lalbatros

1/x + 1/x' = 1/f

with x=0.02m and x'=0.8m

so the focal length (f) of the crystaline lens is appromatively 0.02 m

But you are right, maybe there is no need to know this focal length.

With additional contact lenses (f') you get for a closer distance (x"):

1/x + 1/x" = 1/f + 1/f' = 1/x + 1/x' + 1/f'

and therefore

1/f' = 1/x" - 1/x'

this does not depend on x indeed