The convenient, compact size of digital cameras requires the use of a short-focal-length lens and small image sensor. This means it is best to get close to your target. If closeness is impossible and you try to use digital zoom on your computer screen, you soon run into the dreaded pixelated image. We tried a simple, economical solution to this problem: combining the camera with a pair of binoculars.
Figure 1. Using plastic mounting platforms, we integrated the lens of the digital camera with the right eyepiece of the binoculars. The user sights the shot by putting his or her right eye to the left eyepiece of the binoculars.
We mounted our camera and binoculars on a rigid platform so that the right adjustable eyepiece was close to the camera lens at the low zoom setting (see figure 1). One corner of the 125 x 145 mm plastic platform was sawn off and replaced by an 80-mm white plastic strip, which maintained platform rigidity while ensuring comfortable use. An elastic band secured the binoculars to a small platform mounted at the correct height on a piece of grey plastic tubing attached to the platform. Ideally, the unit should be tripod mounted, but results can be obtained by holding the vertical white strip firmly against the jaw so that the right eye can locate with the left eyepiece of the binoculars in a manner similar to using a rifle with a telescopic sight.
We assessed system performance by measuring the optimum print width (OPW), which compares the resolution capability of the eye with that of the camera using a sector star test pattern (see oemagazine, May 2004). An alternative to quoting the number of pixels in the image sensor, the OPW is the width of a print viewed at an arm's length (500 mm), where the spatial resolution of the eye (taken as 5 cycles/mm) matches the measured resolution of the camera. The OPW takes into account lens performance, pixel size, and signal de-mosaicing. The larger the OPW, the more information the camera provides. Using the sector star pattern provides values independent of focal length, zoom setting, and object range. Measuring the OPW for the camera and camera/binocular combination tells us the extent to which the addition of binoculars degrades or improves the OPW value of the camera.
We took measurements with the camera alone at 1X and 3X settings and with the Pentax 8 x 21 binoculars at 8X and 24X zoom values. The OPW of the FinePix 3 MP camera alone was 177 mm; when used with the binoculars, the OPW was reduced to 130 mm. This drop in OPW, independent of zoom setting, is a fair measure of the reduction in information content by about half. Similar measurements using Nikon 9 x 35 binoculars gave an OPW of 138 mm. This method probably works well with a digital camera because the entrance pupil of the camera matches the exit pupil of the binoculars; the tests did reveal significant lateral chromatic aberration in some binoculars.
Figure 2. Images taken with the combined system using a range of optical zoom ratios were set on the computer to give the same final magnification, showing the improvement possible for a bird image at a range of 12 m.
The system produced images with a range of zoom ratios (see figure 2). At the highest zoom shown in the figure, we were actually able to apply a further digital magnification of 3X before pixels appeared. For convenience, the measurements were taken indoors. To obtain a range of 12 m, we mounted the 150-mm-diameter target behind the digital binoculars and recorded the target as a virtual image from a domestic wall mirror placed at the far end of the room, which was 6 m deep. The camera was a FinePix 6800Z model that used the highest 6 MP setting; the binoculars were Pentax 8 x 21, 6.2°. The arrangement shows promise for wildlife, sporting, and surveillance applications. oe
Lionel Baker is a consultant in Orpington, UK.