Objectives and Microscope
Three objectives were tested. CFI60 LU Plan 20x NA0.45 WD4.5mm, CFI60 LU Plan Fluor 20x NA0.45 WD 4.5mm, CFI60-2 TU Plan ELWD 20x NA0.40 WD19mm (Fig.1). Another CFI 20x MI interferometry objective was used for stage leveling. Microscope was a Nikon based frankenscope for Epi-illumination. A Nikon1J5 1” sensor digital camera was used for imaging.
Subject
Chrome on glass 0.1mm grid micrometer (Fig.1)
Fig.1
Setup
The grid micrometer was placed on a leveling stage, and with the interferometry objective leveled first. The micrometer was further tilted along the long axis of the imager to measure 10 fringes per 200μm under 545nm monochromatic illumination (Fig.2). Since each fringe represents 1/2λ height difference, 10 fringes translate to approximately 2.73μm per 200μm. The objectives to be tested and micrometer were brought to focus at the center of field of view under 545nm illumination.
Fig.2 Note that the lower side is tilted higher than upper side.
Imaging and analysis
Imaging was done with white light, and captured in RAW format. Development was done in Adobe Camera RAW, and RGB channel separation was done in Fiji. Whole area images are shown in Fig.3. Further crops were made of each image, lower left, center, and upper right. Cropped areas are denoted by white rectangles. Enlarged crops are shown in RGB, B,G, and R channels respectively (Fig.4-6)
Fig.3
Fig.4 LU Plan 20x
Fig.5 LU Plan Fluor 20x
Fig.6 TU Plan Fluor ELWD 20x
Results
Though there are two major caveats in testing...
First, achromatic objectives are chromatically corrected in blue and red, and only spherically corrected in green, maybe focus should have been determined in blue or red rather than green.
Second, the spectrum characteristics of the chromatic filters on the Bayer sensor of the Nikon1J5 is unpublished. RGB is thus only a representative term here.
The effects of the phase fresnel element in the TU Plan ELWD 20x is shockingly good, good enough to call it a true "semi-apochromat" whereas the Fluor greatly disappoints. Most members of the forum who are into macro photography would often choose a Mitutoyo MPA 20x as their prime lens. Mounting this objective on a microscope is cumbersome due to its long parfocal distance (unless you set up a dedicated Mitutoyo scope), and in some applications the sheer weight of the objective is problematic. In those kind of applications, I figure the TU Plan ELWD 20x is an objective worth considering.
