An ATF file is a so-called Adobe Transfer Function file, ie a settings file for the transfer function of the image processing software Photoshop. In the German version this file is called a print characteristic. An ATF file stores information that is used for adjustments and corrections of possible image changes when transferring images to eg different print materials such as film.
When using different printing presses, printing materials or printing inks can lead to a dot gain. This can lead to a difference between the original image and the print.
The print characteristic is used to add or subtract pixels or dots depending on the setting. To create an ATF file yourself, select 'Output' in the 'Print' dialog of Photoshop and then the 'Print characteristic' button. Once you have made the desired setting, you can save it in an ATF file via 'save'. In this dialog also ATF files can be loaded.
It should be noted that when transferring multiple images for each image you have to create an ATF file separately.