Most images can benefit from an increase in mid-tone contrast. This is especially true of B&W images, landscapes and high frequency image content such as textured detail. There are a number of different ways to increase the mid-tone contrast. Since most of them are global there is an unwanted increase in the overall contrast of the image which can drive highlights and shadows into clipping and a loss of detail where we do not want that to happen. Restricting the added contrast to the mid-tones or a specific tonal range is fairly simple process.
You can add contrast with curves, shadow/highlight, and other controls, but one of my favorites is the low contrast, high radius unsharp masking technique. First we will add the contrast and then tame down the effect on the high and low values.
Duplicate your background layer and put the new layer in luminosity mode if you want to restrict the effect to the image content and not the color. Using Filter | Sharpen | Unsharp Mask, set the radius to a much higher number than you would normally use to sharpen an image. We want to have the sharpening halos to be so large as to disappear into the image and not be seen. While this is somewhat image resolution dependent, the effect will work without specific radius numbers being important. So, try 50, 87, or 122 pixels and notice that the difference is subtle if even noticeable. Set the threshold to zero to start as we want the effect to influence the entire range of the image. You can play with larger threshold numbers if you need to soften the effect, but that should be easily controlled with layer opacity.
The contrast will be controlled by the Amount slider and the setting for that is lower than normal. It takes about 10% to be noticeable, and somewhere around 20-25% works well. Higher numbers may produce more contrast, but that isn’t really necessary for the effect we are looking for. A reasonably useful set of numbers for me is Amount 20%, Radius 80 pixels, Threshold 0.
Now we control the effect by using the Blend-If sliders in the layer style dialog accessed by double clicking in the blank area of the layer or clicking on the fx at the bottom of the layers pallet and choosing Blending Options. At the bottom of the Layer Style dialog box you will see the following area which is known as the Blend-If control. We are working on the layer we need to control, so This Layer is the place to work. Drag the left slider to the right to about 10-15 and hold down the <alt>[option] key to split the slider and drag the top half of the slider to about 40. Splitting the sliders requires a little getting used to, but soon becomes easy to do. At the right end you want to drag down the highlight slider to about 240, and split the lower half to about 200-210.
Splitting the sliders is the equivalent of blurring a mask edge to make the transition area of the effect softer and less noticeable and helps you blend the sharpening effect out of the brightest and darkest areas of the image. In this case we are telling Photoshop to have no effect on tones above 240 or below 10 and gradually increase the effect so that 100% is applied between values of 40 and 210. The specific numbers are arbitrary and can be modified to suit your image. However, the overall effect is rather subtle and an action can easily be made that would make all of the above settings happen automatically to a layer. The result will be added contrast to the mid-tones of the image imparting more snap and the appearance of a sharper image without clipping highlight and shadows.
Here is a portion of a high frequency image with and without a mid-tone contrast boost. In the finished image the highlight and shadow values are maintained at the level prior to the contrast boost.