Although Auto is an option, consider a more flexible techniique, as well. First, confirm that the Highlight and Shadow eyedroppers are set to your chosen tone value criteria:
Step One: Locating and marking the existing extremes of highlight and shadow
From the symbols at the bottom of the Layers panel, select Adjustment Layer (half-black-half-white circle) and from its drop-down menu choose Threshold. You will be presented with a histogram, and below it, a slider. Move the slider all the way to the right; the image will go completely black. Inch the slider to the left until the first white non-spectral patch appears. It is the location of the highlight extreme. Choose the Color Sampler tool. (It is nested with the Eyedropper tool in the Tools panel.) Click in that highlight location. The Color Sampler will mark the site. We will call it Register Mark 1.
Next, move the slider all the way to the left; the image will go completely white. Inch the slider to the right until the first meaningful black patch appears. (Values darker than the one selected will ultimately go to maximum black. Deciding the position of the shadow end point is not a mindless, mechanical process. It is an important judgment call. Choose the Color Sampler tool. Click in the shadow location. The Color Sampler will mark the site. We will call it Register Mark 2.
You no longer need the Threshold layer, so drag it to the Trash. The points you marked will remain. You have now successfully located and marked the highlight and important shadow end points.
Step Two: Placing your end point criteria in those two areas
Image > Adjustments > Curves
Click on the Highlight eyedropper; then click on Register Mark 1
Click on the Shadow eyedropper; then click on Register Mark 2
You have altered the tonal curves and created a full range image. (An extra goodie: Correcting the end points in this manner often contributes substantially to correcting color balance throughout the image, as it did in the example above).
You may find that now, other areas, perhaps the 3/4 tones, should be lighter or darker. Alter the curve to suit yourself. You may make the adjustments by either clicking and bending the curve or using the TAT (Targeted Adjustment Tool) in the image area. The critical end points, which created the full range image, will not be affected. (In the bottom-right example above, a gentle S curve was added to provide middletone contrast.)
Last, remove the markers: With the Color Sampler tool chosen, click on the word Clear that appears in the Options bar.
One final note about end points: Not all images require meeting endpoint criteria. If the highlights in a photograph are dark or off color or the shadows a bit weak, but you simply like the look of the image, don’t mess with success. Leave it alone.
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