This disclosure describes methods and systems for improving performance of biometric systems that use features in the eye, such as iris or eye-veins, particularly when both biometric performance and user convenience are objectives. The disclosure relates to optimizing biometric performance when the iris or eye veins are neither fully visible, nor fully occluded, but in a partially-occluded state which occurs often when a user's eye is in a relaxed or natural state. In some embodiments, the method comprises a biometric enrollment or training step whereby an original image of a human eye is acquired, and a plurality of synthetic or augmented images are generated that are a combination of the original image and synthesized images that simulate specific ways that the eye can be occluded. A classifier can be trained using the plurality of augmented reference images, and subsequent recognition is performed using the classifier on newly acquired real images.