Now that we have opened the Pandora box of manual photography, so, when exactly would you ever use manual focus with your Nikon D90?
Here's a few suggestions...
1. Macro Work - When doing macro photography I almost exclusively switch to manual focusing and find the results much more pleasing.
The narrow depth of field in these shots mean that you need to be incredibly precise with focusing and being just a smidgen out or having your camera choose to focus on the wrong part of your subject can have a significant impact upon your image (for better or for worse).
Manual focusing puts the control completely in your hands and will get your images with the right parts in focus.
2. Portraits - When shooting portraits focus needs to be precise.
The majority of your shots of people will need to have their eyes in perfect focus.
Switching to manual focus will give you complete control to enable this rather than having to line up the focusing points on your camera on the eyes pre-focusing by pressing halfway down and then having to frame your shot.
3. Shooting Through Glass or Wire Fences - If you’ve ever shot through anything like a window or a mess/wire fence at a zoo or museum you’ll know how cameras will often get confused on where to focus.
Sometimes falsely focusing too closely on the fence or glass instead of your subject.
Manual focusing will avoid this completely and allow you to tell the camera exactly what you want to be in focus and what you want to be blurred.
4. Low Light - Shooting in dimly lit environments can be difficult for some cameras and lenses when it comes to focusing.
You’ll know when your camera is struggling in Auto mode when every time you go to take a shot the lens will whirl from one end of it’s focusing options to the other and back again before deciding on where to focus.
This can really lengthen your shooting process and make taking quick candid shots quite frustrating.