Please ask the expert to show me how to adjust the metering mode and focusing mode of Nikon D90.

See attached figure for adjustment of metering mode. Usually novices use matrix photometry to shoot landscapes, and they need to add -0.3EV exposure compensation. The greener the background, the greater the compensation. Nikon SLR uses the central key to measure light when shooting portraits through point measurement linkage, which can basically ensure the accurate exposure of faces.

AF-C (continuous autofocus) is recommended for autofocus. The adjustment method is shown in the following figure:

In focus mode, it is recommended to use dynamic area AF or 3D tracking (select "dynamic area AF" or "3D tracking" in "A 1: AF area mode" in the personal setting menu), use the arrow keys to move/select the focus point, and press the shutter button halfway. The selected focus is the main focus, and the rest are the auxiliary focus. Even if the main focus deviates from the focus position for a short time, it doesn't matter, and the auxiliary focus. If it is 3D tracking focus, the main focus will automatically track the focus position on the subject, but the response speed of 3D tracking is not as fast as that of dynamic area AF.

You can also set the AE-L/AF-L key as the focus key ("f4: Set AE-L/AF-L key" in the personal setting menu is set to AF-ON, and the autofocus mode of the camera is preferably set to AF-C), and half press the shutter button to lock the exposure ("c 1: Shutter release key AE-L" in the personal setting menu is set. The advantage of this setting is that the focus is completely controlled by the right thumb (that is, as long as the AE-L/AF-L key is not actively pressed, the lens is completely manually operated, so that the focus can be interrupted at will during autofocus, and manual focus correction can be used immediately, so as long as AF can be aimed near the main body, at least the angle required to twist the focus ring can be omitted), the index finger only controls the exposure and shutter, and the left index finger and thumb control the focus ring of the lens, which can be manually operated at any time.