Hello all,
I have read some threads in these forum about the role of the tube correction achromatic lens and I think I understand why is needed. Thing is, we tried to remove it (nothing else changed) and images are more or less the same, no loss of quality, apparently, only a smaller field of view. Is that the expected result when removing this lens? It is a expensive item (for this low-cost design, I mean).
Any comment will be appreciated. Regards,
Adolfo.
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Hi @adolfocobo it’s definitely the case that you can often get away with leaving out the 50mm achromat. It means you’re using the objective further away from its design parameters, so I would expect a little degradation in quality, but it may not be noticeable (particularly if you have a very cheap objective).
I’m reluctant to remove the lens from the instructions because I’m keen to do it properly in the high resolution optics module. However, @j.stirling has done great work adding in explanatory notes in various places - perhaps we could add one explaining a bit further that the design will work without the tube lens, though it’s not recommended for the reasons I outlined above.
@adolfocobo do you have some comparison images? That is always a helpful thing to be able to show…
The quality of the lens from the Pi camera is remarkable (Comparison of a pi-camera and a 20x optical lens)
I think that is a great solution if cost is an issue and very little quality is sacrificed. A high power microscope objective in the optimum configuration with a tube lens will give high resolution. If you move away from optimum performance by not using the tube lens, how much better than the simple Pi camera lens does it end up being?
Thankyou all for your kind replies. We have just got some images with and without the tube correction lens. Both are with the HQ camera at the same shoulder-sensor distance than the v2 camera.
With lens:
Without lens:
Any differences in quality could be to a non-flat slide and non-optimal focus, I think. The objective is a cheap achromatic x40 from Amazon: https://www.amazon.es/dp/B07PVP3GP3
It seems that the main difference is the (intuitive) larger magnification. What I would expect then is a larger working distance, but it seems it is not the case, or, at least, the difference in the focal point is very small (not measured yet, though). I am not sure but I believe the lines are 10 microns apart.
I share here also the design of the HQ camera part, it is a quick and dirty editing of the STL file optics_picamera_2_rms_f50d13.stl with a CS thread added, in Autodesk Inventor.
Link: (the STL file was larger than the 4MB size limit)
Thankyou all and regards!
Adolfo.
The purpose of the tube lens is to change the focal length of the system.
A microscope objective designed for 160mm FL will provide its best performance when the sensor surface is 160mm away from the shoulder of the objective. This isn’t just a matter of field of view or focus, but also spherical and chromatic aberration.
Your system will not be fully corrected as designed if you use any other configuration. But using the achromat gives you a good balance of factors, and better correction than a shorter tube length.
If the difference is small enough that it’s hard to see for the samples you are imaging, then like everyone has already said, how much does it really matter? But now you know what the lens is for. 
I have attempted to capture the key points about both how the lens and camera position are calculated (and how this will change to enable customisation), and what the lens is doing in this Open Flexure Enhancement Proposal
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Thankyou for the reference, everything is very well explained there.
Regards!
Adolfo.
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