Fingerprint sub-image capture

A fingerprint sensing system arranged in a smartcard, the fingerprint sensing system comprising a fingerprint sensor being configured to acquire fingerprint data of a user for biometric authentication, and a processing unit being configured to cause the fingerprint system to be operative to:

• detect a finger of the user contacting a sensing area of the fingerprint sensor;
• initialize the fingerprint sensor with a predetermined sensor setting;
• acquire, for the finger being detected to contact the sensing area of the fingerprint sensor, a calibration sub-image which is confined in size to a subarea of the sensing area;
• determine whether or not a quality criterion is met for the acquired calibration sub-image, and if so to;
• acquire one or more further sub-images, each of the one or more further sub-images being confined in size to a further subarea of the sensing area;
• combine a plurality of the acquired further sub-images into a representation of a fingerprint of the user, wherein the further sub-images are acquired during a time period when the smartcard is not engaged in waiting time extension, WTX, request signalling with a card reader with which the smartcard performs contactless communication.