Epsom Salt at 5x Magnification

A small piece of Epsom Salt appears as large as a bolder when imaged at 5x on a high density APS-C sensor. Tack sharp images are elusive at 5x. Diffraction becomes noticeable at the f/16 aperture setting I used to get adequate depth of field.
Camera and Lens Settings
65mm  f/16.0  1/160s
ISO 100
5184 x 3456px
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