Lateral Chromatic Aberration from Using a Noncompensating Coolpix Adapter with a BH2 Microscope
In a previously published article, I noted the serious loss in resolution from using a noncompensating eyepiece with the LOMO Biolam microscope, instead of a required compensating eyepiece, to couple a Nikon Coolpix 995 digital camera to the microscope. My test specimen for this observation was the reflected light LOMO stage micrometer. This slide, unlike other reflected light stage micrometers, can be used with both transmitted and reflected light because it is transparent, with gaps in the chrome coating on glass substrate serving as the graduation marks. I cemented a glass cover slip over the test pattern with Canada balsam for transmitted light application. The Canada balsam has yellowed with age, so the graduations have a slight yellow coloration. I noted in my previous article that the graduations were increasingly blurred towards the edge of the field when the stage micrometer was viewed with the 9X LOMO objective using a noncompensating eyepiece. The stage micrometer imaged with a LOMO Edupointer compensating eyepiece is shown in Figure 1. I did not bother then to capture an image of the stage micrometer with a noncompensating eyepiece since demonstrating image degradation from not using the correct compensating eyepiece was not the topic of my previous article. Peter Evennett later provided for the article the missing example of a stage micrometer recorded with a noncompensating eyepiece adapter using a Zeiss objective requiring a Zeiss Kpl compensating eyepiece. Peter used a conventional transmitted light stage micrometer with opaque lines on clear glass substrate. Peter added a filter to the illumination that blocked the green portion of the spectrum so any lateral chromatic aberration would be evidenced by a splitting of the lines into separate red and blue lines; his example in Figure 2 shows this beautifully.
Recently I was part of a team at McCrone Associates testing an Olympus BH2 with Coolpix digital camera adapters of both compensating and noncompensating types using the LOMO stage micrometer with a cemented coverglass as one of the test specimens. With the10X objective we observed the same blurring of graduations away from the center of the field with a noncompensating eyepiece. This time we digitally imaged the stage micrometer to document the chromatic aberration away from the center of the field; it is evident in spreading of the graduations into red, green, and blue color bands in Figure 3. We discovered during this testing that OPTEM couplers fitting the BH2 were either compensating or noncompensating, with no markings on either adapter to note which adapter was compensating. The compensating OPTEM adapter performed well in the chromatic aberration tests with the stage micrometer when used as a high eyepoint, viewing eyepiece. Time limitations prevented us from determining whether the compensating OPTEM adapter eyepiece had the optimal corrections of the Olympus 8X WHK eyepiece used in the BH2 adapter described in my previous article. That article links to Peter Evennett’s RMS article describing his use of a resolution test pattern, which would allow quantitative evaluation of the OPTEM eyepiece performance versus the WHK eyepiece with the BH2 microscope. The compensating OPTEM eyepiece exhibited the characteristic orange fringe at the edge of the field stop of a compensating eyepiece. This orange fringe is recorded in Figure 4 using a Zeiss Kpl eyepiece. An eyepiece can be quickly tested to determine whether the orange fringe characteristic of a compensating eyepiece is present by viewing the field stop when the eyepiece is pointed towards a bright light source.
Some careful observers may note an apparent reversal of the lateral magnifications for blue and red when they compare the stage micrometer images of Figures 1 and 2. The image of the opaque lines of the transmitted light stage micrometer shows blue lines at lower magnification than red lines. Since the object does not emit light through the opaque lines, how can the outer part of image in Figure 2 contain red and blue lines? The answer is that the blue lines are where the lines have been imaged by the objective in red. Since the lines in the stage micrometer are opaque, the image of the micrometer in red will have the lines imaged with no red intensity. The blue images of the lines are from the purple filtered background being imaged in blue in the outer part of the field where the effect of the different magnifications for the two colors is most significant. Peter Evennett has supplied for this article an image of a stage micrometer having clear graduations in an opaque background, like the LOMO stage micrometer. Peter’s image of this stage micrometer taken with the purple filtered light is shown in Figure 5. Note there is no discrepancy when the images are properly interpreted.