Magnifiers: a closer look (XIII)
Professional focal length measurement for amateurs - further results!

In the July issue we had presented the theory behind an advanced focal length measurement method (making up an almost universal micro-optics magnification measurement method!).

The respective practical procedure had been explained in August.

First measurement results had been presented in September and Oktober.
In those two previous issues we had shown several misleading magnifier inscriptions, some of them possibly ranging beyond the borderline of fraud. This time we will focus on the well-behaved instruments, i.e. those with a magnification inscription with 10% deviation or less. Our first measurements of famous brand products (Zeiss and Leitz) hade revealed "deviations" down to 1% and below. In those cases we were not even able to clarify whether the deviation was possibly due to our measurement method!

Here are some further positive examples:

Fig. 1: A very small, 20x "FIVE ELEMEMTS" folding magnifier. This is a true 5 lens system - but be warned that there exist very similar instruments bearing a modest single cylinder lens in their housing! One might call this an anastigmatic magnifier (including achromatic behaviour).
Measured magnification: 18.4x

Technical annotations: We had shown this instrument already in one of our older issues, in order to illustrate that even a misspelled inscription like the one shown in fig. 1 needn't be proof for a fraudulent product. The measured magnification value of 18.4x is well within our trusted range of 10%. And the optical system is made up of two (!) cemented doublets arranged around a central biconvex lens.
Weight 18.14 g, Length of housing (when closed) ca. 2.6 cm, height 1.6 cm.
User evaluation: This is a top-notch optical instrument delivering a crisp image quality without any color fringe. Nevertheless a 20x magnifier with cylindrical housing might turn out as problematic when used in subdued light. Moreover we were not able to assess a noticable visual advantage of the five lens system when compared to a 20x Steinheil triplet system.

Fig. 2: Besides the 20x "FIVE ELEMEMTS" magnifier shown in fig. 1 had a golden sister (or brother) with identical optical construction. Well, it is golden and not merely glary but instead a true five lens instrument!

But let's turn our attention to a much more sober Zeiss magnifier:

Fig. 3: 10x Zeiss magnifier with ring fitting for use with various optical heads. Measured magnification 9.98x (!)

Technical annotations: The optical head is not restricted to be used within the ring fitting. Alternatively it could be mounted on an old-fashioned dissecting microscope stand. The magnification specification is absolutley correct (what appears to be typical for those old Zeiss products). The optical system is consisting of a two lens cemented doublet (an achromatic system). Diameter of the optical head 34.6 g, overall length with handle ca. 16 cm.
User evaluation: This is a high quality lab and field use instrument - though lacking any glamour. The top is blackened and recessed which might be of great advantage under adverse lighting conditions. The field of view is relatively narrow for a 10x magnifier, measuring merely 1.5 cm. As to be expected the image quality is flawless.

Fig. 4: The foldable 20x "Betamag" magnifier - a quality product with honest inscripton. We measured a 20.0x (!) magnification. This is true precision!

Technical annotations: This is a special type of magnifier, intended to be placed above the object at a fixed distance. The optical system is made up of a cemented doublet in combination with a single convex lens (i.e., a true triplet).
Measured magnification: 20.0x. Length of housing 5.6 cm, weight 90.45 g.
User evaluation: The Betamag's primary use might be the study of print products, like e.g. old engravings at fixed distance. But the optical head could as well be unscrewed and used separately, as a stand-alone, more universaol instrument. Solid feeling, all glass and metal.

Fig. 5: Another vintage instrument, probably made in the 1850s - the "Zirkel K" 16x magnifier.
Measured magnification 15.7x - almost perfect!

Technical annotations: The Zirkel K magnifier is marking a record with respect to tinyness and featherweight: its length is 3.6 cm and the weight 5.24 g! Its optical system is relatively simple, made up of two lenses (this a generally named as an aplanatic system). Measured magnification: 15.7x.
User evaluation: Due to the relatively simple construction the image can be neither wide-angle nor achromatic but it appears to be fully appropriate for casual outdoors' use. Moreover the plastics housing's ergonomy and haptics are quite agreable. Please keep in mind that in cold weather plastics feel much better than icy metal!

