High resolution version - e-Plan objective touching slide/no sharp image


I am using a “BMS Objective e-Plan 40/0.65, 160/0.17” and a “12.7mm diameter, 50mm focal length achromatic doublet lens” with an “optics_picamera_2_rms_f50d13” optics module casing.

When I try to focus my preparation I can’t get it sharp because the objective is touching the preparation before I can get a sharp image.

Any idea?


Make sure the sample is placed with the cover glass down.

@ImAreO Yes, I did that but it didn’t help. I can focus the dust only, not the preparation.

Edit: I am using a finite conjugate microscope objective, therefore this shouldn’t be the problem.

Hello, Florian!
After the sample is dry, remove the cover glass. Can you see the remaining objects on the slide?
If yes how much distance from probe to objective? in my case approximately 0.25mm. I can’t measure more precisely - the front lens of the objective on the sprin.

That is very strange - do you know the intended working distance of the objective, and the thickness of your coverglass? 160/0.17 should meant that at its designed tube length,you can easily focus through 0.17mm of glass, so as @ImAreO says there ought to be 0.25mm from the objective to the focal plane (leaving around 100um from the outside of the coverglass to the objective). One thing you can check is whether the 50mm focal length lens is correct - if you remove the objective and point it at a brightly-illuminated object some metres away (e.g. point it out the window), it should not form a sharp image. If you then unscrew the two small screws holding on the camera module, and move the camera module back by around 12mm, you should get a sharp (if very zoomed in and thus quite wobbly) image. If that is the case, then the optics module seems to be working correctly and I am very confused! If that’s not the case, perhaps it is possible that your doublet lens does not have the same focal length as the one I’ve used. Is it a ThorLabs AC127-050-A, or a generic one sourced elsewhere?

@ImAreO and @r.w.bowman Thanks for your replies. I checked whether the 50mm focal length lens is correct, but it doesn‘t seem to be . It‘s a generic one from aliexpress.
I‘ll buy one from Thorlabs…


Florian, try without a 50mm lens, it works? maybe your objective has a very short working distance? in my case it became 2 times shorter (40x)

if the working distance is like that of the second (60x) in the picture, then when shortened by 2 times, the cover glass really does not fit.

Good suggestion, that’s definitely worth a try. @Florian-HH if you hold the lens underneath a ceiling light and form an image of the light bulb on a piece of white paper, what’s the distance from the lens to the paper? It’s always worth trying the simple version of the protocol before worrying about doing it with the camera! Sorry I didn’t suggest that one earlier…