1. An information processing apparatus comprising: a processor to communicate with a memory; an image acquirer configured to acquire a captured image obtained by an image sensor that captures an image; a storage configured to store relationship information indicating a relationship between a luminance value of an image captured by the image sensor and an amount of variation of the luminance value; a variation amount deriving unit configured to derive an amount of variation of a luminance value of respective pixels in the acquired captured image based on luminance values in the acquired captured image and the relationship information; a coefficient acquirer configured to acquire a filter coefficient of a feature detection filter used to detect an image feature from the captured image; a calculation unit configured to calculate a luminance gradient value which is a gradient value of the luminance value and a changing rate of the luminance gradient value in accordance with a change of the luminance based on the luminance values of the captured image and the filter coefficient of the feature detection filter; an estimation unit configured to estimate an amount of variation of the luminance gradient value based on the derived amount of variation of the luminance value and the changing rate of the luminance gradient value in accordance with the change of the luminance value; a threshold value deriving unit configured to derive a threshold value of the luminance gradient value to be used in detecting an image feature from an image based on the estimated amount of variation of the luminance gradient value; and a detector configured to detect a feature from the acquired captured image using the feature detection filter based on the threshold value and a luminance gradient value obtained from the acquired captured image, wherein the variation amount deriving unit, the calculation unit, the estimation unit, and the threshold value deriving unit are implemented by the processor.