When looking into a clear blue sky, no cloud, birds, trees or any object to give a frame of reference. When a person is actively looking, as opposed to day dreaming or gazing.

The sky goes for a long distance, and with uninterrupted view, and I guess the first question would be: - At what point into the atmosphere can the human eye see (which is not a topic for this site), following this,

What I want to know; is at what point to the eyes come to rest and How is the focal point determined?

Another consideration is:

How can this be measured?
Would any attempt to measure this would involve putting a device between the eyes and focal point?

  • $\begingroup$ I can provide some details to answer the question about how far into the atmosphere we can see, if required. $\endgroup$
    – user10932
    Nov 14, 2013 at 2:24

1 Answer 1


Accommodation is the process where the lens in the eye changes shape to focus on an object. The eyes have a default accommodation distance, called the resting point of accommodation (RPA). The RPA is the distance at which the eyes focus when there is nothing to focus on. The RPA is not identical to the acommodation state of the eye when focusing at infinity. Instead, the RPA can be measured in total darkness. There, the eyes' focal distance will equal the RPA after accommodation is complete. If the lights were then turned on, an object placed at that distance would be in clear focus. The RPA averages 30 inches (approximately 760 mm) for younger people and gets farther away with age (source: D. Ankrum).

The RPA can be measured by using infrared optometry, which is a non-invasive technique to measure the refractive state of the eye and the accommodation (Roorda et al., 1998).

- Roorda et al., Vis Res (1998); 38(13): 1913–24


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