Focus is something else to consider when you take a silhouette. Depending upon how you frame the shot and what settings you use to set the exposure, your camera might accidentally lock the focus on the background. For your silhouette to have dramatic impact, though, it needs to be sharp. In most cases, fixing this problem is just a matter of ensuring that the focus locks on the subject when you press the shutter release. You might want to check your camera's user guide and make sure that the exposure lock button doesn't also lock the focus, for example. Worst case, you might need to switch to manual focus and set it yourself.
Perfect the silhouette on your computer
Finally, keep in mind that it's rare to capture a perfect silhouette in-camera. Most silhouettes will require some touchup in a photo editing program. The most common problem you'll have is that the silhouette isn't perfectly black—you'll still see some color or detail. Fix that with your photo editor's Burn tool. Burn is a brush that darkens the scene wherever you paint. So select the Burn tool (if you're using Photoshop Elements, it's in the second cubby from the bottom of the toolbar) and paint over the subject to remove all trace of color and detail.
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