as an opportunity for artistic effect. As the subject moves during the scan, distortions are caused along the axis of the scan head's movement, as it captures different periods of the subject's movement line by line in a manner similar to slit-scan photography; these are forms of strip photography. The artist can use this by aligning the direction of the scan head's movement to deliberately caused the desired distortion. Stereoscopic scanning A variation of macrophotography involves using the scanner to produce stereoscopic or "3D" images of small objects. This is made possible because of the optical system of a typical scanner, which uses prisms to put the sensor at an optical distance from the glass of 3 to 4 feet, allowing a small sensor to cover the entire width of the bed, while keeping the bed physically shallow. This also gives better than expected depth of field, and introduces a certain amount of parallax when the same object appears at different positions on the bed. This allows the generation of stereo pairs, much like the "shift" technique where a single camera is shifted to produce right and left views of a still life scene. This technique probably goes back to the earliest days of flatbed scanners and was mentioned on the photo-3D mail list by Bob Wier on December 14, 1995, though he makes vague reference to earlier experiments by others. Though it could be described as a trivial application of a centuries-old technique to a new device, the concept is not widely known, even among stereo photography enthusiasts. This may be due to the common misconception that the typical flatbed scanner uses an imager that spans the width of the bed, thus leading to the assumption that shifting objects would not produce parallax. The most basic version of this technique involves simply placing the object upside down on the scanner and moving it by hand, but this leads to irregularities between the two images. Better results can be obtained by placing the object in a glass front display box and sliding the box against a straight edge. Smaller objects such as seeds can be placed on a microscope slide and secured using small adhesive labels. Another, more involved technique is to remove the lid and turn the scanner upside down, then move the scanner rather than the subject. This allows the imaging of extremely flexible objects as well as