●   Use key F to maximise image on a grey background;

●   Click on the eyedropper tool to set a 3 x 3 average sample;

●   Open a curves adjustment layer;

●   Set preferences for sample droppers;

●   Double click on the shadow dropper and set RGB to 20;

●   Double click on the highlight dropper and set RGB to 244;

●   Double click on the mid-tone dropper and set RGB to 133;

●   OK and exit curves;

●   Open a threshold adjustment layer;

●   Move the slider to the left to find shadow points and zoom in;

●   Choose the colour sampler tool and mark a shadow point (p1);

●   Zoom out and move the slider to the right to find the highlight points and zoom in;

●   Choose the colour sampler to mark highlight point (p2);

●   Zoom out;

●   Open a new layer just above the background;

●   Fill with 50% grey and set the blend mode to difference;

●   Open a threshold layer and move the slider fully to the left, then slowly move it back to the right until the mid-grey points appear;

●   Zoom in and choose the colour sampler to mark a mid-tone point (p3);

●   Zoom out – dump the grey-filled and threshold layers;

●   Open a curves adjustment layer;

●   Choose the shadow sampler and click on point (1) – note that the alignment is shown when the brush point disappears;

●   Choose the highlight sampler and click on point (2);

●   Choose the mid-tone sampler and click on point (3);

●   Clear the colour sampler points; and

●   Use the curves adjustment layer to adjust brightness and contrast.

Note that using a mid-grey target card in an helps – especially in a studio environment.