Mammoth Memory

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.

Principal focal length of a distant object

But this needs to be pulled back to 18.5mm with an object at 250mm from the front of the eye.

Principal focal length of a near object

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


Thin Lens

Principal focal length of a thin lens


Thick Lens (exaggerated)

Principal focal length of a thick lens


To find out how the eye changes the thickness of the lens see Mammoth Memory Biology – The eye.



Lens thickness of a human eye when focusing on distant and near objects


More Info