1. 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 second line scan detector comprising at least one focus shifting element such that each focus scan line is simultaneously obtained at respective first and second focus levels along the focus scan line, wherein the first focus level is different from the second focus level, the method comprising: obtaining at least one focus scan line of the target using the second line scan detector; obtaining at least one image scan line of the target using the first line scan detector, the image scan line being obtained at a respective third focus level; calculating at least one focus parameter using the at least one focus scan line; and estimating a nominal in-focus level of the target using the at least one focus parameter.