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Objective measurements

Last Updated: 22nd Feb 2016

By Stephan Wolfsried

In my last article I found the Sony Nex 7 to be most appropriate camera in my setup with a bellows and microscope objectives.
Now I wanted to figure out which objectives cover best the range from 1 mm to appr. 20 mm FOV in terms of resolution and working distance to obtain best stacking results. For that purpose I bought a special target from Carl Zeiss which is capable to measure the resolution of objectives up to 3000 line pairs per mm. It is not a bargain, CZ Göttingen sells it for about 1500 €, but I think it is worth while, because there is no other way getting resolution measurement immideately.

Carl Zeiss Target 3000, FOV is 22 mm

An exemplary crop of a measurement is shown here:

Resolution of a Mitutoyo M Plan Apo 20x

This is the proof of the capability of my setup to come into the specified resolution areas of the objectives, which means that the overall stiffness and increments of the linear actor are sufficient.

With that Carl Zeiss target I measured my whole set of Luminars with and without tubus lens and as well my infinite objectives, most of them are Mitutoyos. The result is shown in the following table:

objective measurements

The really interesting figure is the amount of line pairs related to the FOV. This term represents the information density of a macro photo. Here is the graph containing this special value from the above table:

objective resolution graph

In a nutshell I found out, that only four different objectives cover the range from 1 mm FOV to 22 mm FOV with a reasonable value of 1500 to 3000 line pairs per FOV. The Luminars 25 mm and 40 mm are not bad in resolution, but they have not the appropriate working distance for good stacking results. The Seiwa Plan Apo 2,5 begins with vignetting at FOV around 10mm. Therefore the Luminar 63 mm seems to be the best choice for FOVs greater than 8 mm.

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