When taking pictures in insufficient light and without a tripod, it's usually not possible to use a very long exposure time, and a high ISO is usually needed too. This leads to photos with a lot of noise. If, however, you have taken several such photos in a row, you can use this function to combine them into one picture. The program averages their pixels, reducing noise. When taking the source pictures, make sure to move the camera as little as possible, because even a small change in its position can cause quite large changes to perspective, making it difficult to align the pictures so they can be sharpened. Cameras' functions for taking quick series of pictures are very useful here. About 4-6 shots is best. With fewer, the noise reduction is smaller than it could be. With more, the risk of nudging the camera during the series is greater.
After selecting source pictures in the Browser, use Create | Join Multi-exposures | Remove Noise… to start the wizard. To fix any mistakes you might have in your selection, use the wizard's first step. In the next step, the pictures are automatically aligned. Double-check the alignment and manually fix it here if needed. For advice on checking and fixing alignment, see the Help section for Align Pictures , which contains a similar window. In the next step, the sharpened picture is generated. Use the settings here to influence the results.
Use Crop to crop away edge areas where the source shots have not perfectly overlapped. Small shifts of the camera between pictures are normal for shots taken by hand, so pictures need to be aligned. This process is not always perfect, so edge areas may be blurred. Use the Sharpen option to eliminate this blurring. If some objects have changed position between shots, use the option named Remove differences between pictures and if necessary use the Amount to remove slider to fix the problem. Use the lowest slider level that works, because higher levels can mean less noise reduction: with strong reduction, in pictures with a high degree of noise, the noise can be mistaken for a change in the picture, and thus not reduced. But areas that contain removed differences between pictures always receive less noise reduction than other areas. This option can also be useful even when a scene contained no moving objects. If the camera position shifted heavily between shots, it can be hard for the program to align them well. Use this option to eliminate e.g. doubled contours caused by poorly aligned source pictures.
In the last step, you can either save the final picture to file or open it in the Editor, where you can continue editing it.