The invention relates to a treatment apparatus having a modular intraocular lens, which comprises a first part having a haptic and a marking, visible in optical coherence tomography, with known dimensions of said marking and a second part having an optics body and which has a convergence state, in which the second part is in contact with the first part, and a spaced-apart state, in which the second part is spaced apart from the first part, having an optical coherence tomography device, which is configured to record, by means of optical coherence tomography, an OCT cross-sectional image which shows the first part arranged in a capsular bag of an eye and which shows dimensions of the marking in the OCT cross-sectional image, and having an evaluation unit which is configured to create a corrected OCT cross-sectional image by transforming coordinates of the OCT cross-sectional image in such a way that dimensions of the marking in the corrected OCT cross-sectional image are closer to the known dimensions of the marking than the dimensions of the marking in the OCT cross-sectional image.