Which part of the eye acts as a secondary area of refraction to "fine tune" the focus? the iris the cornea the lens the retina

The part of the eye that acts as a secondary area of refraction to fine tune the focus is the lens. The lens of the eye is a biconvex, transparent structure of the eye that helps in refraction along with the cornea. Most of the refracting of the light rays happens in the cornea, but the lens help refract light to be focused on the retina by changing its shape to focus on objects at various distances. 

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