Eye lens changes
To accommodate a change in principal focal length the eye lens must change to provide less or more refraction.
We have learnt that when the eye focuses on an object that is very far away, the principal focal length is 20mm.
But this needs to be pulled back to 18.5mm with an object at 250mm from the front of the eye.
To do this we have learnt in Mammoth Memory "Principal focal length and centre of curvature" that
|Thin lens||= long focal length|
|Thick lens||= short focal length|
Thick Lens (exaggerated)
To find out how the eye changes the thickness of the lens see Mammoth Memory Biology – The eye.