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<p>its optical power and focal length change.</p>

<p>For two or more <a href="page.php?w=thin_lens">thin lens</a>es close together, the optical power of the combined lenses is approximately equal to the sum of the optical powers of each lens: . Similarly, the optical power of a single lens is roughly equal to the sum of the powers of each surface. These approximations are commonly used in <a href="page.php?w=optometry">optometry</a>.</p>

<p>An <a href="page.php?w=eye">eye</a> that has too much or too little refractive power to <a href="page.php?w=focus_%28optics%29">focus</a></p><p>
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