I too am a regular user of Midtones for various purposes. First things first though… How do you get to midtones to isolate them. I use the TK Luminosity masking tools which makes life much easier. Here are 3 images to help visualize midtones.
For this example I started with a straight Black to White Gradient:
Selecting the Brightest 50% of the pixels via a Lights 1 selection yields the following selection:
Then Selecting the Darkest 50% via a Darks 1 selection of the pixels yields the following Selection:
To get to a Midtones 1 selection you subtract the Darks 1 from the Lights 1 which you might think leaves nothing. The visual issue is that marching ants in Photoshop only show the pixels which are at least 50% selected, so there is a feathered edge to both the Darks Selection and the Light Selection which leaves the midtones.
Here’s the Mask for a Midtones 1.
Remember this is a mask so the part of the mask that is the lightest in tone is the most selected which is the middle tones of the original Black to White Gradient. The parts of the mask that are Black are not selected at all. There isn’t a way to show the marching ants for a midtones 1 selection because there are no pixels which are at least 50% selected.
So how do I use this on a regular basis? As the last step in processing I always take a look at a subtle contrast addition to an image via a midtones 1 mask.
I create a midtones 1 mask, create a curves adjustment layer with the midtones 1 mask associated with the adjustment layer and change the blending mode to Soft Light. Since the Soft Light Blending mode is designed to ignore 50% gray, tones that are brighter than 50% in luminance get a touch brighter, and tones that are darker than 50% luminance get a touch darker. Therefore you get a very soft subtle contrast boost through a midtones 1 mask. Note, I don’t adjust the properties of the curve at all. It is left as a 45 degree line. (This technique also works fine with a Levels Adjustment Layer, or any other layer than deals with luminance adjustments).
If you want a slightly stronger contrast adjustment, change the blending mode to Overlay as it is just a variant of Soft Light that is a bit stronger.
A very cool technique that I use regularly. On most images I like it with no further adjustments. Sometimes I’ll drop the opacity of the layer just a bit if I feel like it introduced too much contrast or conversely on very rare occasions, I’ll increase the contrast by adjusting the curve, but that is really rare.