According to the Wikipedia article on eye accomodation, the range in which a human eye can focus is from 7 centimetres away up to essentially an infinite distance. Of course, people can't see things an infinite distance away, but at a certain point the light reflected off an object will come at the eye in effectively parallel paths.
As an eye focuses on objects progressively farther away, it seems to me there would be a point when the lens hits maximum flatness, after which it everything will be equally sharp in focus. Looking at an object 500 metres away or 1000 metres away probably doesn't require any change in the lens. However, going from one metre away to seven requires the ciliary muscles adjust to just the right amount.
My question is, what is a normal minimum distance from the eye at which the lens will reach maximum flatness? Everything else in this question is just for context, so an answer to this, stated in metres from the eye, is all I am seeking.
The article I linked to above mentioned the human eye seeing "13 dioptres", and I have tried to understand dioptres, but it's a little confusing for me. If I understand correctly, the higher the dioptre, the closer you focus, so the 13 dioptres refers to how close the human eye can see. If I have this backwards, please let me know.
In any case, my purpose in asking is to determine the range in which ciliary muscles would be required to do more nuanced work in order to hold the lens at in the right shape. Focusing on something 7 centimetres away, the ciliary muscles are at maximum exertion, and at a certain point and beyond the ciliary muscles are completely relaxed. To hold somewhere in between would require more subtly nuanced, and, I suspect, more difficult work on the part of the ciliary muscles.
At least, I assume that to be the case because I am hypothesizing that the act of contraction for the ciliary muscles would be similar to other muscles. For example, it is easier to do a bicep curl with weight when going from full resting position to maximum contraction, as opposed to stopping at a particular spot somewhere in the middle of the motion. To stop mid-motion requires more muscle control, and, depending on how fast you wanted to do it, can be difficult to be exact.