Fig. 6: A no name 20x 6LED magnifier, made in China. Measured magnification 18x.

Technical annotations: We already presented this magnifier in one of our older issues but at this time were not able to perform precision magnification measurements. With a true 18x magnification value this devide ranges among the instruments with honest inscription, as a 10% deviation can be considered to be still accepptable. The design is usable for various tasks: it can be positioned in fixed distance mode, e.g. to study the print quality of engravings but also freely, with the optical head hand-held. Furthermore, there is the 6LED light providing a flawless, shadow-free illumination in any environment. The optical system is consisting of a cemented achromatic doublet, in combination with an additional biconvex lens. The outer lens surfaces appear to be coated. Length of housing 5.22 cm, weight close to 30 g.
User evaluation: The optical surfaces appear to be finished to a high standard. The resulting image quality can be considered as perfect for practical use. And it is free of color fringes. The 6LED illumination is providing an additional bonus and does guarantee that the instrument can be always used, also in very dark environment. What we do like less: the battery housing is made of cheap silver colored plastics. But, to sum up and considering the typically requested low price, this instrument must be recommended as one of the most bizarre and rewarding consumer bargains, thanks to international trade and globalization.

Fig. 7: One of the classics - a BelOMO 10x "Made in Belarus" magnifier - a true triplet! Measured magnification 8.9x.

Technical annotations: Please be warned that fake BelOMO "TRIPLETs" are marketed as well. These are non-achromatic, two-lens (doublet) systems. They can be fairly easily discerned by their markedly lower weight (23.3 g in case of the fake vs. 41 g in case of the real thing!) BelOMO magnifiers have a very good reputation among amateurs. One reason behind is that they were available in high numbers and at low prices at the time when the Soviet Unions fragmented into its individual members. As we all know the typical microscopist is an elderly man and therefore tremendously price-conscious. So we are not surprised that the BelOMOs found their fan basis - and why not: they are solid heavy metal constructions with a honest, true cemented triplet lens interior! When folded the length of the housing is 3.6 cm, its height 2.5 cm. The weight is 41 g. The field of view is quite large, measuring ca. 2.3 cm in diameter.
User evaluation: The mechanically solid instrument is delivering a professional, achromatic image quality and a wide field of view. So there is no need for serious criticism. Nevertheless users might be not fully convinced with the housing's screws as those have a tendency to losen and fall off. But of course it is possible to fix this by means of a tiny droplet of glue. Moreover we detected traces of rust on some of those screws on older instruments, a damage type which is quite rare in the world of magnifiers. Last but not least a metallic housing like the one used here doesn't feel nice when used in cold weather.

Resume:
It is not possible to give a clear recommendation for any user and any application. Some people tend to emphasize a wide field of view, others prefer a metallic feeling to the one of Bakelite. Last but not least some persons are not able to focus on a given object through a typical 10x magnifier but will be happy with an ordinary 3x reading glass!

For a quick overview and general use we are preferring an old 3x "Steinheil" branded magnifier. It boasts with an extremely large field of view and an excellent resolution from edge to edge. One should understand that the use of such a true Steinheil system within a 3x magnifier is extremely rare and a solid basis for paradise-like image quality - possibly bordering to image quality overkill!
Our tardigrade workhorse magnifier will always be the 10x 6LED true triplet magnifier as shown in our previous magazine issue (there fig. 1).
Higher magnification instruments are intended for use by people who know very well what to look for (pinpointing very fine object detail without the need of comfortable focus depth or wide field of view).

But be warned to buy relabeled Chinese 6LED 10x mgnifiers from local dealers at highly elevated prices!

© 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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