Objective too close to slide

I just put my preliminary build together and now has a problem. In order to focus the sample has to be really close to the objective (x40). So much that I had to put the premade slide upside down to focus properly and run calibrate.
Does that mean my parfocal distance is not 45mm? (default in build script) Would changing it to 35mm fix this issue? Do I need to print optic part at various setting 35-40-50 to test?

The objective is generic ones from Aliexpress. When I ask for more details the seller sent this:
Which I think is the proper focusing distance!? But x100 should be much shorter right?


You probably have the par-focal distance correct, 45mm is the distance from the shoulder of the screw at the back of the lens to the focus. The numbers in your post will be the effective focal length of the lenses, which relates to the nominal position of optical middle of the lens.

The working distance from the face of the objective to the sample is very short for high power objectives Objective Working Distance | Nikon’s MicroscopyU. Typically it is less than 1mm for 40x and 100x lenses and microscope slides are 1mm thick. So yes, you do need to turn the slide over so that you are only looking through the coverslip. Long working distance objectives do exist, but they are much more expensive.

If you have samples that physically have to face upwards for some reason then you could consider something like the Upright microscope prototype.