A tonal selection curve is always more complicated than a master (or base) curve. (The master base curve on this example image is shown above.) Use the tonal selector tool on a new curve adjustment layer, above the master curve, to do this. Every tonal selector (or selection) curve is different. The goal of a tonal selector curve is to interpret the image in your specific artistic way. Don’t make too sharp angles in the curve or you will degrade the image.
For the image below I want the shadows to be slightly more open and I want the highlights on the onion to be more flat and more detailed at the same time. That means adding contrast to the highlights and flattening the mid-highlights. I don’t mind if the table goes flat.
1. Start in the top highlight and make a point. Then do this for the next two highlights going down.
2. Select the middle highlight point and simply pull it down a few notches with your down arrow on the keyboard. This will both flatten and add contrast (separation) to the highlights. Notice the histogram location.
3. In the next two points you can pull your mid-tones back to normal and bring up your shadows a smidgen. Then mix and modify all those adjustments. In my case I ended up making the adjustment much more extreme but notice how the curve is not all squiggly. Even though I have many data points on the curve, they are not destroying the tonal integrity of the image.