<p>This mainly depends on the strength of your prescription. The higher the prescription, the higher index is required for the lenses. Selecting a higher index will ensure your lenses are as light and thin as possible.&nbsp;</p> <p>&nbsp;</p> <p>Here is a picture for reference:&nbsp;</p> <p><img alt="" src="https://df5apg8r0m634.cloudfront.net/images/2018/0301/IFQzyzZ17w.jpg" /></p> <p>See the example below for how the thickness of a -4.00 prescription lens changes with each of our available indexes.</p> <p><img alt="" src="https://df5apg8r0m634.cloudfront.net/images/2018/0301/ynVX5zeRsD.jpg" /></p> <p>Why?&nbsp;Each lens type is made from different materials. The higher index materials allow for flatter curved lenses that are still able to maintain an aspheric design. Both of these features help produce thinner and lighter lenses.</p> <p><strong>Notice</strong>: The ultimate thickness of the lenses is determined by the frame size (including lens height, lens width), frame material and your PD. If you choose a big frame the lens edge will undoubtedly be thick. Frames with thin metal rim will also make the lens look thicker. If the frame is not the best for your PD, the lens may not be beautiful and thin as you expect.&nbsp;</p>
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