I like to make my display shutoff when I leave the desktop. My iMac obviously does not have a specific physical button to shutoff the display only. So here’s the solution I found. This tip is more useful for those with a desktop than a laptop.
If you have a Mac keyboard with an Eject key you can use Ctrl-Shft-Eject to put your MacOS to sleep, but since my Logitech keyboard does not have an Eject button that makes it difficult. I used System Preferences/Desktop & Screen Saver/Hot Corners/Put Display to Sleep to handle this, but my wife got upset every time she would move the mouse to a particular corner, the screen would shut off.
This is the solution I found, which basically is the same, but you can add a key to the Hot Corner. Simply follow the same directions above, but before you click “Put Display to Sleep” hold down your desired hotkey such as Ctrl, Cmd, Alt key and you will get that option. It’s a perfect solution and I hope this helps some people =)
Source: Discussion Apple