The main challenge in night photography is the lack of light.
As such, you need to increase shutter speeds (open the shutter longer) to allow more light to come in and hit the sensor.
But as human hands are shaky, the longer the sensor is open, the more the image is prone to be blury, due to camera shake.
One way to overcome that is by increasing the ISO (sensitivity of the sensor). But this also has a drawback, as most entry-level DSLR sensors don't perform well in low light. Even though the D3100 is a new generation camera, its sensor's low-light capabilities do not match two-year old professional models like the D700 or D3s/x.
The problem is noise, or what you see as tiny grains in the image. When you push your ISO up, it amplifies the sensitivity, which in turn, generates noise if the ambient light is insufficient. Think of it this way - when you turn the volume of your speakers to full blast, you're likely to hear crackling sounds or interference.
There are several methods you can overcome this. 1. Use an external flash gun. Unlike the camera's built-in flash which only points forward, an external flashgun allows you to point your light to the exact area where it's dark, or bounce the flash off a surface to generate more natural tones.
2. Use a wide aperture lens. This helps, but F2.8 lenses are typically very expensive, big and heavy. As Nikon does not have any cheap or lightweight F2.8 variable zoom lens at the moment, the Tamron 17-50mm f2.8 is great third party alternative that is reasonably priced.
3. Use a tripod and remote trigger. If you're into shooting landscapes, this can be very, very useful. It allows you to open you shutter for longer periods (up to 30 secs) and create beautiful effects.
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