AE/ BBF button

I use BBF and use the AE/ AL button, so how can i lock exposure on my camera?

On my 7100 I just assign fn button to exposure lock

I use the Fn button for the viewfinder virtual horizon.

The pv button can also be programmed, or try menu c1. That will let a half press of the shutter lock exposure.

Another user of fn button for exposure lock

Never used exposure lock,

Just read this article about it.
https://support.nikonusa.com/app/answers/detail/a_id/440/~/when-should-i-use-ae-lock?

One of my favorite things on the D500 is that with the AF-ON button on the back you can assign exposure lock to the shutter button 1/2 press. They should do that on everything.

Like folks said, Fn button on the D7100.