Checking and calibrating a cheapo microscope.

[DIYSwede]

Are you forgetting that 20 bucks was what you told your wife it cost and not what you actually paid?

Nope - it was on BG at some kind of 30-ish % introductory discount back then.
Today's prices are quite a bit steeper as container shipping costs from China has skyrocketed in the last 6 months,
and their prices also seem to have gone up quite a bit too since then (AFAIK).

I don't have to blow hot air into my dearest's ears, as she sees me being cheap-to-the-bone thru all my daily "honey-dos" too...

BTW: The foil I've scored is from OLD monitors, with CCFT backlighters (which went outta fashion some 10 years ago),
I cannot say what contemporary LED-fed monitors use inside.
Also: My foil HAS only 20 lines/ mm - but when lying with its ridges upwards,
and lit from above each ridge gives 2 light maxima, thus 40 lines/ mm resolution...

BONUS: For HMT:ers checking their microscopes (incl SEMs) with higher magnification,
an ordinary CD has a pitch of 1,6 µm whereas a DVD has a pitch of 0.74 µm



Even simpler and less cumbersome is just zooming in your cell phone display (which resolution could be found out),
as this is a pic of my worn iPhone SE's 640x1136 px display (showing an overexposed white pic):



Note: Vertical resolution IS better than horizontal

Cheers
Johan