the scale on the condenser's aperture

[iconoclastica]

s-l500.jpg (19.52 KiB) Viewed 5157 times

Often the aperture rings of the condensers are provided with a scale. The figures on those scale at least suggest that setting it by the scale would more or less be the correct setting given the system's NA. However, whe I close the aperture until it starts showing in the periphery of the objective's BFP, the chosen value doesn't even come near the NA I thought to have (much lower usually). For example, with a 1.30NA objective used with immersion and a 1.0 achromat condenser without, I expect to get NA to be c. 1.0, but I set the aperture to 0.65. Same objective, 1.40 achro-aplan condenser, immersion on either side, 0.75 seems to be the right setting. What am I missing?

[LouiseScot]

s-l500.jpgOften the aperture rings of the condensers are provided with a scale. The figures on those scale at least suggest that setting it by the scale would more or less be the correct setting given the system's NA. However, whe I close the aperture until it starts showing in the periphery of the objective's BFP, the chosen value doesn't even come near the NA I thought to have (much lower usually). For example, with a 1.30NA objective used with immersion and a 1.0 achromat condenser without, I expect to get NA to be c. 1.0, but I set the aperture to 0.65. Same objective, 1.40 achro-aplan condenser, immersion on either side, 0.75 seems to be the right setting. What am I missing?

Hi

I think, in general, you should initially set the condenser aperture NA to match the objective NA. The condenser on its own is essentially a contrast device so you can reduce the aperture diaphragm to get a more contrasty view, at the expense of resolution. If you have a field iris you should follow the Kohler procedure to get even illumination. You already realise that max overall aperture in air is 0.9-1, and the highest NA is achieved with immersion oil and an oil condenser (though I've never personally tried the latter!).

Louise

[apochronaut]

This might have something to do with the BFP not being in the same location as the aperture stop or pupil plane in the objective used. Generally that is not necessarily the case with finite objectives but is more often the case with infinity objectives.

[woyjwjl]

For immersion objective lens, NA adjustment of condenser lens becomes insensitive, at least in my BHS. Of course, it is related to your expectation of contrast.

In addition, high NA means low depth of field. I suggest you first ask yourself these two questions:

1. Do you know the resolution of 1.0na objective lens?

Resolution and Numerical .jpg (84.59 KiB) Viewed 5067 times

2. Do you really have the ability to make full use of 1.3 or 1.4na objective lens?

Popular science -- Resolution limit of optical microscope 200nm.jpg (102.25 KiB) Viewed 5067 times

[iconoclastica]

These are tha lat steps of the Köhler-instrucions if routinely follow:

• Remove one of the eyepieces or replace it with a centering telescope. You can now observe the objective’s BFP in the tube.
• Close the aperture diaphragmwhich is visible within the objective’s BFP, until its diameter is about 3/4 1 of the size of the BFP

I set out today to get some measured data and what I found is revealing. I aligned the aperture diafragm with the illuminated disk of the BFP, while observing he BFP through a phase telescope. These are the readings of the condensor's scale I got:

Code: Select all

objective			condenser scale
-----------			------------------
10x/0.25			0.25
20x/0.40			0.40
40x/0.65			0.65
100x/1.30 (dry)			0.80
ibid. (oil )			1.00 
ibid (oil 2x)			1.30		i.e. oil on the condenser top too

In the latter case, the objective's aperture for adjusting to cover glass thickness reduced the read NA to 0.50 at most.
Am I correct to think that this is a way to measure the overall NA of the system? If so, what happens if one follows the Köhler instructions as above?

For example:
Objective 40x, NA=0.65. Interpreting 'size' in the instructions as diameter (not surface area) I set the aperture diafragme to 75% of its max. For once I do this precisely by clamping a transparent ruler to the bottom of the condenser, so the scale is focused in the phase telescope. When I reduse the aperture diameter from 10mm on the ruler to 7.5mm, the NA-scale on the condenser drops from 0.65 to 0.45. Does the same happen to the effective NA?

