A method of estimating an in-focus level of a target in an image scanning apparatus, wherein the image scanning apparatus comprises a first line scan detector configured to obtain one or more image scan lines of the target and a second line scan detector configured to obtain one or more focus scan lines of the target, the method comprising: obtaining at least one image scan line of the target using the first line scan detector , each at least one image scan line being obtained at a respective focus level; obtaining at least one focus scan line of the target using the second line scan detector , each at least one focus scan line being obtained at a respective focus level and wherein the focus level of the at least one image scan line is different from the focus level of the at least one focus scan line; calculating at least one focus parameter using at least the at least one focus scan line; and, estimating a nominal in-focus level of the target using the calculated focus parameter(s); wherein the image and focus scan lines are obtained from different positions in the target and wherein image information is obtained by the said first and second line scan detectors along different optic axes from the target so as to produce the said respective image and focus scan lines; wherein image information is reflected to the second line scan detector using a mirror; characterised in that : the mirror is rotated so as to provide focus scan lines of the target at different focus levels; the first line scan detector is placed in the focal plane of the imaging lens and the first line scan detector is perpendicular to the optical axis. obtaining at least one image scan line of the target using the first line scan detector , each at least one image scan line being obtained at a respective focus level; obtaining at least one focus scan line of the target using the second line scan detector , each at least one focus scan line being obtained at a respective focus level and wherein the focus level of the at least one image scan line is different from the focus level of the at least one focus scan line; calculating at least one focus parameter using at least the at least one focus scan line; and, estimating a nominal in-focus level of the target using the calculated focus parameter(s); wherein the image and focus scan lines are obtained from different positions in the target and wherein image information is obtained by the said first and second line scan detectors along different optic axes from the target so as to produce the said respective image and focus scan lines; wherein image information is reflected to the second line scan detector using a mirror; characterised in that : the mirror is rotated so as to provide focus scan lines of the target at different focus levels; the first line scan detector is placed in the focal plane of the imaging lens and the first line scan detector is perpendicular to the optical axis. the mirror is rotated so as to provide focus scan lines of the target at different focus levels; the first line scan detector is placed in the focal plane of the imaging lens and the first line scan detector is perpendicular to the optical axis. <EOS>