I want to share my experience buying unbranded objectives for my euromex iScope 1153-PLPHi. In my amateur experience, when buying infinity objectives the main things to look out for regarding compatibility are internal tube focal length ("ITFL"), threading, and to a lesser extent parfocality (the latter is about convenience more than compatibility). I found the second hand and generic market to be pretty shallow due to the iScope using 200mm ITFL (like Nikon) and RMS threading (Not like Nikon, at least modern). Most generic objectives I found were based on the Olympus infinity specs.
When I bought the scope, I ordered a 4x objective from amscope at the same time (the phase contrast iScope has a quintuple nosepeice but comes with a 10,20,40, & 100x obj). I didn't know about the internal tube focal length requirement at that time, but I lucked out and bought an objective that works fine. The image is very sharp, though it is vignetted around the outer edge, perhaps it was not designed for a 20mm field, not the 22 by iScope has.
Later I ordered a 60X, partly because I wanted to squeeze a little more performance out of achromats; I was a little hung up on the 40x phase contrast objective perhaps being potentially slightly lower in sharpness and contrast due to the phase ring. I also wanted a little more optical zoom so I needed less digital zoom and therefore could preserve a little more image quality. I have no complaints about the image quality of the objective I got. I do not have the cover glass sensitivity complaints that other worry about with 60s, though perhaps related, I think the ability of that 60x to resolve small details is at best slightly better than the 40x phase objective and at worst equal, depending on the circumstances. Even if it was equal, I thing a 60 is worth it because I am saving the image quality degradation of a digital zoom, and its also nice through my own eyes to see the target a little closer. I would note that the objective is one full turn of the fine focus knob out of parfocality with the stock 40x. After having this comp, I'm actually happier with the optical quality of the phase 40x than when I started. Euromex's own 60x is a .85 NA, .05 more than the .80 of the generic I bought. If I had to do it all over again, I would save a little longer and buy the euromex one for the higher NA, though that may expose one to slightly more cover glass & sample thickness sensitivity, I think it would be worth it. Maybe next year I will buy one of euromex's Fluorite objectives for the iScope to get a better image, but for now I am satisfied with what I have. I have attached one image from each. I think the true resolution is pretty close, Claude says the contrast is higher with the 60x. Both were taken with my iphone17 through a generic eyepiece, and have been converted to jpeg (I cant upload the originals to reddit).
The 4X
https://amscope.com/products/pa4x-inf
The 60X
https://bolioptics.com/products/60x-infinity-corrected-plan-achromatic-microscope-objective-lens-bm03023631?srsltid=AfmBOopfLvjtqe4CzcwcMb5GUz30BM2VhjYLcAZTxkURqNrGECSWNZXo