The next task is to correct the file’s color cast. Use the Threshold adjustment layer approach, as explained in Chapter 1.
Remember to set your black eyedropper value to R: 7, B: 7, G: 7 and your white eyedropper value to R: 247, G: 247, B: 247.
Press Cmd-0/Ctrl-0 to zoom out, so the entire image is visible in your window.
Add a temporary Threshold adjustment layer. While the Threshold dialog is still open, drag the slider to the left to find the image’s black point, and to the right to find the white point. Shift-click in each spot to place a Color Sampler. Click Cancel.
Create two Curves adjustment layers, one for the light (or white) point and one ...