The AE-L/AF-L button on my Nikon D7200 is set to focus the camera. I like this setting, but then I don't have a quick way to lock the exposure settings. The only way I know how to do it is to switch the camera to manual mode, turn off Auto ISO, then adjust the ISO to get the exposure I want. Is there a quicker way to lock the exposure and still use back button focusing? I also don't want to use half-pressing the shutter button to lock the exposure. Can I set that "L" on the back that locks to focusing point to lock the exposure instead?
Engaging depth-of-field preview (usually mapped to the Pv button on the front of the camera) will also engage AE lock as long as the button is held down. Of course, you can also set the Pv button to perform AE lock only, and by doing so you can also set AE lock to stay engaged until the button is pressed a second time. But I like to leave the Pv button mapped to DoF preview and take advantage of the AE lock side effect as a bonus.
According to page 107 of The Nikon D7200 User's Manual, the default action of the AE-L/AF-L is to lock both focus and exposure when the button is pressed.
Custom settings a4 (page 277), c1 (p. 279), and f4 (p. 285) affect which buttons do and do not affect AE lock and AF lock. If you are using the MB-D15 battery pack (battery grip), you can assign the function of the AE-L/AF-L button on the grip using custom function f10 (p. 286).