Selecting the exposures
To get good results with HDR processing, your capture sequence must include photos that correctly expose highlights and photos that correctly expose shadows. The latter is especially important to avoid noise showing in the processed HDR image.
In the lightest photo of the sequence, the darkest shadows should be at least in the mid-tones. You can check that with your camera's histogram preview in playback mode. In your most overexposed photo, the left part of the histogram should be empty until 1/3rd of the histogram's width. If this is not the case, you will have to add one or more photos taken with longer exposure times. Another option is to re-shoot the exposure sequence with the normal exposure set one or more EVs higher if your most underexposed image in the exposure sequence was too dark. This is the case when the histogram of your darkest image is completely empty on the right half.
The number of exposures needed depends on the dynamic range of the scene, in addition to the exposure increment. For most outdoor scenes, three exposures taken at +/- 2 exposure increments will be sufficient, provided the scene does not include the sun. However, for the interior of a room with a bright view out the window, you will need at least five images taken with an exposure increment of +/- 2, or nine images taken with an exposure increment of +/- 1.
In scenes with extreme differences between light and dark details, you should change the exposures manually to ensure you capture a wide enough range to cover your scene.
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