Hello,
I’m a newbe and try to build a microscope for my son. I bought these objectives but the image is a disk rather than a square and very difficult to focus.
Could someone explain how to read this reference on the photo?
I guess:
But what means those values on the right: “0.65” and “0.17”?
Is this objective suited for the Openflexure microscope?
Thanks for your help.
40 is x40 magnification.
/0.65 is the numerical aperture - tells you about the illumination angle needed and the resolution possible.
Infinity is the back focal distance. There are two common types: 160mm and infinity. You need the correct optics module as the tube lens needs to be in a different place for the two types. The infinity optics module is longer, and needs the 45mm Pi base.
/0.17 is that the lens is designed for best performance when focusing through a 0.17mm thick glass cover slip.
Thanks for your answer.
So those labelling are not related to parfocal distance nor focal length…
On the Chinese website, they said “Conjugate Distance :195”. Do I have to understand the distance between object to observe and camera sensor shall be 195 mm?? If so, I bought the wrong lenses then… Please undeceive me 
Conjugate distance :195 mm does not match an objective marked infinity. The lens will be fine in the microscope, you will just need to print the infinity version of the optics module optics_picamera_2_RMS_infinity_f50d13 (if you have the 13mm diameter, 50mmfocal length tube lens.
Thanks so much!
195mm usually means a 45mm parfocal distance (that’s what I usually use) plus a 150mm sensor-to-image distance (which means 160mm “tube length”). As @WilliamW says, I wouldn’t expect the infinity sign. Usually infinity-corrected objectives are more expensive, so you may have gotten a good deal! Use William’s suggestion of the right optics module and base, and you should be good to go The image is pretty much always a disc, but the disc is bigger than the camera’s rectangular sensor so it should all work out fine.