ScreenScope Handheld Adjustable Stereoscope
The ScreenScope is a mirror stereoscope for viewing side-by-side images in the parallel format and suits computer monitors. The ScreenScope features chrome plated first surface glass mirrors that present very clear images without ghost reflections or chromatic aberration (color fringing) which is inherent in prism based viewers. The mirrors are set at a based angle around 45 degrees and at this angle the mirrors are parallel and the viewer suits 3.5 inch wide by 6.5 inch high images. The angle of the outer mirrors can be decreased via screw adjustments to deflect light for more widely spaced image pairs and this allows larger images to be viewed from a greater viewing distance. A stereoscope can project two slightly different images to each eye creating a perception of depth in an image. Some people can master viewing stereoscopic images unaided by looking wall-eyed or cross-eyed but it is difficult to master and can be a strain on the eyes. A stereoscope deflects the images to allow more natural viewing which makes them accessible to the untrained viewer and relieves eye strain. This 3D viewer includes removable lenses which help to focus at close range and these can be handy when zooming in.
The ScreenScope is made by Stereo Aids in Australia. The adjustable ScreenScope version shares many features with the standard non-adjustable model including the strong ABS plastic casing and handle. The ScreenScope has a detachable handle making it convenient to package and store and it is supplied in a cardboard box.
A removable eye mask is supplied which includes optional lenses and the eye mask is a light press fit onto the body of the viewer. The eye mask has a concave shape that blocks distracting light from the side view and this helps to increase immersion. The concave shape of the eye mask does not suit people wearing glass but the mask can be removed when not using the lenses.
The adjustable model uses the same high quality first surface glass mirrors as the non-adjustable model. These special mirrors have the reflective metal coating on the front surface of the mirror avoiding ghost reflections and yielding crystal clear images. Typical household mirrors have the reflective metal coating on the back of the mirror and a faint ghost reflection occurs from the front glass surface which blurs the reflected image. The mirrors use chroma for the reflective surface which is much harder than aluminium but it not as reflective and the reflected view is noticeably darker than the direct view. Since the delicate reflective coating is on the front of the mirrors it is quite easy to damage them and great care is recommended in handling and cleaning them. Avoid touching the mirrors, use a blower to remove dust, and if absolutely necessary then use damp cotton wool to very light clear the mirrors.
The adjustable model allows the angle of the large outer mirrors to be decreased which deflects the light from more widely spaced images and allows the viewing of stereoscopic images that are more widely spaced and also assists the viewing of larger images by increasing the viewing distance. The adjustment makes the viewer more versatile but does not not increase the field of view. Although larger images can be viewer it is necessary to view them from a larger distance so the field of view remains the same. Support for larger images may assist use with a computer monitor by increasing the coverage of the view to use a larger area of the monitor and thus more of the monitors resolution. If you are mainly using the stereoscope to view prints with good resolution and if you have control over their size and layout then you may find the non-adjustable model adequate and more economical at half the price.
The adjustable mirrors are positioned through adjustment screws added behind the outer mirrors, and the mirrors pivot about their outer edge thought the addition of a plastic flexure hinges. The adjustment screws can be seen on each side of the viewer in the photographs above. Springs are used to apply a restoring pressure to the mirrors and these are cleverly placed out of the light path and do not interfere with the view. The mechanism is illustrated below in a partly disassembled viewer.
The range of the adjustment is illustrated below. The left image illustrates the natural position with the mirrors flat against the casing and in this position the mirrors have the same placement as in the non-adjustable model. The right image illustrates the extent of the adjustment achievable by turning the adjustment screws and deflecting the mirrors. It is not clear if the back of the mirrors are a plate to spread the pressure from the screw, and it may be safest to store and transport the viewer with the in mirrors in their natural position to reduce the risk of damage.
Adjusting the angle of the outer mirror is a common technique for increasing the versatility of mirror stereoscopes however the mirrors are deflected from the parallel configuration and this introduces keystone distortion into the view and makes the view sensitive to the viewing distance. Keystone distortion is a stretching of the images where one side become smaller and the opposite side becomes larger, and the deflection of the mirrors causes opposing distortion in the left and right views which magnifies the problem. With this distortion the features in the left and right images are no longer horizontally aligned and this diminishes the quality of the view. At small angles of deflection this distortion does not appear to be signification in the ScreenScope but it is quite noticeable at the full extent of the adjustment and does make viewing less pleasant. Under controlled viewing condition it may be practical to compensate for the distortion but applying a corrective distortion to the source images. The the mirrors are parallel they introduce a fixed horizontal displacement independent of the viewing distance and this make it comfortable to vary the viewing distance, zooming in to see detail, or zooming out to take in the full photograph. However when the outer mirrors are deflected they are no longer parallel with the inner mirrors and the horizontal displacement changes with the viewing distance. The eyes can often adjust to some variation in the horizontal displacement but it can require experience and cause eye strain. A positioning arm is sold separately and includes the same adjustable viewer and this is handy for maintaining a fixed position and adjustment. The ScreenScope has quite tight baffles which are important but the baffles are fixed and as the viewing distance changes the natural convergence of the eyes also changes and without a change in the baffles they may block part of the image and reduce the coverage - this is quite difficult to communicate but I hope to offer a quantitative analysis in future. Ideally are larger stereoscope with the mirrors parallel would be used for viewing more widely spaced or larger images. Note that prism viewers also suffer from keystone distortion and from variation in the horizontal displacement with viewing distance.