Nikon scope objectives.

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lothman
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Post by lothman »

Charles Krebs wrote:Chris,

A problem is that Nikon did not mark all of their CF objectives as such on the objective itself. This is certainly true of the majority of the CF 210mm tube length M and BD objectives we like so much, where most bear no "CF" designation.
Nikon M Plan 10/0,21 210/0 SLWD on Ebay 4 from 5 still available :wink:

The seller gave me about 30% rebate.

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Post by ChrisR »

That'll be this one, then?:
Image

Oooh kay,
For the 10x, NA 0.21 scope lens
the "f" number is about
1/(2*0.21)
=f2.38
WD 20.4

So that looks very similar to an Olympus 20mm F2.0, if (big "IF"!) there's one lying around.
I believe there's a test hereabouts showing the Oly isn't quite as sharp as a scope lens, even though its aperture is wider. *

doing the other sum, comparing the best ordinary 10x objective which has NA of 0.3,
Resolution being given by (0.61 *0.55)/2NA,
the SLWD comes out at 0.8 micron (μ)
versus for the NA0.3, 0.56μ.
Multiplying by the magnification gives 8μ and 5.6μ.
On Charles' excellent adventure with a spreadsheet ** he points out we really need 3 pixels to satisfactorily resolve a detail one pixel big.
SO to see the benefit of the difference in NA we need a pixel size smaller than, say two and a half microns.
The smallest-pixel DSLR sensor I'm aware of currently (4/3rds Olympus sensor) has pixels 4μ across, and mine are double that.
So it would seem from this alone, that some of us might begin to see a difference in resolution, others wouldn't.


Other factors I can think of -
>>F numbers and NAs aren't all that they appear to be - where's that post where Rik shone lasers through his lenses...? ***

>>Something like an Olympus macro lens is designed to cover a 35mm frame - scope lenses aren't - but some do pretty well.

>>Something like a NCF 10x/0.3 scope lens for 160 tube is 16mm.
[Edit: That was wrong. Parfocal 45mm 160 tube length, FL = approx 20.5mm] We know that it can be wound out to give 15x magnification quite well. That's getting a bit long on the bellows for a 21mm [Edit: Wrong again, it would be longer] (210 tube length) lens such as the SLWD.

( * means I have to find a link )


The same vendor is currently offering NCF10x 0.3 infinity objectives. Now I wonder how much difference that really makes?


Edited to correct errors in focal lengths of the scope objectives.
Last edited by ChrisR on Mon Sep 21, 2009 10:21 am, edited 1 time in total.

rjlittlefield
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Post by rjlittlefield »

ChrisR wrote:where's that post where Rik shone lasers through his lenses...?
It's at http://www.photomacrography.net/forum/v ... =2046#2046.

The big issue in that case is pupillary magnification factor (PMF). The Olympus 38 mm and 20 mm lenses are moderately asymmetric, with exit pupils smaller than entrance pupils. Their f/2.8 and f/2.0 ratings are based on FL and entrance pupil size at infinity focus. But when you add extensions as required to put them in their design range with magnifications much greater than 1:1, then the exit pupil size dominates and the effective f-number becomes smaller than you would anticipate not knowing about the PMF. I've done some more study of those lenses in the meantime, but I see I haven't written up the results yet. Truly boredom will never be a problem...

--Rik

elf
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Post by elf »

As a side note, where is the entrance pupil located on the Nikon M Plan 10/0,21 210/0 SLWD (measured from the base of the threads)?

Another side note: How will this lens work reversed?

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Post by rjlittlefield »

elf wrote:As a side note, where is the entrance pupil located on the Nikon M Plan 10/0,21 210/0 SLWD (measured from the base of the threads)?
I have no idea. However, every microscope objective I've played with at 10X and above has had such shallow DOF that it works fine to turn off scale adjustment in stacking software. As soon as you turn off scale adjustment, then it no longer matters where the entrance pupil is actually located, because the stacking process essentially imposes orthographic projection like you'd get with a lens that's telecentric on the object side. For purposes of stack-and-stitch, that means you don't need to rotate around the entrance pupil; just sliding sideways works fine.
Another side note: How will this lens work reversed?
These lenses are designed to be used nose-forward at 10X. If you were to reverse one, it would work really well at 0.10X for projecting a 25 mm subject onto 2.5 mm of sensor. Beyond that circle, the image quality would fall way off. What application are you thinking of?

--Rik

elf
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Post by elf »

rjlittlefield wrote:
elf wrote:As a side note, where is the entrance pupil located on the Nikon M Plan 10/0,21 210/0 SLWD (measured from the base of the threads)?
I have no idea. However, every microscope objective I've played with at 10X and above has had such shallow DOF that it works fine to turn off scale adjustment in stacking software. As soon as you turn off scale adjustment, then it no longer matters where the entrance pupil is actually located, because the stacking process essentially imposes orthographic projection like you'd get with a lens that's telecentric on the object side. For purposes of stack-and-stitch, that means you don't need to rotate around the entrance pupil; just sliding sideways works fine.
--Rik
Is the diaphragm visible when viewing from the front?
I'm planning on getting one of these (if I ever find one at the right price), so was curious how it would work for large panos.
rjlittlefield wrote: These lenses are designed to be used nose-forward at 10X. If you were to reverse one, it would work really well at 0.10X for projecting a 25 mm subject onto 2.5 mm of sensor. Beyond that circle, the image quality would fall way off. What application are you thinking of?

