Some Wild stereo microscope numerical aperture measurements

[jfiresto]

Northern Europe has been enjoying some fair springtime weather these last couple days, so naturally a (not really so) young man's fancy turns to — measuring the numerical aperture of his microscopes! He has been patiently waiting for a cloud free day.

The microscopes' lower, objective / magnification units all use infinity optics, so I mounted each one upside down, found a nearly collimated, natural light source, the sun, reflected it into the left and right optical outlets and measured the diameters of the focused beams, exiting the objective at maximum magnification:

setup.jpg (78.82 KiB) Viewed 5423 times

I then calculated the numerical aperture with: N.A. = sin(arctan(beam-diameter / 2 / working-distance))

Here are the beams out of a Wild M3:

m3.jpg (84.2 KiB) Viewed 5423 times

The horizontal scale marks are about 5.03 mm apart (rather than 5). Accounting for that, I measure a numerical aperture for the M3 of 0.75.

Here are the beams out of a Wild M7A (my M7S is similar):

m7a.jpg (78.34 KiB) Viewed 5423 times

This M7A is missing the accessory ring that spins onto the end of its objective; without it, has a 92mm working distance. (Has anyone got an extra ring?) Using that and accounting for the spread scale marks, I measure a numerical aperture for the M7A/M7S of 0.070.

Leica Microsytems told Walter P. at the Mikroskop Forum that the M3Z with a 1X achromatic objective has a numerical aperture of 0.075.

I believe these NA's of 0.07 to 0.075, for a ca. 90 mm working distance, were fixed by the common, 14mm diameter infinity optical paths chosen for the M3 in 1972.

[75RR]

Very interesting. Will have to attempt that with my M8

Do you know what the NA should be?

[jfiresto]

Given the pattern, I would posit 0.07 ~ 0.075 for the 1X, 87mm working distance objective.

[75RR]

Given the pattern, I would posit 0.07 ~ 0.075 for the 1X, 87mm working distance objective.

Thanks

[Scarodactyl]

Does the NA not drop at lower mag settings? I know it does on some systems (macroscopes like the az100 or the leica z series), presumably since the objective is only really high res at the center where it counts at higher zoom settings, and stopping down at lower mags makes the ce ter match the lower resolution periphery while increasing DoF.
Edit: whoops I misread the 'maximum magnification' as 'minimum.' It has been a long day.

[75RR]

Leica Microsytems told Walter P. at the Mikroskop Forum that the M3Z with a 1X achromatic objective has a numerical aperture of 0.075.

This is a translated extract from the Walter P. link: