As suggested by @Maggpi, it would be advantageous to have a generic easy to get calibration card for microscopy. To that end, I imaged the corner of a 5$ note using two magnifications. I then imaged a USAF 1951 calibration target. Imaging was done by a Galaxy S7 and a microscope adapter. when I started to analyze the images I noticed that some of the phone camera parameters were not properly set and will repost with better images.

When addressing microscope calibration there are several parameters which are relevant:

* Pixel to mm conversion

* smallest discernable features (optical resolution)

While doing this, it's worthwhile to also inspect the distortion (which is noticeable in some of the images)

* Optical distortions in the image (straight lines appear to have a curvature)

Low magnification images:

High magnification images:

The target images are inverted (had to do that to reduce shadow) and the distortion in the high magnification is very noticeable.

To asses what are the smallest objects which can be discerned I have cropped, flipped and changed the dynamic range.

The smallest Group / Element is 5 / 6 (just above the number 4 in the bottom right) the width of such a line in 8.77 um (see here).