[LouiseScot]

I think the numerical aperture on the condenser scale is just indicative of the setting to use with a given objective. The scale on my T720 displays objective magnification rather than NA. So goes 4, 10, 20, 40, 60, 100. I think it presumes the positions for the diaphragm are for achromatic objectives. Plan Fluors or Apos generally have larger apertures for a given magnification, whereas long working distance objectives have smaller NAs. Obviously the diameter of the iris aperture influences the light cone.
Thorlabs have quite a good explanation:

https://www.thorlabs.com/newgrouppage9. ... e%20sample

Louise

Edit:
Oliver has a short article about using the condenser diaphragm
https://www.microbehunter.com/the-conde ... diaphragm/

[iconoclastica]

I think the numerical aperture on the condenser scale is just indicative of the setting to use with a given objective. The scale on my T720 displays objective magnification rather than NA. So goes 4, 10, 20, 40, 60, 100.

Exactly what I was given to think by I don't recall what textbook. I even have another condenser that shows the diameter in millimeters. Yet my figures exactly match the scale. If I had made them up, I would have introduced some random error...

Still, I don't get the entire picture:

• the condenser cannot increase the NA, but it can reduce the NA

How does it reduce the NA? (If well focused) only by closing the aperture.

• closing the aperture too much reduces the NA

So not closing it too much won't, I assume. I think now, it starts becoming limiting once the aperture appears in the BFP.

If this is correct, the position where the condenser's aperture equals the next most limiting NA is once it equals the diameter of the light circle.

• the diameter of the light circle in the BFP increases when other NA limiting factors are eliminated.

I mean, adding oil immersion and to the condenser's top. This means then that you can actually see the effective NA (but not yet measure it directly). Considering the condenser scale as an indirect measurement, I'd say that the correspondence between the expected values and the read figures is just too good to be called 'indicative'. This may of cource vary between condersers of various makes. At least two of my Nikons are pretty exact conforming the objective's NA (the 3rd one (Abbe) has the mm-scale).

But questions remain:

• the NA is defined as a sinusoidal function of the top angle of the light cone

The sinus is maximal (1.0) when that angle equals 90°, when the NA equals RI. How can the condenser 'know' which media are applied?

• Close the aperture diaphragm (..) until its diameter is about 3/4 of the size of the BFP

The condenser's effective NA is linear with its aperture's diameter. Closing it to 80% therefore reduces the NA to 80%. Assuming everything else is optimally adjusted, at e.g. 40 you're not having NA=.65 but more likely .50. Beats me why this is never mentioned (makes me doubt my whole theroy, in fact).

• there have been theory wars about whether the effective NA equals the most limiting factor, or whether it's the average of below and above stage NA.

This would be a rather pointless dispute if it could be so simple to observe as I did. But this doesn't help me to see exactly what I am observing...

[LouiseScot]

Hi
I'm a bit busy at the mo and can't remember if I posted this link https://zeiss-campus.magnet.fsu.edu/tut ... flash.html

You have to remember that NA depends on refractive index of the medium which is limited to about 1 in air. You'll potentially get the highest resolution with a condenser oil and objective oil interface - but not necessarily the best view since lowering the NA increases contrast and depth of field. There's also a decent wiki on the topic: https://en.wikipedia.org/wiki/Condenser ... l_aperture

Hope that helps

Louise

PS Don't forget that if you have a field lens and diaphragm, this can be adjusted with the condenser to achieve Kohler illumination which gives even illumination of the fov

[iconoclastica]

I ... can't remember if I posted this link [url]https://zeiss-campus.magnet.fsu.edu/tut ... flash.html[/url

A beautiful animation, but plain wrong. Here is the same in reality:

NA set by scale to 0.2

NA0.2.jpg (20.71 KiB) Viewed 4936 times

NA set by scale to 0.7

NA0.7.jpg (21.05 KiB) Viewed 4936 times

NA set by scale to 1.0

NA1.0.jpg (19.87 KiB) Viewed 4936 times

Their mistake is that theta is to be taken from the normal vector. instead they measure the entire angle and so double the value. Compare with your link to Thorlabs.


Note that at 1.0 the angle is still less than 90°.

[LouiseScot]

I ... can't remember if I posted this link [url]https://zeiss-campus.magnet.fsu.edu/tut ... flash.html[/url

A beautiful animation, but plain wrong. Here is the same in reality:

NA0.2.jpg

NA0.7.jpg

NA1.0.jpg

Their mistake is that theta is to be taken from the normal vector. instead they measure the entire angle and so double the value. Compare with your link to Thorlabs.


Note that at 1.0 the angle is still less than 90°.

Hi
The animation on that page doesn't run in my Edge browser so I've no idea what it shows! I was really only linking to the text anyway.
Louise

[hans]

• the NA is defined as a sinusoidal function of the top angle of the light cone

The sinus is maximal (1.0) when that angle equals 90°, when the NA equals RI. How can the condenser 'know' which media are applied?

