Method for determining image reconstruction parameters in an optical biometric imaging device comprising a plurality of microlenses forming a microlens array and an image sensor arranged to receive light having passed through the microlenses, the method comprising: by the image sensor, capturing a plurality of sub-images together representing an image of a biometric object in contact with a sensing surface of the biometric imaging device, each sub-image corresponding to a respective microlens; and determining a demagnification factor based on at least a subset of the plurality of sub-images.