Implantation of an accommodating intraocular lens in an eye may require the accurate measurement of the size, circumference, or diameter of the capsular bag of the eye. After the natural crystalline lens has been surgically removed, a flexible ring may be temporarily or permanently inserted into the capsular bag for measuring the circumference of the capsular bag. The ring is generally compressible to fit through a surgical incision, then expands to fill the capsular bag along an equatorial region. The ring has a central component that changes shape as the ring is compressed. The shape change is generally correlated to the size of the capsular bag and may be measured visually or with a camera through the cornea, the measurement being generally independent of corneal magnification.