NA is not affected by the media applied to the condenser if the the selected NA is less than the RI of the media. When the selected NA is equal to the RI of the media the most extreme rays of the light cone are at the critical angle for the interface where total internal reflection begins. Increasing the NA setting of the aperture beyond the RI of the media gives no further increase in NA as rays corresponding to NA greater than the RI of the media are internally reflected at the condenser -> media interface.

[iconoclastica]

Hi
The animation on that page doesn't run in my Edge browser so I've no idea what it shows! I was really only linking to the text anyway.
Louise

shifting the slider turns the ring and narrows the bundle

flash.jpg (89.78 KiB) Viewed 4922 times

[LouiseScot]

Hi
The animation on that page doesn't run in my Edge browser so I've no idea what it shows! I was really only linking to the text anyway.
Louise

That's ok - I don't need to see it!

[apochronaut]

I ... can't remember if I posted this link [url]https://zeiss-campus.magnet.fsu.edu/tut ... flash.html[/url

A beautiful animation, but plain wrong. Here is the same in reality:

NA0.2.jpg

NA0.7.jpg

NA1.0.jpg

Their mistake is that theta is to be taken from the normal vector. instead they measure the entire angle and so double the value. Compare with your link to Thorlabs.


Note that at 1.0 the angle is still less than 90°.

Looking at the last picture you supplied, what appear to be the two linear branches of the angle, portray more than 90° .

[iconoclastica]

Looking at the last picture you supplied, what appear to be the two linear branches of the angle, portray more than 90° .

Then the 'shadows' of the edge would have been off screen..?!

Θ = approx. 60°

file.jpg (41.55 KiB) Viewed 4856 times

[iconoclastica]

NA is not affected by the media applied to the condenser if the the selected NA is less than the RI of the media.

I don't think I can understand that statement. The NA is the product of the RI and the sine. Mathematically it is easy to prove that NA <= RI * sin(theta) is always true, so the NA can never be greater than the RI

[Chas]

Deleted.

[hans]

I don't think I can understand that statement. The NA is the product of the RI and the sine. Mathematically it is easy to prove that NA <= RI * sin(theta) is always true, so the NA can never be greater than the RI

Yes, maybe I misunderstood the question, if the the NA set by the condenser aperture exceeds the RI of the media after the condenser then the excess high-angle rays are internally reflected and don't make it out of the condenser. But if the NA set by the aperture is less than the RI of the media that follows then NA stays the same across the interface regardless of the RI. For example if you set 0.8 on the condenser you get 0.8 in either air or glass/oil. So in that sense the condenser scale doesn't "need to know" about the following media other than the hard upper limit NA <= RI.

[wabutter]

This is a great discussion when it comes to comparing theoretical vs. practical real world application of the condenser diaphragm adjustment. Most of the major manufacturers have used some form of reference to the objective, in use, on the condenser, giving the user a reference point. It doesn’t matter if it is NA or Mag. The previous discussions have adequately addressed the variables, and possible permutations that can be applied to the condenser adjustment.

One element that has not been mentioned is the specimen. As noted earlier, the optical ray path competes between resolution and contrast. The condenser is responsible for the control of both. An aperture that is open too far results in max resolution, but possibly not enough contrast to see the details of the specimen. Contrarily, if the aperture is reduced too far below the objective aperture size, there is an increased contrast with resultant loss of resolution.

I was taught, on any objective, start with an open aperture and slowly close the aperture until you see contrast begin. If you look at the back aperture of the objective you will find that the aperture will be about 7/8 th of the aperture of the objective. Optimum contrast with optimum resolution. Of course, you can also look at the back aperture by removing an eyepiece and looking down the eye tube to set the aperture for the same result.

Using this method, you can easily distinguish a 40x plan Achro, plan Fluoro, plan apo from each other. With out have any descriptive information on the objective.

[Hobbyst46]

...Of course, you can also look at the back aperture by removing an eyepiece and looking down the eye tube to set the aperture for the same result.

Using this method, you can easily distinguish a 40x plan Achro, plan Fluoro, plan apo from each other. With out have any descriptive information on the objective.

Good point !

[apochronaut]

The discussion however, has meandered a long way from the original question which was why the condenser diaphragm's graduated markings do not represent the full aperture of the objective when the condenser diaphragm's aperture size when viewed at the bfp of the objective, physically equals the aperture at the bfp of the objective. In order to figure that out, one needs to determine if the aperture that is being viewed at the bfp of the objective equals the full aperture of the objective.

[iconoclastica]

on any objective, start with an open aperture and slowly close the aperture until you see contrast begin. If you look at the back aperture of the objective you will find that the aperture will be about 7/8 th of the aperture of the objective. Optimum contrast with optimum resolution.

Excellent! We probably all have learned to proceed that way, but rarely it is explicit why. So, to find the optimum between resolution and contrast. Optimum, so neither is maximal.

So, is it correct to draw this conclusion, that the objective's engraved NA is only the maximum NA, not obtainable unless all other settings and media are optimal including the condenser's aperture being viewed at the bfp of the objective equalling the full aperture of the objective? Or rather, that the operational NA is maximal about 7/8th of that in the reality of normal use?

[iconoclastica]

In order to figure that out, one needs to determine if the aperture that is being viewed at the bfp of the objective equals the full aperture of the objective.

So:

• inspect the BFP

• make sure the condenser's aperture is focused

• make sure neither field aperture not the objective's aperture (if any) are interfering

• observe when changing the condenser's aperture where it just starts reducing the illuminated circle

• read the scale

If this procedure s correct, the scale reports the real NA. And I am more confused than when it would have been no more than a loose indication.

[apochronaut]

It isn't so much a question of procedure but whether the visual field you are seeing at the bfp is the total aperture of the objective.

[iconoclastica]

Is there a way to find that out?

[apochronaut]

It might be more straightforward for infinity objectives because of the design prerogative to produce more coherent parallel ray bundles but for objectives that are converging the rays it might be harder to determine and some objectives have the bfp inside the objective and others external to the lens pack. In many cases those rear lenses are just correcting lenses and don't really affect the focus.
It is something to think on. I've recognized for years that adjusting the diaphragm is a tradeoff between contrast and resolution but did not ever consider the degree of aperture limitation that you are implying with your tests. It just isn't possible that a 1.30 N.A. objective has aperture limitations of .50 N.A. or more when the condenser diaphragm aperture physically equals the objective aperture. That objective aperture visible at the bfp must already be only partial.

[LouiseScot]

Maybe this diagram helps (from Murphy and Davidson, 2013)

RoleofCondenserDiaphragm.JPG (52.34 KiB) Viewed 4551 times

You can view an enlarged BFP with a 'Bertrand Lens' or 'Phase Telescope' (which you can knock up with a couple of suitable lenses).
I'd have thought you'd be able to see how the aperture varies at the bfp or at the exit pupil, with, say, a 40/0.65 objective. Having read around a bit I believe the scales on a condenser are intended to give a useful aperture compromise (between resolution, depth of focus and contrast) for the particular objective. So, at maybe 70% of the aperture that would theoretically give the max resolution. At the end of the day, once Kohler adjustment has been done, one generally just adjusts the aperture diaphragm for the best view.
There are ways and means of making an apertometer to use with a condenser. Does anyone ever bother?
Is this helpful? Maybe not!

Louise

[patta]

Some random comments to sway further the subject:

- The "Back Focal Plane" is not well defined in the objective itself; objectives don't usually have an aperture stop (!!!)
The max light cone is stopped somehow by all lens rims; I'd take as official BFP the image of the condenser iris. Only occasionally it matches some lens rim "stop".
In case of Phase Contrast objectives, the position of the phase ring in the objective matches the image of the condenser annulus, and we can take it as the BFP.
I have some objectives with Iris, and the image of the condenser iris never fall there exactly (maybe because I'm mixing different brands of condenser and objectives).
This issue become relevant when trying to get a neat darkfield/COL.

- I've used extensively the method as from Iconoclastica posts #9 & #15, to look at the condenser actual aperture; I think it works properly; to make it better one should use a "pinhole slide", a black slide with a small hole, so the cone of light is neater.
Condenser apertometers, somebody bothers ....

- The Position of the condenser and its aberrations can play a substantial role in the actual working aperture, but I'll say that mostly they tend to reduce it. I never had a true "aplanatic" condenser, but still doubtful that it can be truly aplanatic at high NA, so some mismatches may be allowed.

- Apart from lasers, the illumination is never really collimated, but more like a messy bundle of rays; coupled with the above aberrations, some light may shine at higher NA than expected?

We've discussed some in this messy thread:
viewtopic.php?f=5&t=12402