Its what it says on the tin
If you're going for a more realistic look: keep in mind that eyes are not just oval openings in the head. You have a specific -3 dimensional- shape. The tear duct is an important part of the eye, do many drawings of the eye with all it's COMPLETE features (lids, lashes, ducts) in different angles and with the correct line weight (darker.. lighter.. steer clear of making everything one tone and then outlining it with a dark line). And with 3 dimensions you need proper rendering, shadows and such. Usually this means more than one variation of darkness (or atleast a gradual progression from dark to light); a gradient.
Paying attention to details of facial features in its entirety will help you. Especially with the jaw and cheek bones as that usually plays a big part in making a photo-reffed picture look similar.
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