Magnifiers: a closer look (IV)
Octoscop 28x hand magnifier vs. Leitz stereo (dissecting) microscope 64x

A few dacades ago, stereo microscopes (i.e. low mag stereoscopic microscopes - not to be mixed up with binocular but non-stereoscopic microscopes!) were extra-ordinarily expensive, far beyond the financial reach of a typical amateur. Nowadays, used top level instruments of the 1970s can be found at dramatically lower prices via internet or local sellers. Here we are going to illustrate how those elitarian instruments compare with respect to the Octoscop magnifier presented in the previous issue of our magazine.

Fig. 1: The noble Leitz "Großfeld-Stereomikroskop TS" with a modern digital camera attached to one of its eyepieces (the second eyepiece is not visible here, hiding behind the camera mount). The Leitz instrument has three integrated objective pairs. In the situation above the 1x objective pair is active. Due to its high working distance it is very far recessed in the stand, not visible on the photograph. Two higher mag objective pairs (4x and 10x) are mounted in conical housings which can slide laterally into the central working position. Overall, with well-fitted Leitz 16x eyepieces, magnifications of 16x, 64x and 160x can be achieved. Nevertheless we think that the 160x is slightly overdone ...

Even today, 50 years after its production, the Leitz Großfeld stereo microscope can provide a fascinating visual object immersion feeling. But there are some potential annoyances as well: whereas many older Leitz stereo microscopes can be perfectly dust-proof, this is not the case with this instrument. The 1x objective lens pair on the underside of the objective turret is not sealed, thus being prone to dust. Moreover the objective change mechanism is relying on a retaining spring which might be not strong enough to function after decades - when the lubricants have a nasty tendency to become stiff. As a consequence professional cleaning might be necessary.

Just as a flashback, for comparison, we are going to show the optical output of the Octoscop hand magnifier at its maximum 28x magnification (fig. 2) and directly below the respective output of the Großfeld stereo microscope (fig. 3):

Fig. 2: Detail view of a German 1 Cent coin, photographed through the Octoscop in its 28x configuration. As already explained in the previous issue the Octoscop has an extremely low working distance when used in its 20x and 28x setting. Moreover the lens fitting is large, thus hindering illumination light to shine on the object at an optimum angle. Furthermore disturbing reflexes appear in the field of view.

Fig. 3: The same detail as photographed by means of the Großfeld stereo microscope, with the 4x objective and the 16x eyepiece in place. The detail view is magnified by factor 2, thus showing only 50% of the visual field diameter. As with all stereo microscopes in general, also this photomicrograph is only a modest representation of the actual visual stereoscopic impression. As mentioned previously the height of the number "1" is about the same as the length of a typical tardigrade.

Technical annotations for photographers: the camera, a Sony Nex-5N was adapted in a purely mechanical manner, being connected by standard photographic M42 spacer rings plus a camera-specific M42 > Sony adapter. For fig. 3 the light was merely passing through one of the 4x objectives, then through one of the 16x eyepieces and from there directly on the CMOS camera chip (no additional, dedicated optics and no camera objective needed). As we are dealing with twin pairs of objectives, each looking slightly laterally on the object from both sides, the object plate had to be slightly inclined towards the photographing objective, at an angle of ca. 7.5°.

As already explained previously the stereo microscope can provide a range of 16x - 64x - 160x (!) with the 16x eyepieces in place. But when looking at the respective numerical apertures of the objectives (0.05, 0.08 und 0.1 - being a measure for their individual resolving power) it will become clear that the resolution step between the 4x and 10x objective cannot be very impressive. So in most cases the photographer will be better off with the 4x/16x (64x) combination than with the 10x/16x (160x) maximum configuration.

Resume: it comes as no surprise that the Großfeld stereo microscope is the winner in this competition. Nevertheless we were quite impressed by the fact that the stereo microscope is actually performing much better, even revealing tiny flaws and scratches on the figures. In the next issue we will demonstrate that a high grade single mag dedicated folding magnifier can do a much better job. But still, the Octoscop is a nice instrument with a beautiful design!

© Text, images and video clips by  Martin Mach  (webmaster@baertierchen.de).
The Water Bear web base is a licensed and revised version of the German language monthly magazine  Bärtierchen-Journal . Style and grammar amendments by native speakers are warmly welcomed.

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