A technique for modifying data of an image, such as can be implemented in a still camera or video recorder in order to correct for defects in its optical and/or electronic systems, includes generating data to modify the image as a function of radial position across it. A variation of the intensity across an image (lens shading) that appears in data from a two-dimensional detector is an example of an application of the technique. In order to make modifications to the data, positions of a two-dimensional raster scan pattern of an image sensor are converted to radial positions and this is then used to generate the modification data. The modification data is generated on the fly, at the same rate as the image data is being acquired, so that the modification takes place without slowing down data transfer from the image sensor.