Scan Results in an Image with Greatly Exaggerated Contrast

You scan a negative with a uniform background expecting something like:

but get:

an image with highly exaggerated contrast.  The preview window appears as

Preview window.  The scanning program will determine the black - white points from pixels contained within the crop area (dashed lines).  In the course of performing a scan, some images will require two crop areas to be defined: one to perform color/tone adjustments and another to delimit the scan area.  This one shows the crop area used to adjust tone and color. It encloses neither a true black nor white

Cause: The black-white point function assumes that the image contains a true black and a true white.  Applying this to a scan that doesn't contain one or both will result in distortions.

Remedy: The user must manually determine and set the black and white points at offsets from the darkest and lightest elements of the image. Detail Link Pending

Comment: Another common beginner's error.  Most frequently this happens with low-contrast images that do not contain a true black and/or white.  The black-white function in either the scanning program or the image editor assumes that the lightest tone is white and the darkest is black.  The entire tonal scale is transformed.  For low contrast images, the dynamic range is greatly extended.   Posterization may also result from this error.