Advice on 20x finite objective

[Tonikon]

Hi everyone,
my setup for daily focus stacking is actually composed by a modified microscope stand (paired with the WeMacro Micro-Mate) and four Nikon finite objectives (Nikon Plan 2x/0.08, Nikon CFN 4x/0.13, Nikon U10/0.22 and Nikon CFN 10x/0.30). I'm quite happy with this setup, but now I'd like to get into the 20x range. So I 'd need a good finite objective 20x with a comfortable w.d. and good corner performance on APS-C camera. Can you advice me on what to look for?
Thanks in advance
Toni

[seta666]

Nikon BD plan 20/0.40 210/0; I prefer this one over the ELWD version. WD is 3-4 mm or so but once you remove the barrel that is plenty.
If you want to try 40X afterwards get the nikon BD 40/0.65, again only 1.5mm WD (maybe 1mm) but it covers FF pretty well

The only thing is that unless I am wrong your other lenses are RMS and for 160mm tube, these are M26 and for 210mm tube.
Anyway, I would use that 4/0.13 with extra extension because of poor corner performance, and maybe also those 10x

[Tonikon]

Hi Seta666 and many thanks for your very helpful (and superfast) reply.

I suppose that in order to use the Nikon BD Plan 20/0.40 I have to buy also an adapter (M26 to RMS), but this in not a problem at all...it is very a common and cheap piece.

All my objectives are more or less for 160mm tube length and I use a focusable adapter (36-90mm) to my Panasonic camera that allow to vary the final magnification of my objectives (2-3x for the Nikon Plan 2x, 4-5x for the Nikon CFN 4x/0.13, 6-9x for the Nikon U10/0.22 and 10-13x for the Nikon CFN 10x/0.30).
So, I suppose thate using a 210mm finite objective, I'll obtain a minor magnification than the nominal, because my system vares from 140mm to 190mm of "total tube lenght" (from the nosepiece to the sensor plane)...I hope this is not a problem for corner performance, at least on my MFT camera (mostly used).
Do I have to "shield" the internal light path on the Nikon BD Plan series? If I'm not mistaken, it is an "Epi" series of objectives...

But just out of curiosity...why do you prefer the BD Plan over the M Plan ELWD version?

[seta666]

If you are getting 10-13x with the 10/0.30 you should get almost 16-20x with the BD 20, at 15x it still should perform well on a MFT camera.

I do not like nikon elwd lenses because of purple fringing, are pretty bad on all of the ELWD.I had the 20, the 40 and the 60, the only one I still have. My favourite one is the 60/0.70 ELWD but now that I work with APS-C I will probably end up selling it.

Also I find it easier to get good lighting on the BD plans, narrow tip makes it easy to get light in. I also use a BD plan 60/0.80 with 0.5mm WD.. but it needs really flat subjects.

[Tonikon]

Ok Seta666...you have convinced me
Now the hunt starts...if you know of any BD 20, just hit me up!
Thanks again
Toni

[seta666]

No hurries, wait for some more recommendations..
The used to be easy to get for under 100$

[ray_parkhurst]

If you are interested in using the BD Plan with the built-in brightfield illumination, I have built illuminators that work very well, and interface to M42 system. You need to be a little crafty with cardboard and glue, but generally it's quite easy to build. Would be a great project for someone with 3D printer. I've thought of building some for folks and offering them for sale but they are just a bit too DIY for that. I'd be happy to post their construction here if you're interested.

Regarding the various BDs, I have both the 20x regular and EWLD. I don't remember the CAs being much worse on the EWLD, but I am now encouraged to test them and will report back.

[seta666]

If you are interested in using the BD Plan with the built-in brightfield illumination, I have built illuminators that work very well, and interface to M42 system.

I would be interested in such project, the more options the better.

Purple fringing is worst on the ELWD, or at least that is what I remember; also is more difficult to fix.

[ray_parkhurst]

I will draw up a cross-section and take some pics of the adapter.

I just shot the D mintmark on a silver dime with both the regular and ELWD 20x BD Plans. I normally shoot copper coins but they seriously skew any color problems due to their strong red hue, but silver is very grey and is better for color comparisons. What I am seeing was surprising...a weird yellow cast that I am not sure the origin of yet. It may be something wrong in the lighting adapter. I thought maybe one or more of the LEDs was having issues, but the whole ring seems to be OK. Anyway, it affects both objectives so the comparison is still valid I think. Bottom line is @seta666 is correct that the EWLD has more purple fringing than the regular objective. I had not compared the two before since it was irrelevant to my application, where the regular objective has far too short a WD to be useable. But if you can tolerate the short WD, looks like the regular objective has better CA correction.

Regular 20x

ELWD 20x

[ray_parkhurst]

I added a diffuser within the BD adapter and eliminated much of the yellow cast. It is also now showing the differences between objectives better, with the EWLD having noticeably more purple fringing. The regular 20x has some purple fringing but the EWLD is showing more of it.

I'm glad I went through this exercise as I did not know about the yellow cast caused by the adapter. This helps explain some odd color behavior I've experienced with my stylus images.

Regular 20x

EWLD 20x

[Tonikon]

Thank you very much Ray, what you wrote is very helpful and the images you made speak for themselves! The "normal" BD version has significantly less purple fringing than the ELWD version.
You sparked my curiosity about the lighting system ... but are you saying that you have made a DIY system for "coaxial lighting"? I would be really interested in understanding how you did it and I would like to try to implement this system. It could be useful in many applications with difficult lighting, at least on subjects that are not too reflective.
At this point, the only doubt I still have about the Nikon BD Plan 20x/0.40 is about working distance ... I wouldn't want it to be too uncomfortable. Nikon declares a working distance of only 2.5mm (8.5mm for the ELWD version) but by removing the and cap can you recover a few millimeters (how many)?
Ciao
T

[ray_parkhurst]

I measured the 20x and removing the frontpiece gives you additional 1.1mm of WD, so I guess it becomes 3.6mm.

I'll work on putting together some drawings and photos to show the BD lighting fixture. Note that shining light through the BD light pipe surrounding the optics is not coaxial, which is typically achieved with beamsplitters. The results are very different between the two methods.

[Tonikon]

Quite right! You are absolutely right ... coaxial lighting is a whole other story! This is more comparable to a miniaturized ring led ...

[seta666]

Thank for the test, my memory is not so bad then!!

If you could post a picture of both lenses side by side without the barrel this would show Tonikon that BD20 although having a short working distance is very easy to work with and can use the best light diffusers there are, ping pong balls.

It has a very narrow tip (All BD Plans do), I have no problems working with the BD40 either but needs flat subject and not to hairy.

[Tonikon]

Reading an "old" brochure on Nikon CF 210mm objectives, I found that the Nikon M Plan 20x / 0.40 210 has a working distance of 2.7mm, which is practically the same as that of the BD Plan. I think the optical scheme is the same. An adapter would not be needed in this case. Only the form factor at this point can make a difference...