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 and coverage width of 60% of the size of aerial photographs however measurements below suggest the coverage width is very sensitive to the eye separation of the person using the stereoscope and the usable width could be as low as 3.7 inches for some people. The coverage width is quite good for people with an eye separation close to 65mm and if your eyes suit the Geoscope then it may well make a good personal stereoscopic photograph viewer. Even if your eye separation is small or large than 64mm then the Geoscope is still good for inspecting large stereoscopic pairs and the viewer can be panned around to study the wider image and higher magnification eyepieces are also available.

The low and inconsistent coverage width makes this viewer less attractive for the presentation of stereoscopic photographs where it is important to cater for a wide range of people with a wide range of eye separation and where it is desired that the full image can be observed without panning. The results below suggest that when using the magnifying eyepiece that the maximum usable image width would be just 3.7 inches and when using the viewer without the eyepiece the usable image width would be 4 inches, and at the optical path length of around 10.8 inches this gives a rather poor field of view and makes the Geoscope uncompetitive for the presentation of stereoscopic photographs. The Geoscope does not include baffles to block secondary views and these can be very distracting, particularly for people inexperienced at viewing stereoscopic pairs, although this can be correct by adding baffles to the Geoscope. A mirror stereoscope with such a large separation between the mirrors has room for larger mirrors and this would increase the coverage, however larger mirrors would increased the size of the viewer and the cost so the Geoscope is a good compromise for the inspection of large stereoscopic pairs.

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. 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.

Measuring the coverage without the magnifying eyepiece

The eyes are focused at about 11.8 inches without the eyepiece 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 separationCoverage width
LeftRightRangeUsable
75mm-68mm+108mm176mm or 6.93 in136mm or 5.35 in
70mm-78mm+105mm183mm or 7.20 in156mm or 6.14 in
65mm-88mm+94mm182mm or 7.17 in176mm or 6.93 in
60mm-94mm+74mm168mm or 6.61 in148mm or 5.83 in
55mm-95mm+51mm146mm or 5.75 in102mm or 4.02 in

 

 

Measuring the coverage with the magnifying eyepiece

The eyes are focuses at a couple of meters when using the magnifying eye piece and a wider 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 separationCoverage width
LeftRightRangeUsable
75mm-47mm+92mm139mm or 5.47in94mm or 3.70in
70mm-56mm+83mm139mm or 5.47in112mm or 4.41in
65mm-67mm+72mm139mm or 5.47in134mm or 5.28in
60mm-69mm+62mm131mm or 5.16in124mm or 4.88in
55mm-71mm+53mm124mm or 4.88in106mm or 4.17in