Magnifiers: a closer look (I)

Meanwhile, after a few decades, we feel confident to share some informations about magnifiers - most of which you won't find elsewhere in the internet. As usual we will preserve the entertainment character of this website but will provide some serious (and partially forgotten) optical knowledge as well. Be warned that we are not talking about reading glasses but about loupes in a more strict sense, i.e. small diameter, close-focus instruments, typically with higher magnifications (5x ... 10x ... 20x ... 30x ...).

In a rather simplified, purely practical demonstration we had already shown previously that the usability difference between a very simple, single lens magnifier and more elaborate three-lens systems needn't be great. You might have a look back to remember, e.g. here.

And there are in-between quality solutions as well: the funny, tiny two-lens "K Zirkel" magnifiers are among the tiniest instruments we know and still perfectly usable to check the characteristic of a sea sand sample or a bug.

Fig. 1: "K Zirkel" (or "Zirkel K" ?) magnifiers can be found in several variations (we know only 8x, 10x and 16x instruments). One reason why we are showing them here is that we were unable to trace the signature logo to a specific optical company. But we have reasons to assume that they were made in former West-Germany, in the 1950s. Perhaps one of our readers does know?
The Zirkel K magnifier optical system consists of two very thin plano-convex lenses, kept at distance by a spacer. Not only the size is definitely minimalistic, but also the weight: 5 grams! This tinyness comes at a price, though: they all have a rather narrow field of view, e.g. 15 mm in case of the 10x magnifier (whereas other, bigger 10fold magnifiers easily provide a view diameter of 20 mm and more).

If you are simply interested in your biological samples, not so much in the gadget factor of the instrument, those Zirkel systems will perform the task.

On the other hand it can be quite relaxing and revealing, or shere fun, to compare with the luxury glamour view through an old-fashioned Steinheil 3x magnifier. The one shown in fig. 2 boasts with a true cemented "Steinheil" triplet system. This might be considered as the utmost magnifier quality overkill when seen in relation to the comparatively low magnification. But there is no doubt that the image quality of those Steinheil 3x magnifiers is fascinating, extremely clear and crisp, with no color fringes, from edge to edge. We even think that the visual experience is a kind of adventure. Some of you might even wonder if they should discard their 10x magnifiers!

Fig. 2: Steinheil 3x magnifier, with true 35 mm diameter cemented triplet optics, probably dating back to the 1930s. Bakelite housing.

In case you are not in the mood to dismantle your own precious objects in order to scrutinize their interior composition there are analytical alternatives: You might perform a non-destructive plausibility test for cemented achromatic character by means of a 365 nm UV flashlight. When passing through a cemented lens system the UV light should cause it to look milky (cf.  fig. 4 and fig. 5). Let's try this with the two magnifiers shown in fig. 3:

Fig. 3: Two typical folding loupes. On top, a ZEISS 6x magnifier based on an achromatic (cemented) doublet. Below, a 10x ENURO magnifier made up of to plano-convex lenses separated by a spacer (i.e., a simpler, non-cemented, non-achromatic system).

Fig. 4: GIF animation. The glass of the 10x ENURO magnifier (left side) is remaining clear under UV influence, whereas the ZEISS 6x magnifier glass is turning milky.

Sometimes you might stumble across statements that this milky character of the achromatic systems might be caused by one of its special glasses (the flintglass). But this is not true. You see, those lenses are cemented together by means of a natural resin called Canada balsam. It is this resin which is causing the milky (fluorescence) effect - not one of the lenses:

Fig. 5: The doublet of the ZEISS 6x magnifier under UV light of wavelength 365 nm. It is becoming obvious that the balsam glue is causing the fluorescence, appearing as a bright line running around the walls of the glass cylinder.

But we warned that also very old or very primitive glasses might show bright fluorescence under UV light. Though normally their obvious old age or primitve fitting will signal that they belong to a very different class of object.

Fig. 6: Single lens "Stanhope" magnifier dating back to the 19th century. Its glass body appears in a green color when illuminated by means of 365 nm UV light (under ordinary white light the glass looks perfectly clear and colorless). In this case there is no glue and the glass itself is causing the fluorescence effect.

Okay, now we can use our fresh knowledge in order to investigate a further object. Just have a look at this one:

Fig. 7: a further candidate for the UV light check - an "aspheric" and "achromatic" 10x magnifier (according to its inscription) sold as an inexpensive tool for stamp collectors.

The UV check result is looking positive (see fig. 8). So, is this actually a bargain, true achromatic system?

Fig. 8: The same magnifier as shown in fig. 7, but in detail top view and under UV light. It is clearly showing the milky fluorescence effect as discussed above. But when checking the actual image quality of this instrument by means of black letters on white paper, it is becoming obvious that the optical performance is rather poor, with lots of color fringing, by no means an achromatic system. Besides, also the surface of the black plastics fitting looks slighty wavy and there is some ugly, creaking noise when adjusting the focus.

Possibly some of our readers will already know the reason behind this diagnostic problem. All the others, please remain patient, we will explain the cause in the next issue of our magazine.

© 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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