I usually measure pinholes with a scanner. Since the scanner knows exactly where the pinhole is and is scanning at a precisely known resolution, you can just read the actual size right off the screen. The highest I can scan is 4800 dpi, which makes this .3mm pinhole about 60 pixels across. A bit of a problem is that the scanner software doesn't deal well with bright highlights and the pinhole is made from brass, so it tries to fix that and makes a way too high contrast image and it's often difficult to see just where the edge of the hole is. This example is better than most. Anyway, it works but it could be better.
I just happen to need a .2mm and a .15mm pinhole this week, so I got my brass, needles and emery paper out and tried to see what they looked like on the Teslong and see if I could measure them.
200x magnification means movement gets magnified 200x too, so pointing is a little tricky. That swirly mark that's on the screen in the picture above was made when my drilling needle, which I was using to hold down the brass against the table, swept across the screen. Pretty exciting. By carefully moving it by nudging the base around you can get enough control moving it to find what you're looking for. The field is much less than that illuminated area, but it's possible to find the approximate center of the spot of light. You don't need the pinhole to be in the center, you just need it on the screen.
The camera uses the shareware video capture program VLC to display the image in a 1280x720 pixel window. VLC wants to capture video, so the easiest way to get a still image is to do a screen capture.
I happen to have several sizes of Gilder EMS pinholes. They come in vials of 100, but Earl Johnson, I think, still resells small quantities for about $1 USD apiece. I'll let him give you contact info in the comments if he wants to. Anyway, my stash includes some .2mm apertures - just what I'm trying to measure.
They look pretty good under magnification.
Selecting just the pinhole, it measures 100 pixels across (how convenient), so now I can measure with it and it's a little better than twice as sharp as the scanner (recall that my scanner had 60 pixels for a .3mm hole).
Now I've got to try to drill a .15mm.
Both pinholes are now mounted on cameras so we'll see how they do when I get some film through them.
The Teslong seems like a pretty good deal for $41.