1. An iris registration method, comprising: at least one processor performing the steps of: a first step of capturing, by a capture device, a plurality of iris images at different pupil openings of an iris of a person to be registered and storing the plurality of iris images in a computer-readable recording medium; a second step of acquiring aggregation of iris images by performing at least one of duplication and deletion of at least one iris image of the plurality of iris images in the computer-readable recording medium so that distribution of the number of iris images with respect to the pupil opening is uniform; and a third step of obtaining, while evaluating authentication performance, a predetermined number of registration features from the aggregation of the iris images which is obtained in the second step and storing the predetermined number of registration features in a computer-readable recording medium, and wherein the third step further includes the steps of: generating features from the respective iris images that belong to the aggregation; and selecting a registration feature from the features with the use of authentication performance as an evaluation index, such that X registration features are selected from M features, wherein M is an integer larger than 1 and X is an integer equal to or larger than 1 and equal to or smaller M in such a manner that: X features are temporarily selected from the M features, authentication for remaining (M-X) features is performed on the assumption that the X features are registered, and evaluation for obtaining a predetermined evaluation value indicating authentication performance is executed iteratively while temporarily selected X features are changed; and the X registration features are determined based on the predetermined evaluation value obtained through the iteration of the evaluation.