Nikon plan apo 20x 0.75

[JH]

I got a copy of the Nikon plan apo 20x 0.75. Many thanks to Adalbert that made a post about it here https://www.photomacrography.net/forum/ ... 554#258554

Used as intended, with a coverslip, this objective performs nicely. This is from a microscope preparate of male genitalia from a chironomide.



The lens performs ok without a coverslip and some extension. Here is a picture of some butterfly scales.



I used a Nikon 300mm and bellows to increase the distance 8cm between the Nikon and the sensor. 300m does not clear the corners. The resolution is better than what I get with my Mitutoyo 20x 0.42 but less than I get with my Nikon plan apo BD 40x 0.80.
I have some more pictures on my website. https://www.hellberg.photo/2020/03/01/n ... objective/

Best regards
Jörgen Hellberg

[JH]

center at 100%

[Robert Berdan]

Nice photos with the Plan 20X objective. I purchased a Nikon 20X Fluor NA 0.75 recently for fluorescence microscopy an it very bright. I might try it for some macrophotography as well.

RB

[JH]

Thanks RB

To follow up on compenasting variation with the Nikon plan apo 20x / 0.75 I did a series of pictures showing the difference between 1) a coverslip attached to the objective, 2) no coverslip and 3) no coverslip but 86 mm extra distance between sensor and tube lens - compensating variation:

The different setups give different enlargement on sensor
Tube lens Apo Ronar 360mm, diffused LED light. The subject is the metal surface of calliper.
This is single photos - no stacking. Camera FF Canon 6D.


For comparison my usual 20x setup.
Left Nikon Plan Fluor ELWD 20x / 0.45 WD 7.4mm
Right Mitutoyo 20x / 0.42

/Jörgen

[Pau]

Very interesting!

3) no coverslip but 86 mm extra distance between sensor and tube lens - compensating variation:

Did you apply any kind of formula to obtain those 86mm, or is it just experimental?

[JH]

Hi Pau
No formula - more trial and error with combinations of tubes with fixed lengths, My best guess from the distances I did test is that optimum might be a little shorter.
/Jörgen

[rjlittlefield]

Did you apply any kind of formula to obtain those 86mm, or is it just experimental?

The classic paper OPTICS OF THE OBJECT SPACE IN MICROSCOPY, by B. M. Spinell and R. P. Loveland (https://onlinelibrary.wiley.com/doi/abs ... .tb04454.x), addresses this issue by experiment. Their conclusion was that "the slope [of the correction curve] seems to be proportional to the N.A. of the objective and inversely proportional to the square of the focal length of the objective".

However, that relationship was only approximate, with the measured best correction varying over a range about +-20% from the value predicted by formula, even in the best-behaved central portions of the curves, and for only the 6 objectives that they tested. (Their Table II, Optimum Tube Length versus Cover-slip Thickness.)

Having seen that result, I pretty much stopped investigating the theory because it seemed that any simple calculations would barely be useful for getting in the ballpark.

--Rik

[Pau]

Thanks both Jörgen and Rik.

Increasing the tube lens to sensor distance (actually focusing it closer than infinite), how it does affect the free working distance? I assume that it will be shorter than the nominal 1mm, am I right?

Jörgen, any chance to view your tests at full resolution?

[JH]

Thanks both Jörgen and Rik.

Increasing the tube lens to sensor distance (actually focusing it closer than infinite), how it does affect the free working distance? I assume that it will be shorter than the nominal 1mm, am I right?

Jörgen, any chance to view your tests at full resolution?

Pau
That is correct you must refocus closer when you remove the coverslip and then when you increase the distance between the tube lens and the sensor you have to refocus even closer.
I tried to measure the effect using a bellow and a Nikon 300mm as a tube lens. I refocused 0.06 mm when I removed the cover slip then after increasing the distance between the sensor and the tube lens approximately 8,5 cm, I had to refocus another 0.06 mm. So, in total 0.12 mm closer.
BUT this is a little less than the cover glass that I removed so in this case I actually ended up with a tiny bit more WD.

Pictures at 100%
1) a coverslip attached to the objective, 2) no coverslip and 3) no coverslip but 86 mm extra distance

/Jörgen

[Sym P. le]

Only a microscopist can consider .05 mm a success.

[Pau]

Jörgen, thanks so much for the complementary info and sample images!

Only a microscopist can consider .05 mm a success.

Well, we are microscopists
In nanotech it also will matter a lot and in corpuscle Physics it would be a whole world

[JH]

Nikon plan apo 20/ 0.75 Nikon 300 mm tube lens and 8 cm extra distance between tube lens and sensor (compensating variation). Diffused led.
The short WD distance made lighting troublesome, - I had to adjust the colours a little more than usual in Adobe PS (saturation and more contrast in the blue channel)

100%

/Jörgen