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…

Sorry for the late reply, I had to regulate a few things and couldn’t go on with my microscope.

I don’t want to remove the 50mm lens because it would be damaged as it sits very tight in the plastic holder.

I am using a different objective(60x from aliexpress, look for 32913301769) now and with the cover glass down I get it sharp and I am very happy with it. The objective says 60/0.65 160/0.17. I think 0.65 is the numerical aperture and 0.17(mm) the working distance.
@ImAreO I understand that the cover glass doesn’t fit with a 0.17mm working distance.

An idea: If I shorten or extend the objective holder is it possible to extend the working distance? Sorry for this noob question. I don’t know much about optics… clearly I’ll need a new 50mm lens in this case, too :wink:

When I am using fresh preparation of living material(like an oral mucosal cell) with the cover glass down it’s difficult to keep it in place…


Hi Florian!
The my image with numbers, just from such a objective, the working distance is 0.13, the objective is calculated to work with 0.17mm cover glasses.
I gave an example table from the same lens, only from another seller.
Distance 0.13 is the distance to the cover glass. I somewhat incorrectly performed the calculation above, did not include the thickness of this glass. i.e. in my case (for 40x) 0.53 + 0.17 = 0.7mm total from the edge of the lens to the object.
Thus, I have an almost 3x shortening of distance.
Did you try to see without a cover glass? It could be a blood smear. Is there any gap between the edge of the objective and the slide?
I also want to say that a 60x lens with an aperture number of 0.65 does not give you advantages in resolution over 40x. The apparent linear size will be larger, but this will not add sharpness. 60x should be 0.85, why the manufacturer released such a model I do not know ((
Removing the lens is not necessary if you have the opportunity to print another part.

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@Florian-HH it is not really possible to increase the working distance of an objective and still have good sharp images. There are microscope objectives that are designed to have a long working distance. The internal designs are somewhat more complicated and they are usually more expensive. Mostly they are designated for metallurgical microscopes, without cover glass.

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@ImAreO Without a cover glass I am able to see the sample sharp with a little space in between. But the space between the the objective and the slide is just a little bit more than the thickness of a paper sheet(80 g/m2). Therefore I think that the space is about 0.1mm and the cover glass should fit(as it does!).
I didn’t know about the aperture. Therefore I will look for an objective 60/0.85. There are a lot of objectives on aliexpress, do you have any suggestions? As well for a 100x objective. I found 100/1.25 in cheap and expensive but a plan corrected should be more sharp, is that right?

@WilliamW Ok, I understand that. Therefore always cover glass down :slight_smile:

Florian, I’ve seen lenses of the same design 0.85, but I don’t trust them, because in china they release oem and can mark as the reseller requests.
I have a 20x lens from this series. The field is very not flat. I really didn’t like it. But when I gave it a users for try with a conventional microscope, users were very happy. Center sharpness is good.
The plan objectives are not sharper, they are sharper on all the plane. In the center of the image, good achromatic lenses show better.
100x is most likely an oil immersion lens, in that table its working distance is the same as yours at 60x. I have a 90x/1.25 LOMO immersion objective and it works. But some inconvenience of using oil during maintenance after use (you need to clean the lens from oil)

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Hmm, ok, thanks for your thoughts I think I will look for a good 60x objective, that should be sufficient for my usage(personal non profesional) :slight_smile: