Wow, quite a lot of discussion here. To get back to the first point, it seems the calculations of stop diameters to match different NA objectives might need an adjustment for the field-of-view. For 40x 0.65NA objective the field-of-view is small relative to the calculated stop diameter, but it becomes significant for lower power, lower NA objectives. Table 1 in the Olympus article
https://www.olympus-lifescience.com/en/ ... darkfield/ does show a large jump up between the 1x and 2x objectives. I have made dark field stops for a stereo scope which require fairly large stops to match the large field of view but here condenser mechanism was different and so the comparison is not direct
https://youtu.be/TIXAr5cCDOc.
I made dark field stops for my Amscope 120 compound scope from transparent plastic food containers and black electrical tape. They work well with the 10x 0.25NA objective and are OK with the 40X 0.65NA but the latter is more difficult to achieve. This scope was binocular, and after a year’s use it became apparent that a trinocular scope would be more useful. The primary driving force was triggered by Covid and the need the transmit live video images over Zoom to a class of community college students who were monitoring plankton. The cheapest option was to buy a Swift 350T scope which shares some parts with the Amscope. I prefer the condenser on the Amscope 120 which has a rack and pinion drive, rather than the helical twist lever on the Swift 350 (which is like the 380, I believe). Optically they are very similar and probably come from the same factory. I attach a dark field image made with the Swift 350T with 40x objective, a 21 mm diameter stop and a 1.3 MP USB camera which sees the central 1/5 of the field cf. the eyepieces. The height of the condenser is very critical – almost but not quite at the top position with a 1 mm thick slide. I was looking at live organisms from my pond. There seems to be quite a lot of chromatic aberration in the images which was not so obvious by eye (may be the camera color balance is off when operating with low light intensities). The color fringes were not apparent with the 10x objective, so they may also arise from the condenser operating with a full aperture. Also, it is important to look at rather sparse samples – if there is too much scattering material nearby you can get a high background fog in dark field.
it's a basic finite microscope without upgrades. It's not meant to compete with anything more professional or sophisticated - it is what it is. As I said, if one really needs a DF condenser then one should get another microscope. But there's no harm fiddling with what you have and trying to get the best out of it! For me, fiddling about is half the fun!
As far as buying economy scopes, I agree with Louise on this point. My first scope was a $30,O00 Zeiss Axiovert with fluorescence and DIC. It was a great scope for routine biology but insufficient for our purposes of looking at single fluorophores The Zeiss emission filters were not good enough and we ended up on spending double that amount of grant money on additions (lasers, emCCD camera etc.) and upgrades. When I retired, I took up plankton sampling as a hobby and bought a $200 Amscope. I considered this good value for money and certainly good enough for purpose of IDing live plankton at low magnification. I became involved with a local community college and joined students in their weekly tows in the Monterey Bay. The college has just bought 6 Swift 350T’s, partly based on my recommendation, so some students can explore their own plankton and show to the rest of the class in Zoom sessions. I have never been with such an enthusiastic bunch of students who are eager to learn the basics of microscopy. And if a few screws come loose and they have to take the scopes apart, an even better education!