Fig 1. OMAX v434b stereomicroscope
Fig 1. OMAX v434b stereomicroscope
This trinocular stereomicroscope (reviewed here) was paired with a Canon EOS 600D / Rebel T3i camera connected by a MicroscopeNet Canon DSLR 2x adapter (SKU A14CanonSLR, labelled with 'NDPL-2(2X)'). It has a Greenough-design with a continuous zoom range of 0.7x to 4.5x, and an angle of convergence of ~12°.
As per the Focus Analyser procedure, the test target for these analyses was a carbon-soot-coated straight-edge razor blade inclined (with respect to the optical axis) by 12° (6.5° provided by a wedge, plus the 6.5° angle of the optical path with respect to the stage) and tilted (with respect to the camera frame) by ~5°, elevated about 5mm above an out-of-focus white paper background. The target was illuminated obliquely by a two LED lamps at ~45° to each side and one halogen lamp at ~45° from the back. Raw files were processed with dcraw ('-D -4 -t 0') to produce PGM files used by Focus Analyser.
The camera was operated remotely (using Canon remote capture software) with 'mirror lock-up' enabled. The camera's white balance was set to Auto.
Fig 2a. Blacked razor blade, inclined with respect to the optical axis. |
Fig 2b. Blacked razor blade, inclined with respect to the optical axis (red line). |
Below are Focus Analyser reports for selected zoom settings from the stereomicroscope's range of 0.7x to 4.5x (magnification of 1.7x to 10.4x via the camera arrangement here, taking the EOS 600D sensor as 22.3 mm wide). The field of view (FOV) for each analysis was determined by taking a photo of a scale positioned perpendicular to the optical axis, at each zoom level.
Ideally, the microscope + camera adapter would focus all three colours identically; the curves would overlap. At lowest zoom levels, the red and green channels are fairly well matched, but as the zoom level increases, the best-focus distance for the three channels drift apart, exhibiting increasing amounts of longitudinal chromatic aberration.
Also, especially at high zoom levels, the three channels don't come to the same degree of focus; at all zoom levels, the blue channel has less resolution, and green is best-focused.
Figure 3 below charts edge spread vs zoom level, in terms of pixels (blue) and microns, for the green channel.
Fig 3. Focus Analyser edge spread vs microscope zoom level, in terms of pixels (blue) and microns (red).
Figure 4 is a photo of an integrated circuit, lit by a ring LED light. The teeth in the comb-like feature in the image at the right are spaced by approximately 11 microns. Compare this with a similar photo taken with a good macro lens and a high-end microscope.
Fig 4. 4.5x zoom, downsampled to 1733 px, cropped and viewed at 1:1 pixel level.