People who have normal vision, emmetrope, have the ability to accommodate objects from distances of 25 cm on average, to distances in visual infinity.
The first distance (25cm) corresponds to the near pointwhich is the minimum distance a person can see correctly. What characterizes this situation is that the ciliary muscles are totally contracted.
In this case, by the Gauss equation:
Considering the eye with a distance between the lens and retina of 15mm, ie p '= 15mm:
In this case, the focus of the image will be found 14.1mm far from the lens.
As for the infinite distance, it corresponds to the remote point, which is the maximum distance reached for a focused image. In this situation, the ciliary muscles are totally relaxed.
As for the near point, we can use the Gauss equation to determine the focus of the image.
However, It is an undetermined value, but if we think that infinity corresponds to a very high value, we will see that this division will result in a very small value and can be neglected.
So we have to: