Adapting aps-C to nikon

[Scarodactyl]

This is something I've been meaning to try for a bit and finally got around to actually doing. Though Nikon microscopes are generally easy to adapt cameras to, like most makers there isn't really a good dedicated solution for adapting aps-c cameras, either in their finite or infinite lineup. The closest is the 2x photo eyepiece (Pl or PLI) which is rare and very expensive. As far as accessible "official" options you have a 1x photo eyepiece which only covers a 16mm diagonal, and a 2.5x which gives an image sized for full frame. Neither is a very good solution.

In terms of unofficial options the best is just directly putting the intermediate image onto an aps-c sensor. A ~27mm diagonal is sized just right to squeeze out every bit of good image circle from a Nikon objective this way without adding any additional aberrations.

However, direct projection isn't so easy on every Nikon microscope or head. Some of their heads are annoying to modify and some of their stereo microscopes (which use the same adapting hardware as their compound microscopes) make it almost a geometric impossibility. So there is some value in figuring out a solution that meshes better with the official hardware.

From getting different nikon stuff over the years I have Nikon PL photo eyepieces in 1x, 2.5x, 8x and 10x mags. I decided to try afocal imaging with the 8x and 10x using a 40x pancake lens on my canon T6, as well as using the 1x and 2.5x normally for comparison, and in addition afocal with the 10x/22 Chinese eyepieces I have mentioned in other threads. I also did direct projection with my UW head, but while the direct projection mod worked fine on the other scope I was using it on I'm getting bad corner shading on this one. This isn't a limitation of the objectives as they cover 26.5mm FN UW eyepieces no problem, it must be to do with the weird layout of the UW head.

For this test I used a Nikon optiphot a BD plan 10X objective, imaging a dusty wafer with coaxial illumination. These BD plans are known for having significant axial CA but not much lateral CA.

Here's a quick view of the imaged area along with a summary of image coverage:


Now let's dive in to the images.
Direct projection:



Aside from the awful corner/edge shading this is excellent. It's sharp in the center and there isn't really any lateral CA towards the edges, suggesting any CA we see is added by intermediate optics, either the eyepiece or the camera lens or both.

1x TV PL photo eyepiece:



Good image quality in the center, an unfortunate amount of lateral CA towards the edge, and severe vignetting just as expected.

2.5x PL photo eyepiece:



As expected there's a ton of cropping here. There is also a very unfortunate amount of lateral CA, especially since this is cropping off the edges. It might be even worse on full frame.

8x PL photo eyepiece imaged afocally with a 40mm canon pancake lens:



Wider FoV even with the vignetting, but similar levels of CA to those seen in the 2.5x. This one might be viable with a longer focal length lens though.

10x PL photo eyepiece imaged afocally with a 40mm canon pancake lens:



Similar coverage but better CA performance.

Note I think the lens might have been placed a bit too high above the photo eyepieces. I may try it again with the camera lens just set onto the trinoc port to see if coverage or CA improves.

Finally, the 10x/22 Chinese eyepiece imaged with the 40mm pancake lens:



Aside from direct projection this gave the best result. Little to no added CA, good coverage and no vignetting. This eyepiece could even be used with a shorter FL lens as the intro image shows--the diameter of the eyepiece's FOV is about 130% the diagonal of the image taken with a 40mm pancake. So 30mm might be a good fit if you can find a good one.

So, I guess I'd say I recommend direct projection if possible, and if not I'd recommend getting one of these eyepieces and using it afocally.

these results were a bit of a surprise--I've never been a huge fan of Nikon PL eyepieces, but mostly because of their awkwardness for APS-C usage. I did not expect them to perform so badly from a CA perspective, but it's hard to imagine all of my eyepieces have gone bad in the same way. A while back I also tried a 10x/22 eyepiece raised up to project an image directly onto the camera sensor but the results were so bad I didn't feel like posting about it, so I'm really glad this worked out!

[hans]

In the afocal configurations using the photo eyepieces I am curious how exactly you are positioning things. Is the distance between the objective and photo eyepiece the same as it would be when working normally at the marked magnification, and focusing is done with the camera lens, if that is even possible? Or are those configurations truly afocal in the sense that the camera lens is focused at infinity and the photo eyepieces moved closer to the objective than normal?

Lots of work to take and present so many comparison photos, and interesting even though I known very little about Nikon microscopes. Thanks for posting.

