Mitutoyo M plan Apo 20x NA 0.42

Some people regard the Mitutoyo M plan Apo’s to be the gold standard for photographing trough a microscope objective from 5x and up. As can be seen in the example pictures below the resolution is a little better than the Mitutoyo 10x but not very much.

Objective Mitutoyo M plan Apo
Magnification 20x
Numerical apperture (NA) 0.42
Resolution 0.65 µm
Depth of Field (DOF) 3 µm or 0.003 mm
Working distance (WD) 20 mm

Resolution r = λ / (2 * NA)

When the numerical apperture (NA) is 0.42 and the wavelength of light (λ) is asumed to be 0.550 µm (green light) the resolution (r) can be estimated to approximately 0.65µm;
r = 0.550µm / (2 * 0,42) ≈ 0.65µm

Depth of Field DOF = λ / NA^2

Depth of Field (DOF) is approximately 3µm or 0.003 mm;
DOF = 0.550µm / 0,42^2 ≈ 3µm or 0.003 mm

Example photo below, butterfly scales from a Peacock butterfly or European peacock (Aglais io), Påfågelöga in Swedish. Mitutoyo M plan apo 20x,  Raynox DCR-150 (208 mm) tube lens, Zerene Stacker, Adobe LR & PS. The second picture shows an enlargement just to show the detail you can photographed with this objectives with a 0.65 µm resolution. The stripes on the butterfly scales are quite visible, but not on all the scales. This is little better resolution than the Mitutoyo 10x.

Peacock butterfly
Butterfly scales from a Peacock butterfly or European peacock (Aglais io), Påfågelöga, photo Jörgen Hellberg
Visible stripes with Mitutoyo 20x
Visible stripes with Mitutoyo 20x, photo Jörgen Hellberg

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