1. A method comprising: calculating image capture parameters of an image capture device for an ocular area including at least one eye of a subject, the image capture parameters used for enhancing a corneal glint from the eye, the corneal glint being a specular reflection from a portion of the eye's cornea caused by an illumination source of the image capture device, the enhancing comprising differentiating an intensity of the corneal glint relative to intensities other features in the ocular area using a grayscale transformation to adjust amplitudes from color pixels captured by the image capture device; capturing at least three different test images of the ocular area, each of the different test images recorded at a different measurement position thereby indicating a different degree to which the test images are in or out of focus on relative to the corneal glint, the different measurement positions are along an optical axis of the image capture device, and at least one of the different measurement positions producing an out-of-focus test image; calculating, for each of the at least three different test images, a defocus measurement based on the corneal glint information from the corresponding test image, each defocus measurement corresponding to a different measurement position; determining an estimated focal point of the corneal glint based on the at least three defocus measurements; and capturing an iris image of the subject by changing the focal length based on the estimated focal point, the iris image used for biometric identification, wherein the grayscale transformation adjusts the iris image to be linear over a range of grayscale of an iris captured in the iris image and adjusts the iris image to be nonlinear outside of the range.