Mitutoyo M plan Apo 10x NA 0.28

Some people regard the Mitutoyo M plan Apo’s to be the gold standard for photographing trough a microscope objective from 5x and up. Having problems with the chromatic aberration with some subjects when I used other non-apo microscope objectives and after seeing some great photos with the 10x lens I bought a 5x, 10x and 20x from a known seller on Ebay.

Mitutoyo 10x

Having read that these objectives are sensitive – the lenses inside the objective can be knock out of alignment I was a little worried about the transportation. When I got the objectives, I compared them with other lenses that I have and concluded that all three objectives were ok.

Objective Mitutoyo M plan Apo
Magnification 10x
Numerical apperture (NA) 0.28
Resolution 1µm
Depth of Field (DOF) 7 µm or 0.007 mm
Working distance (WD) 34mm

Example photo below, an Issoria lathonia, Mitutoyo M plan apo 10x, Canon 400mm 5,6 tube lens, LPL Copy stand, Canon 6D, Zerene Stacker, Adobe LR & PS. I used a 400 mm lens instead of the nominal 200m because I wanted the good image circle of the objective to cover a full frame sensor. The second picture shows an enlargement just to show the detail you can photographed with this objectives with a 1 µm resolution. The stripes on the butterfly scales are just visible on some but not on all the scales.

Issoria lathonia
Queen of Spain fritillary (Issoria lathonia), Storfläckig pärlemorfjäril, photo Jörgen Hellberg

Some math

The distance between two stripes are approximately 7 pixels. A pixel on the camera I used a Canon 6D is approximately 0.0066 mm (36 mm / 5472 pixels). The distance between two stripes are approximately 7 pixels or 0.046mm (0.066 mm/pixel * 7 pixel). Using a 400 mm tube lens that have twice the focal length of the nominal 200 mm the butterfly wing is enlarged 20x instead of 10x on sensor so the distance between two stripes is approximately 0.0023 mm (0.046 mm / 20). This means that one stripe is a little larger than 1 µm or on pair with the smallest thing this lens can resolve.

Resolution r = λ / (2 * NA)

When the numerical apperture (NA) is 0.28 and the wavelength of light (λ) is asumed to be 0.550 µm (green light) the resolution (r) can be estimated to approximately 1µm;
r = 0.550µm / (2 * 0,28) ≈ 1µm

Depth of Field DOF = λ / NA^2

Depth of Field (DOF) is approximately 7µm or 0.007 mm;
DOF = 0.550µm / 0,28^2 ≈ 7µm or 0.007 mm

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