A macro lens will allow you to best capture the detail of the eye. Macro lenses vary in focal length from 50 to 200mm. You can still take a good close-up photo of the eye with a regular lens, but you may not be able to get the eye to fill the entire frame or capture all the detail you want. If you don’t have a macro lens and don’t want to spend the money to get one, try using a close-up filter as an alternative. Many cellphones have a macro mode, which will allow you to take a more detailed photograph of the eye than with your camera’s standard settings. A macro lens attachment for your cellphone will produce even more detailed shots.  You can find a macro lens attachment for your cellphone at most major retailers and electronics stores. If you’re planning to buy a macro lens attachment, make sure you choose one that fits your phone model. Extension tubes fit between the rear mount of the lens and the camera body. Using an extension tube will magnify the eye in your shot, allowing you to capture more intricate details.
Put a macro lens on your camera. Use a macro mode or macro lens attachment when using a cellphone. Consider adding an extension tube to make the lens focus closer.