Refocusing your eyesight

[Kristofer_02]

I am currently working on my thesis in navigation at sea, and a major part of my discussion relies on our ability to perceive objects at sea. I am desperate (as the due date is approaching rapidly) to know how long it takes for your eyes to adapt from fixing on an object up close (closer than 0.5m) to then switch to looking for something further away, anywhere from 200-2000m. I need either an answer from a medical specialist or a reference to some literature that could give me an estimate of how long this process takes. Any help or pointers would be massively appreciated. Thank you.

[JohnHaganMD]

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[SoftwareDeveloper]

I should note that I took a quick glance out of curiosity about whether there was data out there. However it isn't clear to me why the extremely fast change of focus of eye's would have much relevance in terms of navigation at sea. I can see it being relevant in the world of fighter jets and in general the military dealing with hitting targets, but not in terms of much related to the sort of speeds involved in navigation at sea.

[SoftwareDeveloper]

That is an interesting question. To give you a start on searching for info, the process of changing focus is called accommodation.

I don't have time to search for more info, but out of curiosity I found some representative hits on related work below to give you a start in a quick search. It seems that some folks in the military world are interested in the issue of times to shift focus, and that some studies that might give a clue seem to be focused on the change of time to accommodate as we get older. The first paper gives one representative results. A couple of the results are just the abstracts, I hadn't checked for ungated versions of the papers.


https://www.diva-portal.org/smash/get/diva2:827043/FULLTEXT01.pdf
"The mean reaction times when changing focus from Near-to-Far and from Far-to-Near within viewing condition and group are displayed in Fig 4."

http://www.sciencedirect.com/science/article/pii/S0042698900002820
"Dynamics of the accommodation response to abrupt changes in target vergence as a function of age"

http://www.ncbi.nlm.nih.gov/pubmed/2818399
"The time required for U.S. Navy fighter pilots to shift gaze and identify near and far targets."

http://www.ncbi.nlm.nih.gov/pubmed/2710507
"Speed of accommodation and age"

http://onlinelibrary.wiley.com/store/10.1111/j.1444-0938.2008.00256.x/asset/j.1444-0938.2008.00256.x.pdf?v=1&t=iod55ita&s=00fbb8a933cdf084f1a3c77b582d69d24c4ee66a
"The eye in focus: accommodation and presbyopia"


Often the change in focus is discussed in terms of a change in number of diopters of focus, which are a measurement of lens power/focal point rather than in meters, but you can convert them. A lens power of 0 on an eye that is focused at distance would be focused at infinity, and otherwise the distance_in_meters = (1 / lens_power_in_diopters), or conversely lens_power_in_diopters = (1 / distance_in_meters).  So a lens focused at 0.5 meter is 2 diopters and at the far distance you refer to its close enough to 0 diopters.

[JohnHaganMD]

OMG you are desperate.  If you were born in 2002 you are about 14. If you focus on an object 12 inches from your nose then quickly look away out the window you will note that that in a young person changing focus from near to distance (greater than 20 feet since light rays are parallel at all distances further than that) occurs in a fraction of a second.

In someone older than 40 the eye loses its ability to focus at near and its called presbyopia and why they wear reading glasses or bifocals.

Other than that the focus-refocus will depend on a person's age and refractive error.  So your answer will varry depending o the age of the mariner. For a young sailor that is not hyperopia they should be able to focus at near-focus at distance-refocus at near in less than a second.

JCH MD