Do you mean which magnification? Either 40x or 100x are good. I think the 40x is slightly easier to start with, since it's easier to find your material in the field of view. At 100x, any slight vibration changes the view dramatically, and it requires a significantly lighter touch to get the hang of being able to focus on anything specific
I'd love to work through a build at 4x or 10x, which should a much simpler in terms of onboarding and just learning how to use the scope. At those magnifications you wouldn't be able to see PM, I don't think.
The cost of all of them actually vary a lot. We took a 40x apart at the barnraising and realized it only has two lenses inside of it, which seemed strange, but it still works. We weren't able to get the high right on the 10x to actually see anything, which seems weird and is a thing worth troubleshooting. I don't think it will work as-is if we just hook up a 10x on the current setup
@kgradow1 are you using one of the openflexure optics modules? If so, that might explain why the 10x lens didn't work as expected. Finite-conjugates objectives are designed to produce an image 150mm from the "shoulder" of the objective, with the sample a set distance (usually 35mm or 45mm) from the "shoulder", allowing you to easily swap objectives. If your camera sensor is closer than that (and you're not using a correcting lens) then different objectives will focus at different heights - specifically, the 10x will focus much further away than the 40x because it has a longer focal length. My apologies if that's obvious to you, just thought I'd mention it in case it helps debug what the issue was.
Hey, for the Openflexure Microscope I tend to buy the lenses from Aliexpress, usually Sunlight Optical but there are many suppliers. The mass-produced objectives are generally all RMS-threaded and designed for "finite conjugates" (they produce an image 150mm from the back of the objectives), and the cost mostly reflects the level of correction (which is related to the number of lenses inside). The cheapest "achromatic" objectives may only have one doublet lens inside (corrected for colour imaging, but not for a wide/flat field of view). These typically cost around $20-$25 and are 35mm from the "shoulder" at the top of the thread to the sample. "plan" or "semi-plan" correction requires at least one more lens inside the objective, and usually is found in 45mm high objectives, costing $25-$50. These will give a much bigger in-focus field of view.
It's possible to add in more correction, or to image at "infinite conjugates", both of which are done in modern lab microscopes - and that's why you can pay thousands, rather than tens, of dollars for an objective if you want. However, I think for the PM work, a basic plan corrected lens will do. My experience so far is that semi-plan is probably acceptable at 40x, but for 100x you really want plan correction (and you need to use immersion oil too!)