Magnifiers: a closer look (XIV)
A bad triplet? - and a little bit of spectroscopy!

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.

Some measurement results had been presented in September, October and November. Well, were are at no. 14 of our magnifiers' series - possibly too far away from our core topic, the tardigrades? Yes and no. The click numbers are telling us that there is some interest in an enhanced discussion of those magnifier optics among the tardigrade community, so we will continue on this pathway for some time. Besides we are smuggling some MINT information into our e-zine ;-)

This time we will concentrate on photographic magnifiers - those which are used to focus on a matte screen or to check the quality of an old-fashioned slide.

There are many forum discussions dealing with those photographic magnifiers, so we are not going to copy zhe information within those threads. Instead we will apply basic materials' science and spectroscopy in order to better understand some of those magnifiers.

It goes without saying that those magnifiers come at very different prices. We will start with one of the more expensive products:

Fig. 1: The 4x transparency (slide) checking magnifier by the Schneider-Kreuznach company. This instrument is sometimes regarded as a quality reference object against which other magnifiers have to compete. It is ca. 7 cm in height and weighs 86.7 g (including the cord which weighs 11.5 g alone ;-).
Obviously the lens surfaces are coated. We used our Leica test slide in order to check field of view, sharpness, contrast, aberration etc. and were positively impressed. The 365 nm UV test for cemented (achromatic) lenses ended positively as well - no surprise. The focus ring is working smoothly. So, no serious complaints from our side, though we do not like silvery plastics in general ...

But there are still some alternatives for those people who are seeking optical quality on a slim budget:

Fig. 2: A KMZ "HORIZON" 4x transpareny viewer. Very solid build (103 g!), height 6.5 cm. Smooth focusing, no visible distortion, crisp image free of color fringes, best suited for medium format (square) transparencies. The 365 nm UV test is indicating that cemented (achromatic) lenses are included.

Well, are we producing nonsense here, simply checking the obvious? Can we expect utmost optical quality just on the basis of the 365 nm UV test?. Not quite, as the following example is indicating:

Fig. 3: A "PEAK 2018" 8x transparency viewer with transparent stand and inbuilt measuring scale. According to the data sheet it houses an achromatic optical element. This instrument has a focus wheel. But when compared to the two instruments above the finish and feeling of the housing material can't compete.

We checked for the achromatic element by means of our 365 nm UV test:

Fig. 4: The PEAK 2018 8x magnifier in the 365 nm UV-check. It appears that there is an achromatic optical element included. And as the lens material is clearly glass one might expect a superior, achromatic image quality as well.

But when looking through the PEAK instrument on our Leica test transparency the outcome is very disappointing:

Fig. 5: View through the PEAK 2018 magnifier. There are several flaws: a severe cushion type distortion, disturbing reflexes and color fringing (see detail window in the red frame).

In order to find out the reason behind those flaws we removed the magnifier eye-piece and checked it separately: in this case the image was perfect. We think that the mentioned problems are arising from an additional rectangular lens below the eyepiece which is probably intended to expand the field of view. To sum up it appears that this additional lens is spoiling the overall optical quality. We are learning from this that the inclusion of an achromatic element within the light path doesn't necessarily guarantee a good image quality!

When looking further for affordable magnifiers we can find many instruments (aplanatic systems) with still acceptable quality, like the 8x no-name magnifier shown in fig. 6:

Fig. 6: No-name 8x transpareny magnifier with nice design and good haptics. It ist not achromatic but focusable and quite usable. Even the smallest inscriptions on our Leica test transparency are clearly readable.

But their are limits: harsh production cost restrictions have a tendency to end up in plastics lenses and even a lacking focus ring:

Fig. 7: Eschenbach 8x transpareny magnifier with plastics lenses. The image resolution is still usable but there are strong chromatic aberrations when looking at the edges of the field of view. Furthermore, without any focus adjustment some users will not be able to get a sharp image, e.g. due to individual eye defects.

On the other hand the plastic lenses of the Eschenbach magnifier appear to be of still acceptable quality in the image center. When looking through those lenses under crossed polarizing filters the material looks rather homogeneous:

Fig. 8: The plastic lenses of the Eschenbach magnifier as seen under crossed polarizing filters: apart from the in-gate the material looks fairly homogeneous indicating a professional production process.

The problem with plastics lenses is that their outer surfaces will always be more prone to scratches than glass lenses and that they might have a stronger tendendy to accumulate dust.

Besides, our UV test might help the plastics lenses to mimic cemented glass systems, as both the plastics lenses and the cemented doublets and triplets will look "milky" under UV light.

In order to get a better diagnosis we might use more advanced technologies to discern between plastics and glass. Morover, those advanced methods will reveal the type of artificial resins as well. About five years ago we presented a DIY Raman spectrometer here. This instrument is well able to deliver a spectral fingerprint which is characteristic for the type of plastics used. For this sake we simply placed the Eschenbach magnifier in sunny-side-down manner on our microscope spectrometer table and took a Raman spectrum:

Fig. 9: The DIY Raman microscope spectrometer (cf. our magazine issues #200 to #207) in action. The laser beam is focused on the rear lens surface here.

Even though the object geometry is slightly tricky, our Raman spectrometer is well able to detect the plastics type of the investigated lens - without any sampling, burning, scratching etc.: the respective lens apparently is an acrylic lens (polymethylmethacrylate, PMMA).

Fig. 10: The raman spectrum of an acrylic reference sample (black line) in comparison with the Raman spectrum of our magnifier lens. The two spectra look fairly similar, identifying the lens as an acrylic lens.

Now you might say that this is a very expensive method in order to solve such a little problem. And in fact there exists an other spectroscopic method which will come to the same result, faster and cheaper - do you have any idea? We will present this method in the February 2025 issue. See you!

© 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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