Nikon plan apo 20x 0.75

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seta666
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Re: Nikon plan apo 20x 0.75

Post by seta666 »

I rather post it here than start a new thread if Jörgen does not mind.

I tried my OEM 1501-9398 20/0.75 APO VC with Raynox DCR-250, 45-50mm extension past infinity gives a 15/0.75 objective which covers APS-C pretty well. Tried to do a stack of a sunset moth but could not, DOF is so thin that out of focus reflections destroy surrounding details, maybe works better with a flatter and not so reflective subject.

100% central crops with different extensions, only levels adjusted; camera is Sony NEX-5N
It seems that some shots were affected by vibration, maybe I did not wait enough to shoot.
Sweet spot seems to be somewhere between 45-50 mm extension; I chose 45mm but maybe 50mm is better.

EDIT: I made a mistake, labels are off by 14mm; infinity would be at 14mm EXT (Not shown), and where it reads 45mm EXT should read 31mm EXT.

Image
Full size

One shot at 15X on NEX-5N, reversed DCR-250 with 45mm extension past infinity.
Image
Full size

I wonder if this technique will also work with finite lenses and the other way round, using objectives design for no coverslip with one;
more things to try equals never ending fun!! :mrgreen:

Javier
Last edited by seta666 on Thu Mar 04, 2021 11:46 am, edited 1 time in total.

Pau
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Re: Nikon plan apo 20x 0.75

Post by Pau »

Hola Javier,

There are some things I don't understand:
Jörgen increases the TL-sensor distance, focusing it closer than infinite and this induces increased magnification while, if I have it right, you do just the opposite: focusing the TL past infinite and getting lower magnification. In both cases this is done for compensating the absence of cover glass.
In plus, the OEM 20/0.75 seems designed to work with a much ticker cover than the normal 0.17 version, so it would benefit of focusing the TL closer than in Jörgen's tests.
What am I missing?

Anyway, your tests show excellent resolution
Pau

rjlittlefield
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Re: Nikon plan apo 20x 0.75

Post by rjlittlefield »

Pau wrote:
Thu Mar 04, 2021 9:23 am
There are some things I don't understand:
Jörgen increases the TL-sensor distance, focusing it closer than infinite and this induces increased magnification while, if I have it right, you do just the opposite: focusing the TL past infinite and getting lower magnification. In both cases this is done for compensating the absence of cover glass.
As I read the posts, Jörgen and Javier are doing the same thing, except that Javier is using a much shorter tube lens. In Javier's post, I read about a Raynox DCR-250, which has only 125 mm focal length. In Jörgen's post I see "Nikon 300 mm". At infinity focus, the DCR-250 should give magnification 12.5X with the 20X objective. Then Javier adds some extension, which does focus the TL closer, and increases the magnification, while compensating for the absence of cover slip.

In Javier's block of images with varying extension, I measure the "45MM EXT" image as being about 100:77 larger than the "DCR-250 INF", which then calculates to about 16.2X.

As for the amount of extension, I note that Javier's adding 45 or 50 mm with a 125 mm TL will change the focus much more than Jörgen's adding 8 cm with a 300 mm TL. To me this makes sense in terms of Javier's lens being designed for thicker cover slip.

I hope I have this right. Please correct or clarify as necessary.

--Rik

Pau
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Re: Nikon plan apo 20x 0.75

Post by Pau »

Rik, thanks for the clarifications.
seta666 wrote:
Thu Mar 04, 2021 7:26 am
I tried my OEM 1501-9398 20/0.75 APO VC with Raynox DCR-250, 45-50mm extension past infinity gives a 15/0.75 objective which covers APS-C pretty well.
There are two points that have confused me: one is that I wrongly assumed a near 200mm TL (Raynox 150 is 210mm I think) and the other is the expression past infinity. Now it has sense although I don't find past infinity expression adequate
Pau

seta666
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Re: Nikon plan apo 20x 0.75

Post by seta666 »

Just like Rik says; I do not know if past infinity would be the right term. I focused the DCR-250 to infinity and then added 45mm extra.
But Rik 16.2X estimate made me realize I made a mistake (Thanks); before the test I had PL filter on the bellows, I removed it for the test and instead of adding 7mm (filter thickness) to the bellows extension I removed it.
Labels are off by 14mm, extension added for the scales shot was 31mm from infinity so proportionally almost matches extension used by Jörgen, infinity would be between the 10mm EXT and 20mm EXT shot, did not make it.
I run a test before and I did not need so much extension, thought variation was caused because of removing the filter and its extra glass.

Total magnification is around 14.6X on sensor, I took a picture of a slide with 0.1mm div
Image

Looking at the slide image seems planarity is lost; I do not know if that is true or is just a effect caused by the slide glass, as the focus on the scales I shown before seems to be OK corner to corner.

I should probably make a new test and I try to lighten the scales better this time.

Bob-O-Rama
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Re: Nikon plan apo 20x 0.75

Post by Bob-O-Rama »

All of this is pretty interesting. Having both the 1501-9398 and also the "Plan APO 20X / 0.75 DIC N2 / 0.17 WD 1.0" I've not been able to get nearly as good a result from the 1501 as with the "Plan APO..." branded objective. It did seem to like a longer FL tube lens - though I did not try adding another 100mm of extension tubes. The "Plan APO..." one worked better than it should have without cover glass, adding proper thickness clover glass made things worse even modifying both the tube FL and the rear extension length.

--Bob

Duke
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Re: Nikon plan apo 20x 0.75

Post by Duke »

Hi guys, I've not read the article regarding coverglass thickness mentioned above (my access is limited), I'm sure it's full of calculations only few would understand (I'm not an optical engineer, so me not included).
But I've decided to do my own estimation, based on pure geometry, snells law and logic. Here's it:
OPD_calculation__.jpg
OPD_calculation_.jpg
d (coverglass thickness) usually 0.17 mm, RI - Refractive index (1.52 for standard coverglass), WD - working distance of the objective, F tube - back focal distance.
I've done some calculations on the Nikon CFI60 PlanApo series:
Nikon CFI PlanApo 4x 0.20 inf/0.17 WD=20 mm : ΔD = 1 mm
Nikon CFI PlanApo 10x 0.45 inf/0.17 WD=4 mm : ΔD = 5 mm
Nikon CFI PlanApo 20x 0.75 inf/0.17 WD=1 mm : ΔD = 34mm (OEM VC 1501-... inf/0.375mm ΔD = 75mm)
Nikon CFI PlanApo 40x 0.95 inf/0.17 WD=0.4 mm : ΔD = 220 mm
“Thoroughly conscious ignorance is the prelude to every real advance in science.” - JCM

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