Delphinus,
Try using a clean piece of white PVC piping. If you find that your image is darker then it seems, set your exposure compensation to +1 EV (exposure value). If this is still too dark then +2 etc....make sure that you don't blow out the rest of the image. In other words, keep an eye on areas with highlights so that they don't appear too bright and compete with the subject that you are photographing.
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