Use key F to maximize an 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.