How to Focus a Nikon DSLR Manually
Some subjects confuse even the most sophisticated autofocusing systems, causing a camera's autofocus motor to spend a long time "hunting" for its focus point. Autofocus systems also struggle in dim lighting, although that difficulty is often offset on the Nikon D3100, D5100, or D7000 by the AF-assist lamp, which shoots out a beam of light to help the camera find its focusing target.
When you encounter situations that cause an autofocus hang-up, you can try adjusting the autofocus options. But often, it's simply easier and faster to switch to manual focusing. For best results, follow these manual-focusing steps:
Adjust the viewfinder to your eyesight.
Set the focus switch on the lens to M.
This step assumes that you're using a lens like the Nikon DSLR kit lens.
Select a focus point.
Use the same technique as when selecting a point during autofocusing: Looking through the viewfinder, press the Multi Selector on your D3100, D5100, or D7000 right, left, up, or down until the point you want to use flashes red.
Frame the shot so that your subject is under your selected focus point.
Press and hold the shutter button halfway to initiate exposure metering.
Rotate the focusing ring on the lens to bring the subject into focus.
Press the shutter button the rest of the way to take the shot.
When you first start working with an SLR-style camera, focusing manually is intimidating. But if you practice a little, you'll find that it's really no big deal and saves you the time and aggravation of trying to bend the autofocus system to your will when it has "issues."