how can i measure the focal length of contact lenses?

Answer 1

Start with the thin lens equation 1/f=1/i + 1/o. If the lens is illuminated by parallel rays, or a point source far away, the object distance is infinity and f=i. ie the focal length is the distance to the smallest spot.

Answer 2

It is also possible to convert the power of a glasses prescription to the power of contact lenses with the following formula:
F(contact lens)= F(glass)/(1-dF(glass)
, where
d is the distance of the glassses to the eye and
F = 1 / D are the focal length of the contact lense and the glass,
D is the lens power in diopters.
VR

Answer 3

1