anyone knows what this lense is for?

[osterport]

I bought a trinocular part ( sales say this is from Motic BA600 ), however, one lens is missing

BA600 trinocular without top lens

Weixin Image_20240228174430.jpg (69.19 KiB) Viewed 705 times

If I mount a camera on it, the image is quite blur, appears to be kind of tube lens is missing.

Then I took a look at the trinocular, there is a convex lens at the bottom. I suppose the eyepiece should share this convex lens with camera, then this one is supposed to be tube lens.

Weixin Image_20240228180007.jpg (132.28 KiB) Viewed 705 times

I compared this trinocular with my BA300 trinocular, this lens is the difference:

BA300 trinocular top lens

Weixin Image_20240228174906.jpg (86.84 KiB) Viewed 705 times

Does anyone know about the trinocular structure? Why is the top lens for camera needed when there is already a convex lens?

[apochronaut]

There are a number of reasons for a lens or 2 to be in the photo tube, depending on the original desgn of the microscope.
When the tube is mfg. specifically to receive a certain camera or sensor size, the lens could be a framing lens of a certain magnification , so that the projection matches the sensor size.
When objectives require further corrections or compensation in optics downstream , the optics required need to be situated so that the photo tube can receive their modified output. If one or more of those optics are situated in such a way that their output bypasses the photo tube, they must be duplicated in the photo tube. This is normally duplications of the eyepiece or possibly as well a photo telan lens.
Framing of the photo image is accomplished more easily if the photo frame size is relatively close to the f.n. of the eyepiece., as is the case with APS-C. For sensors that are much smaller or much larger a fractional or magnifying optic must be used in order to frame the image appropriately but the tube length can also be used to either magnify or compress the image. If the mfg. decided to shorten the photo tube for the sake of ergonomics , when in fact a longer tube was required in order to frame the image correctly, they would need a higher mag. photo lens to compensate for the shortened tube.
Single lenses, unless they are very wide with a very large aperture will confer aberrations and distortions on the periphery of an image. This is overcome in a doublet to a large degree. Doublets can be air spaced too.

If I had to guess in the case of your microscope, I would suspect that one is a reduction field lens and the other a correcting and projection lens. One of them could be a cemented doublet. If your telan lens is situated inside the binocular and therefore is outside the optical tube of the trinocular port, then one is a photo tube telan lens but this is uncommon.

[osterport]

There are a number of reasons for a lens or 2 to be in the photo tube, depending on the original desgn of the microscope.
When the tube is mfg. specifically to receive a certain camera or sensor size, the lens could be a framing lens of a certain magnification , so that the projection matches the sensor size.
When objectives require further corrections or compensation in optics downstream , the optics required need to be situated so that the photo tube can receive their modified output. If one or more of those optics are situated in such a way that their output bypasses the photo tube, they must be duplicated in the photo tube. This is normally duplications of the eyepiece or possibly as well a photo telan lens.
Framing of the photo image is accomplished more easily if the photo frame size is relatively close to the f.n. of the eyepiece., as is the case with APS-C. For sensors that are much smaller or much larger a fractional or magnifying optic must be used in order to frame the image appropriately but the tube length can also be used to either magnify or compress the image. If the mfg. decided to shorten the photo tube for the sake of ergonomics , when in fact a longer tube was required in order to frame the image correctly, they would need a higher mag. photo lens to compensate for the shortened tube.
Single lenses, unless they are very wide with a very large aperture will confer aberrations and distortions on the periphery of an image. This is overcome in a doublet to a large degree. Doublets can be air spaced too.

If I had to guess in the case of your microscope, I would suspect that one is a reduction field lens and the other a correcting and projection lens. One of them could be a cemented doublet. If your telan lens is situated inside the binocular and therefore is outside the optical tube of the trinocular port, then one is a photo tube telan lens but this is uncommon.

I tried to extend the camera adaptor even longer, the resolution is much improved! Thanks apochronaut.
It seems to the missing lens is the reduction field lens. Without this lens, an alternative is to make a long camera adaptor.