[Scarodactyl]

Should have clarified that--the photo eyepieces were in their normal position in the trinoc head. Focusing the lens away from infinity made focus worse, but at infinity it only took a tiny tweak to the fine knob to get good focus so I figured it was good enough. I did try afoval with the 1x and 2.5x just for kicks but it was impossible to get them to focus. The lens was first connected by a series of adapters, but for afocal on the 10x/22 I set the lens directly on the trinoc port to avoid vignetting. This didn't solve vignetting from a quick test on the pl 8x and 10x but I didn't do a full test on that variation yet. I might later tonight, the wafer is still under the scope where I left it. I can 3d print a direct adapter from filter threads to the port for a later test too.

[hans]

Might be interesting to try positioning the sensor at the normal projection distance for the 8X and 10X photo eyepieces but somewhat off axis, to capture a small crop from the edge of the projected image, and see if the lateral CA is similar?

[Scarodactyl]

That might be difficult logistically, but I'll have a swing at it at some point.
In the mean time I removed all the adapting hardware and laid the camera with lens directly on the viewing eyepieces for the 8x and 10x, and got interesting results.
for the 8x, sensor coverage got a little worse while the covered image got wider, and CA performance improved significantly (or maybe it's a little masked by difference in color temperature, I think I didn't get the halogen lamp to the same setting, but I think the effect is real.)




Sensor coverage and scale didn't change pretty much at all on the 10x. It has a recessed front element so was a little further from the lens surface. CA seems similar, though it was low to start with. I think the 8x's CA is better than this now.




This seems a lot more viable this way, though either might be better paired with a 50mm lens to avoid vignetting.

[MichaelG.]

A very interesting and informative exercise, Scarodactyl

Thanks for sharing the results.

MichaelG.

[apochronaut]

I have used those Chinese eyepieces in several systems that I judge as requiring neutral eyepieces. Neutral in the sense of being neither correcting or compensating. They seem to be a very slightly correcting eyepiece. They work well where a small degree of edge of field correction is necessary.
One of the problems with afocal systems is in the quality of the camera lens used. Very few lenses being used are apochromatic , so it would be safe to assume that most photo lenses even really good ones are subject to creating off axis chroma of varying degrees. There are numerous tests out there that compare photo lenses on the basis of edge distortion, lateral ca , coma etc. and very few come through with flying colours, at least not too many that don't require a bank loan to acquire.
Does it seem that the Chinese eyepieces just happen to be correcting for the lateral ca of the camera lens?

[Scarodactyl]

I am really not sure. I have generally found the canon 40mm pancake to be a good performer for this usage, but I haven't extensively tested it either. You can see that there is indeed possibly some sort of CA activity from the chinese eyepiece, since the very faint CA present looks to be opposite from that seen on the photo eyepieces (red and green are flipped I mean). But it's pretty faint? Either way I can't be sure of the exact causes of what I'm seeing, only what I'm seeing (and maybe not even that). One way or another I do love these eyepieces.

I had recently ordered a new Nikon PL 2.5x for unrelated reason (going onto a different scope I'm working on), which is in much nicer cosmetic condition than the one I used in this test. I was puzzled by the poor results, so I figured it was worth doing a direct shootout between the two to see how CA compared. The new one is the "XA" variant which I have seen once before on an SMZ-U stereo. I am not sure what the 'XA' means (Xtra Awesome?)
Long story short, it has the same scaling as expected, center performance is identical but the corner CA is reduced relative to the other one, though still troublingly present IMO. The lack of parcentricity between the two photos suggests my mechanical adapter is maybe not quite as slick as I had hoped and may be introducing a bit of tilt. That said, I have tried holding the camera over the 2.5x and tilting it every which way, and while it did introduce remarkable distortions it didn't seem to do anything insane to the CA, so it may not be a major factor here (especially since the CA seems symmetrical). I will have to redesign the adapter and try again though.

Normal 2.5x PL:




2.5x PL XA

[apochronaut]

The XA does appear slightly better.
I suspect the reason for the red green reversal with the Chinese eyepiece is that it has some small degree of peripheral corrections whereas the photo eyepieces are compensating or under corrected. The few Nikon photo eyepieces I have used seemed to be compensating.
What does your eye see when you view the periphery of each eyepiece?

[Scarodactyl]

To the eye the chinese eyepiece looks clean on my nikon and mitutoyo objectives--just looking again on the mitutoyos, there is maybe a tiny touch of added CA visible on brightly lit specks of dust, but it's awfully subtle. Doesn't seem to have reduced planarity. That's probably not surprising, since whatever it's doing is a bit subtle and it needs to be unsubtle to be very noticeable to the eye.

[apochronaut]

I guess the important thing would be whether there is a red green inversion in the ca. Are the Chinese eyepieces still showing ca as red to the outside and green to the inside. As well , how does the colour banding in the photo eyepieces look to the eye. Still green to the outside?