How does D90 lock the exposure?

In AF-A, AF-S and AF-C autofocus modes, press and hold the AE-L/AF-L button, press the shutter half-way to focus, and then lock the exposure and focus. At this time, even if the shutter is released, the exposure and focus are still locked. If the autofocus mode is set to AF-S, the focus will be locked when the shutter is half pressed for focus.

1. exposure lock: measure and record the exposure value at the focus by pressing the exposure lock key. If you look into the viewfinder, you will find that when you move the lens after half pressing the shutter, the aperture size or shutter speed will change accordingly (that is, exposure value). Therefore, it is necessary to use the exposure lock function to lock the exposure value of the object you want to expose normally, so that the change of exposure value will not cause overexposure or underexposure of the object after re-composition.

2. Focus lock: Press the focus lock key to measure and record the distance to the focus.