Geoscope Standard - Coverage Test
The Geoscope Standard model is a relatively large mirror stereoscope making is suitable for viewing large 8 to 10 inch wide images such as aerial photographs. The coverage is an impressive 8.6 inches high and the manufacturer claims a coverage width of 60% of the size of 25cm wide aerial photographs and the results below demonstrate a coverage close to this over a good range of eye separation when using the magnifying eyepiece and when the separation of the images can be varied. If the image positions are fixed, which is likely for a print, then people can not adjust the image separation for the best coverage width to suit their eyes and because eye separation varies between people some will observe a reduced coverage width. The coverage will also decrease as the eyes move further back from the eyepiece so it may be a little lower if wearing glasses - if you are short sighted then you may need to wear glass when using the eyepiece.
Measuring the coverage
The coverage is defined as the area of the print that is viewable and it is measured here by placing two test images with distance scales under the stereoscope and then panning a camera in front of the mirrors and recording the extent of the test image that can be seen. An attempt was made to place the camera so as to emulate the view the eye sees and the results are roughly consistent with the coverage my own eyes see, but please keep in mind that the results are rough. The actual coverage is quite sensitive to the distance between the eye and the first mirror, and a distance of around 20mm has been used which most people should be able to achieve. The camera was aligned with the help of a scale placed on the viewer, the blurry white bar in the photographs below, and it also gives some perspective as to the range measured.
For natural viewing of stereoscopic pairs it is necessary to separate the images so that the eye convergence matches the eye focus. The required focus changes with or without the eyepiece so different image separations are used. The viewable extent to the left and right of the image center is recorded and the minimum of these is the maximum usable width because the images must be viewable in both eyes and the extent is the opposite in the opposite eye. If the separation of the images can be changed to suit the person viewing them and if an unnatural eye convergence can be tolerated then there is some opportunity to take advantage of the coverage by changing the image separation to recenter the images but an unnatural separation will make the images harder for inexperienced people to see quickly and is not a option for general presentation of prints.
Coverage with the magnifying eyepiece
The eyes appear to be focuses at a couple of meters when using the magnifying eye piece and an image separation of 255mm or 10.04 inches was chosen for this test as it gives close to the widest usable coverage width for an eye separation of 65 mm which is in the middle of the test range. A coverage height of 220mm or 8.66 inches was measured - the photograph shown below left is rotated 90 degrees to fit so the width seen is actually the height. The coverage width was measured at an eye separation of 75, 70, 65, 60, and 55mm. The maximum coverage width of 139mm or 5.47 inches was measured for an eye separation of 65, 70, and 75mm, however the coverage width drops of to 124mm or 4.88 inches at an narrow eye separation of 55mm. Without changing the image separation the usable width is twice the minimum of the left and right extents and the measured maximum usable width is 134mm or 5.28 inches for an eye separation of 65mm but drops off to 94mm or 3.70 inches at a wide eye separation of 75mm.
Note that the usable width of 134mm at an eye separation of 65 mm is not quite as good as the available width of 139mm even though this was targeted to use all the available width, by suitably selecting the image separation, and this suggests that with some more fine tuning a slightly better result could have been obtained. Keep in mind that the listed usable widths assume the that separation between the images is fixed, such as in a large print presented to many people, and that when personally viewing two separate images it is possible to adjust the separation to better center them and thus realise the full available width although the required separation may not be natural for the focal distance.
Eye separation Coverage width Left Right Range Usable 75mm -47mm +92mm 139mm or 5.47in 94mm or 3.70in 70mm -56mm +83mm 139mm or 5.47in 112mm or 4.41in 65mm -67mm +72mm 139mm or 5.47in 134mm or 5.28in 60mm -69mm +62mm 131mm or 5.16in 124mm or 4.88in 55mm -71mm +53mm 124mm or 4.88in 106mm or 4.17in Coverage without the magnifying eyepiece
The Geoscope may also be used without the eyepiece installed and without the aid of the lenses the eyes are focused at about 11.8 inches and the ideal image separation was calculated to be around 8.26 inches. A coverage height of 222mm or 8.74 inches was measured - the photograph shown below left is rotated 90 degrees to fit so the width seen is actually the height. The coverage width was measured at an eye separation of 75, 70, 65, 60, and 55mm. The maximum coverage width of 183mm or 7.2 inches is measured for an eye separation of 70mm and this is a respectable width, however the coverage width drops of to 146mm or 5.7 inches at an narrow eye separation of 55mm. Without changing the image separation the usable width is the minimum of the left and right extents and the maximum usable width is 176mm or 6.9 inches for an eye separation of 65mm but drops off to 102mm or 4.0 inches at a narrow eye separation of 55mm.
Eye separation Coverage width Left Right Range Usable 75mm -68mm +108mm 176mm or 6.93 in 136mm or 5.35 in 70mm -78mm +105mm 183mm or 7.20 in 156mm or 6.14 in 65mm -88mm +94mm 182mm or 7.17 in 176mm or 6.93 in 60mm -94mm +74mm 168mm or 6.61 in 148mm or 5.83 in 55mm -95mm +51mm 146mm or 5.75 in 102mm or 4.02 in
