When taking long exposures (or even sometimes, mid-range exposures), you need to reduce the camera shake as much as possible.  A tripod helps, but knowing what can cause camera shake is beneficial to isolating, and addressing, a problem.  Here’s a short list of things that can cause your camera to shake, in no particular order.

1. Unsteady Hands - if you are not blessed with a steady hand, you may need to use a tripod much more often.
2. An undersized tripod - make sure your tripod can support the weight of your camera AND your lens and all other peripherals).
3. Mirror Shake - the mirror in your SLR camera slaps upward just before the shutter opens.  On long exposures, this won’t be noticeable.  But on the medium length exposures, it may be beneficial to utilize your camera’s “mirror lock-up” function.
4. Wind - Strong winds will move your camera just enough to blur your photo.  Shield the camera as much as possible, or try lowering the tripod or spreading it’s legs.
5. Bad Tripod Placement - Be very attentive to where you set up your tripod.  A wooden deck, for example, can potentially cause camera shake if people are moving around on it.  Soggy soils may heave if your weigh shifts nearby.  Consider a different location.
6. Pressing the Shutter Button - Any rifle instructor will tell you to “squeeze the trigger, not pull”.  Same goes for cameras.  Better yet, use a remote trigger and keep your hands away from the camera.
7. Sound - Yes, sound.  Sound, especially low frequency sound, compresses air.  It can shake everything near it, including your camera.  So if you plan on photographing in dimly lit conditions, such as at a dance club, you’ll want to invest in a remote flash and a diffuser so that you can get the shutter speed as high as possible.

This is, by no means, a complete list, but a good primer to help you become more aware of your environment when setting up for a photo shoot.  Feel free to add your own observations by leaving a comment below. 

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