A method of estimating an in-focus level of a target in an image scanning apparatus is provided, 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 comprises 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; calculating at least one focus parameter using at least one focus scan line; and estimating a nominal in-focus level of the target using the calculated focus parameter(s).