Magnifiers: a closer look (II)

In the previous magazine we had learnt that a positive "true triplet" UV reaction (the milky appearance of a lens packet) doesn't necessarily mean that you have a true triplet at hand. Here, once more, is an image showing the tricky mimicry of a false sort of triplet:

Fig. 1: A 10x measuring type magnifier inscribed as "ACHROMATIC" and "ASPHERIC". The field of view has a usable diameter of 2.5 cm. The height of the instrument is 4.5 cm. Weight 36 g. It delivers an image quality which might be characterized as "mediocre" - at best.

Fig. 2: The same instrument as seen under transmitted 365 nm UV light

As you can see the UV reaction is similar to a truly cemented lens system. Nevertheless, when looking through, black letters on white paper show some nasty color fringing, similar to or worse than even the most modest type of a simple glass cylinder magnifier.
Well, the reason behind is quite simple: the optics of this magnifier are not made of glass but of acrylics. A strong diagnostic hint towards plastics is the visual appearance under crossed polarizing filters on a light desk:

Fig. 3: The vivid colors between analyzer and polarizer are clearly indicating plastics (where strain patterns from the casting process and resulting differences in local refraction indices are quite common).

Obviously, wrongly declared magnifiers are by no means restricted to far-east distributors but marketed by domestic companies' salesmen as well. And please keep in mind that e.g. a Chinese seller might be unable to understand the meaning of fake inscriptions whereas the domestic seller should be able to understand the difference, just by reading in his own language.

Acrylic and other synthetic resin based lenses tend to be marketed as "valuable", "aspheric", and, as we have seen, even as "achromatic". And it goes without saying that those lenses can match, sometimes even surpass glass lenses - at least in theory. But one should be aware of the fact that most quality optics are still consisting of glass lenses. Synthetic resin based lenses can be cast in fractions of seconds and the need for additional finishing is not as apparent as in the case of raw glass material.

Fig. 4: Lateral view on a synthetic resin based lens in a magnifier of lowest quality (polystyrene - NOT acrylic!). Please note the general geometry flaw (wavy appearance), moreover the pits on the surface and traces of what looks like small volcanoe eruptions in the inner volume. The latter are commonly attributed to traces of water in the synthetic resin. During the casting process of the lens the temperature rises well over 100° C, resulting in a kind of frozen bubble firework.

Fig. 5: Top view on the optics of a further cheap acrylic lens magnifier, as well labelled to be "achromatic". The problem with this particular magnifier is that it is intended as a stand magnifier with fixed focusing distance but is lacking a focusing wheel. As a consequence users with need of diopter-assisted eyesight cannot focus properly on their subjects of investigation. Similar as the instrument in fig. 1 also this magnifier is delivering hefty color fringes. Furthermore the plastic housing looks "sunken in", apparently due to some volume shrinking after the casting. Last but not least the system as a whole appears to be extremely susceptible against scratching, even in the more recessed areas. It comes as no surprise that acrylic lenses tend to be offered as "scratch-resistent" (poor salesmen!).

We think that acrylic lenses have only little advantage over glass lenses: possibly being more light-weighted and cheaper in production. For the more scientific people among you they offer one further benefit: when inspected by means of polarized light they are delivering interesting insight into their production process.

Just have a look at the acrylic magnifier shown in fig. 6. Its lens and its housing are cast in one piece, in clear, transparent acrylic resin:

Fig. 6: "COMBI" acrylic magnifier, produced by the AGFA company which at that time was famous for their analogue film products. It is interesting to note that in this case a central lens is flanked by two additional acrylic lenses sliding over the central element. The central lens is delivering a 6x magnification, adding up to 9x in combination with the two lateral lenses. Height of housing close to 6 cm, weight 25 g. Overall it is an interesting optical combination and an interesting design as well. Of course the transparent housing doesn't tolerate lateral light and the length of the housing appears to be way too much when considering that it is protecting lenses with barely 2 cm in diameter.
Obviously, in this instrument design is winning over usability - so we do have a nice collector's item.

Fig. 7: Inspection of the two lens 3x element of the AGFA magnifier under crossed polarizer filters. It appears that the resin inlet for casting was positioned at 3 o'clock. Apart from this there are only few further hints for inhomogenities, indicating that the producer was quite good in resin casting.

After the poor and slightly fraudulent products we would like to show the opposite extreme, a Leitz 10x magnifier with achromatic optics NOT declared as achromatic, thus demonstrating that true and honest understatement is possible:

Fig. 8: Leitz 10x magnifier, probably dating back to the first half of the 19th century. Cemented achromatic triplet lens system. Please note the inherent understatement, as there is no inscription claiming achromatic character.

© 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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