Hi Xiaoleiwang,

I don’t think there’s a tutorial for that, but someone else asked a similar question that was answered recently and might help you.

I think you are missing a specification for your microscope because lenses with the same NA can have different focal lengths depending on the aperture size. The NA (in air) can be approximated by the diameter of the aperture divided by twice the focal length. That means, you can reach a NA of 0.2 with a different focal length by changing the diameter of your aperture.

Typically, you would also specify the magnification of your microscope. Let’s say you want a magnification of 10X, the magnification is given by the ratio of the tube lens focal length over the objective lens focal length. Therefore, your objective lens focal length has to be 20 mm (200 mm / 20 mm = 10X). Microscopy objectives typically have an aperture at their back (towards the infinity space) located one objective-focal distance away (for telecentricity). You can infer the size of this aperture knowing the NA as mentioned above. With a NA of 0.2 and an objective focal length f of 20 mm, the diameter of the aperture D should be 8.16 mm (using the exact formula: D = tan( asin ( NA ) ) x 2 x f in Wikipedia).

I made a simple example of this microscope with paraxial lenses in OpticStudio:

You can model this microscope in both orientation depending on what you want to study. I started with the 200-mm tube lens followed by an aperture (using the Float By Stop Size aperture definition). This allows me to specify an aperture Semi-Diameter of 8.16 / 2 = 4.08 as per our specifications. Finally, I have the second 20-mm focal lens objective lens. In the Merit Function, I’m reporting the magnification (notice it is negative) and the Image-Space NA. I’m uploading this file to my answer.

Hope this helps, and take care,

David