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In natural vision when we gaze about, everything we choose to look at is imaged correctly in 3-D as the convergence and focus of our eyes adjust in unison to suit the subject of central attention being looked at. Objects outside of our center of converged viewing are double imaged and out of focus and are perceived with less detail. When taking a stereo photo it is common to consider that the camera's lines of view should be converged to meet with the subject of interest as with natural vision. But when this is done, only the area of central convergence is presented correctly. The heights of objects that are outside of the central area converged apon in the left and right views are different. The image size on the right side of the left image is smaller than the corresponding image size on the right side of the right image, and visa versa. This is referred to as horizontal keystoning. For projection of converged views the keystoning effect may be compensated for by converging the left and right projectors to reverse the keystone effect on screen. This is effective when projecting from a close proximity to the screen and where the degree of camera convergence in the image capture was fixed and constant. Any vertical height difference in the stereo pair hinders the viewability of the 3-D image. Everything in the stereo image should be presented as viewable at the discretion of the observer. The vertical height of all points common to the left and right images must be presented equally as if converged on in the original scene. |
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Height imbalance still remains due to the difference in distance between the subject and the two lenses. Objects on the sides of an image scene are closer to one lens than to the other resulting in different image sizes. This is less apparent when objects are further from the camera. This effect also occurs with natural vision as a secondary cue for the location of objects. Another factor is that the focal length of the lenses in a stereo camera are not exactly matched. One lens will provide a slightly wider angle of view resulting in a smaller image size. When a stereo pair from a stereo camera is scanned, resizing is always required to achieve height equivalence. Parallel imaging results in an acceptable stereo pair for 3-D viewing but the loss of the full frame width is not suitable for full screen display.  As parallel camera views require image height correction anyhow and result in a loss of frame width, the convergence of camera views is then preferable as it enables full frame width to be maintained. Correction of converged camera views is STUDIO 555's preferred method for still or motion stereo pair capture and for the presentation of printed, screen projected or monitor displayed 3-D images. See examples of height equalised 3-D images in the STUDIO 555 (ACB) 3-D ANAGLYPH GALLERY For Still and Motion Stereoscopic Photography and for Alignment of your Stereo Pairs, Contact STUDIO 555 |
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