let me give you an quick example.. Person 1: 18mm from back to front and + 68 dioptries strenght might be perfect for this eye.. Person 2: 26mm eye and 56 Dioptries might be perfect for this eye....Person 3: 22.22mm and 55dioptries means that this person has eye sight issues and needs some sort of correction.. every paramter will vary from each time. but if they match each other perfectly the person still has perfect eyesight
    tomf
    Participant
    November 27, 2013
    From a practical point of view (pun intended!) the center of projection of the "average" eyeball is at its apparent pupil, which is 11mm in front of the center of eyeball rotation, and about 2mm behind the surface of the cornea. This is the model we use for most of our FOV and eye-relief calculations, and it seems to be fairly robust as long as you don't need accuracy below 1mm - which for most applications seems to be good enough. Since eyeballs vary by about that much anyway, asking for higher accuracy is probably just kidding yourself, unless you're going to take a micrometer to every user's eyes!

    I think it is a marvel of biological engineering that although the size of the rest of our bodies ranges dramatically, and even the distance between our eyes changes a lot, the radius of the human eyeball is extremely consistent across human heights, sexes and ages. It certainly makes my life considerably simpler.
    Tom Forsyth - OculusVR coder
    Fredz
    Up and Coming
    November 28, 2013
    "tomf" wrote:
    From a practical point of view (pun intended!) the center of projection of the "average" eyeball is at its apparent pupil, which is 11mm in front of the center of eyeball rotation, and about 2mm behind the surface of the cornea.

    In this article it's said that :
    An incident ray directed toward the first nodal point will appear to emerge from the second nodal point with unchanged direction. [...] The eye is a complex series of refracting surfaces that forms an image in vitreous of an object in air. Therefore, it has a pair of principal planes and a separate pair of nodal points that can be used to represent it.

    To me that's basically the definition of the center of projection, and it seems clear that it's located at the first nodal point and not at the center of the apparent location of the pupil, which is confirmed by several eye models :
    - 7.078 mm from the front surface of the cornea in the Gullstrand's schematic eye (17.309 mm from the retina) ;
    - 5.7 mm from the front surface of the cornea in the Listing's reduced eye (17.2 mm from the retina) ;
    - 5.55 mm from the front surface of the cornea in the Emsley's standard reduced eye (16.67 mm from the retina).

    Previously I had read everywhere (on Wikipedia and in various articles about photography) that the center of projection was the center of the apparent location of the pupil and I tried to find a definitive answer about its location. But I couldn't find any and I started to read more advanced research papers about the eye, which weren't talking about the entrance pupil anymore but about nodal points.

    And it makes sense to me that the center of projection can't be located just behind the cornea. If it was the case it would mean the crystalline lens doesn't have any influence over the refraction of incident rays toward the retina, which can't be true since its refractive index is different from the one of the vitreous humor.

    I initially thought that there was too much inconsistency between the estimated location of the nodal point in the different eye models, but it was because I was considering the distance of the nodal point from the front surface of the cornea. The location of the cornea can't be similar to the one found in an in-vivo eye for the creation of a reduced eye model.

    And in the fact the distance between the first nodal point and the retina is very consistent in the various eye models that have been constructed over the centuries (16,9895 mm ± 0.3195 mm), so that reinforces me in thinking that this measure is indeed valid.

    "tomf" wrote:
    This is the model we use for most of our FOV and eye-relief calculations, and it seems to be fairly robust as long as you don't need accuracy below 1mm

    The problem is that I don't see any way to be sure that the calculation of the location of the center of projection is correct, that's why I've been trying to find a definitive (ie. scientific) answer about its location.

    "tomf" wrote:
    I think it is a marvel of biological engineering that although the size of the rest of our bodies ranges dramatically, and even the distance between our eyes changes a lot, the radius of the human eyeball is extremely consistent across human heights, sexes and ages. It certainly makes my life considerably simpler.

    That's exactly the reason why I started searching a bit about this, to me it seemed that this consistency could help giving definitive calculations in the case of emmetropic eyes. I still need to read more recent papers to see if the numbers I've collected are still valid today with the more advanced tools that are available.
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