Olympus Ach 100x/1.25 Oil iris

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coprinus
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Joined: Thu Mar 26, 2020 7:19 pm

Olympus Ach 100x/1.25 Oil iris

#1 Post by coprinus » Thu Mar 26, 2020 7:32 pm

Hi,

I acquired a second hand Olympus CX31, with 4x, 10x ,40x and 100x objectives.
The 4x, 10x, 40x are Plan C objectives.
The 100x objective is an Ach 100x /1.25 oil with iris (1.25-0.55).
I have the impression that the Ach with aperture set at 1.25 is not that sharp.
Reducing the aperture quickly introduces diffraction.

I will use the microscope for mycology. So bright field only.

My questions :
Would a 100x Plan C 1.25 oil objective be better than the 100x Ach ?
Are Plan C objectives UIS?

PeteM
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Joined: Tue Dec 06, 2016 6:22 am
Location: Santa Cruz, CA

Re: Olympus Ach 100x/1.25 Oil iris

#2 Post by PeteM » Thu Mar 26, 2020 11:44 pm

If it's marked with the infinity symbol, an Olympus objective should be either UIS or UIS2 (very similar).

I'd wonder if your 100x achro is scrupulously clean? In the center it should be as sharp as the Olympus Plan Achro. The latter will be in focus for a wider field of view and preferred for taking photos. The former should be fine for routine observation.

With the iris, I'd open it up wide and then adjust the microscope itself for Kohler illumination if you're not attempting darkfield observation.

coprinus
Posts: 4
Joined: Thu Mar 26, 2020 7:19 pm

Re: Olympus Ach 100x/1.25 Oil iris

#3 Post by coprinus » Fri Mar 27, 2020 12:06 pm

All the objectives are marked with the infinity symbol, also the Ach 100x one.

I followed your suggestions:

- I checked if the Ach objective was clean under a stereo microscope at 40x magnification.
It looked clean no scratches but I could see some shallow dotted craters mostly at the edge of the lens surface, not in the center.
I suppose this is some damage to the coating from corrosion of the oil (I cannot observe those dots when the objective is used in the compound microscope)
Trying to clean did not improve anything. By the way what is the best and safest cleaning method?

- I used a slice with a micrometer pattern to test the sharpness. (Previously I tested on mushroom spores)

- I tried to achieve Kohler illumination. This is not obvious. Up to a magnification of 100x it is easy to regulate the height of the condenser to achieve sharp edges of the diaphragm with the micrometer pattern on the slice also sharp. For a magnification of 400x, at the smallest diaphragm opening the diaphragm edges are still visible but I cannot get the edges sharp. Best result is with the same height of the condenser at 100x magnification. At 1000x magnification the edges of the diaphragm are no longer visible. At 1000x magnification, with the aperture of the condenser at its smallest, there is not enough light, the image is to dark. I have to open the aperture a bit. Opening to much reduces contrast. (I use a 30W light bulb, at least that's what I think is installed)

- I did set the iris of the objective at 1.25. and got the micrometer pattern in focus and the pattern looked sharp. I can observe irregularities in the print of the micrometer pattern smaller than 0.5 micron. Regulating focus on the edges of the field of view does not deteriorate sharpness in the center of the field of view and vice versa. So the objective must be reasonably plan. Closing the iris of the objective a bit does not improve sharpness, closing a lot reduces sharpness. I suppose details smaller than 0.5 micron is sharp.
The micrometer pattern are black stripes against a light background. This is in itself a setup with high contrast, so the perceived sharpness might be higher due to inherent high contrast.


What's your opinion?

viktor j nilsson
Posts: 104
Joined: Thu Aug 23, 2018 10:12 pm

Re: Olympus Ach 100x/1.25 Oil iris

#4 Post by viktor j nilsson » Fri Mar 27, 2020 1:18 pm

coprinus wrote:
Fri Mar 27, 2020 12:06 pm
At 1000x magnification, with the aperture of the condenser at its smallest, there is not enough light, the image is to dark. I have to open the aperture a bit. Opening to much reduces contrast. (I use a 30W light bulb, at least that's what I think is installed)
Based on this, it seems like you begin with the condenser iris fully closed and then open it up slightly. This is the wrong way to go. To use the full numerical aperture of your 40x and 100x objectives, you need to keep the iris fully open or very nearly so. This will allow the objectives to operate at maximum NA and thus resolution.

In practice, this may generate images that lack contrast, as you saw. Closing the condenser iris somewhat can help you find a good balance between high resolution (large aperture) and good contrast (smaller aperture). But starting out with a very small condenser aperture is surely going to destroy the resolving power of your 40x and 100x objectives!

If you don't like your brightfield images when your condenser iris is almost fully open with the 40x and 100x, you might want to look into other illumination techniques like oblique, Gradient universal filter, rheinberg, darkfield, DIC, etc. Or use stains.

coprinus
Posts: 4
Joined: Thu Mar 26, 2020 7:19 pm

Re: Olympus Ach 100x/1.25 Oil iris

#5 Post by coprinus » Fri Mar 27, 2020 4:47 pm

Interesting remark. I do indeed tend to go for the smallest aperture which does not lead to optimal balance between resolution and contrast.
The samples I examine, mushroom cells, spores, usually do not have that much contrast. In some cases you can use stains to enhance the contrast but not always.
Sometimes you need to observe small features on spores. Depending on the species, spores vary in size from 4 to 12 microns and you need to be able to spot details on a spore.

For a given objective and magnification, once you did setup Kohler illumination:
- setting field iris diaphragm to just cover the field of view
- adjusting the optimal height of the condenser
- centering of the condenser

there are still 2 parameters you can control:

- the intensity of the lamp
- the diaphragm of the condenser.

how do you proceed to obtain optimal balance between resolution and contrast?
Is it just trying out or is there a method?

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