--Rik
I was wondering if they worked like other reversed lens where the magnification was increased without increasing the effective aperture.

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Post by rjlittlefield »

elf wrote:Is the diaphragm visible when viewing from the front?
I don't believe these lenses have a diaphragm, in the sense of an adjustable iris. They certainly have some limiting aperture -- every lens does. But you have to be careful to identify that aperture from the standpoint of the in-focus subject. With microscope objectives, it's common that different apertures appear to be limiting if you look into the front of the lens from some comfortable viewing distance (say, 12 inches) versus looking into the front of the lens from where the subject would be. This is a different situation from normal macro and enlarging lenses, where the adjustable diaphragm is the limiting aperture no matter how you look.
was curious how it would work for large panos.

By "large panos", I think you must be meaning several frames wide and several frames high. Again, I'd expect any microscope objective to work OK for that, by just turning off scaling and treating the lens as if it were telecentric (orthographic projection).
worked like other reversed lens where the magnification was increased without increasing the effective aperture.
This part puzzles me. Using any lens, you can increase magnification without changing the f-number by adding closeup lenses. If you focus by extension, then depending on pupillary magnification factor, the effective aperture may be bigger or smaller than you'd predict without considering PMF. It's true that with some lenses you can get higher magnification at the same effective aperture by reversing them, but that's certainly not universal.

In any case, aberrations are the strongest driver of whether to reverse or not. In the case of a 10X microscope objective, you could reverse it to give good quality around 0.1X (over a very small field). But if you tried using it at any magnification much above that, the image quality would become abysmal.

--Rik

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Post by mgoodm3 »

"It's true that with some lenses you can get higher magnification at the same effective aperture by reversing them, but that's certainly not universal"

More likely "atypical" to retain the same or better EffAp when reversed. Since most reversed lenses are short focal length and have a P>1. when reversed, the pupil will be smaller and farther away from the detector. Long focal length lenses with a telephoto design are the potential exception.

lothman
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Post by lothman »

ChrisR wrote:That'll be this one, then?:
Image

Oooh kay,
For the 10x, NA 0.21 scope lens
the "f" number is about
1/(2*0.21)
=f2.38
WD 20.4
it seems to work :D
Image

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Post by rjlittlefield »

This response is a bit late, but maybe it's still worthwhile.

Much earlier in this thread, the following exchange appears:
ChrisR wrote:
Charles Krebs wrote:Actually a 5/0.14 and a 10/0.30 used on bellows are capable of resolution that most sensors cannot fully accommodate.
Oh?
10x NA 0.3

10 x1.13 = 11.3microns.
Many a DSLR sensor pixel is smaller than that?
Am I missing something?
Charles Krebs wrote:The smallest resolvable detail a 10/0.30 can produce is 11.2 micron at the sensor. In order to resolve all the detail there must be an bare minimum of 2 pixels across this detail. But that is only if everything is totally serendipitous with the way pixels line up with detail. Most references I've seen suggest that it is best to have 3 to 3.5 pixels for the smallest resolvable detail size.
...
3 pixels per smallest detail:
largest allowable pixel 3.7 micron
sensor needs 39.4 Mp
...
There is a subtle point hiding behind the numbers.

The smallest resolvable feature size is correct. (Table 1 at microscopyu says that 10/0.30 gives 0.92 microns at the subject, which would be 9.2 microns at the sensor. This difference is not significant and depends on assumptions about what is "resolvable".)

The subtle point is that at that feature size, the lens is creating only a very low contrast image -- someplace around 10% MTF, again depending on various assumptions.

The lens will have more contrast for features that are not so small, but even with no aberrations MTF cannot reach 50% until the features are about twice as big as the smallest resolvable ones. (See the graph HERE and the following discussion.)

So, Charlie's figures correctly indicate the size of pixels that are needed to capture the finest details that the lens can resolve, but this is struggling to capture details that have already been severely degraded -- roughly a 10X reduction in contrast. Much larger pixels, roughly 2 times larger, are sufficient to capture details that have suffered only a 2X reduction in contrast (MTF=50%). If you're looking at a black/white resolution chart, the improvement made by smaller pixels is easy to see. If you're looking at a typical biological subject, it's far less obvious.

I hope this helps. It seems like I work with these formulas and numbers every day, and still I find them confusing and misleading at times. But every discussion helps to clarify them, which is good.

--Rik

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Post by ChrisR »

This is encouraging, I think. :) I have sometimes seen detail that really doesn't show up as I thought it ought to, only to find that I'm at the edge of what the lens "should " achieve. SO that fits.

I remember a pic of Rik's where Charles commented that the observable resolution was very close the the theoretical figure for the NA of the lens. The detail (hairs) would of course have been sharpened, which done right, would bring that low contrast detail much clearer.
A hair is a bit of a special case though, usually a relatively high contrast compared with its surroundings ot the background.
It would be interesting to do tests on a real subject with lenses of different NA, and see how nearly post-sharpening could make them look the same.

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Post by rjlittlefield »

I thought it would be helpful and interesting to add a link to this thread.

http://www.microscopyu.com/tutorials/ja ... alculator/ has a good discussion of sensor requirements versus magnification and NA.

But take care to note that the page contains two very different numbers bearing exactly the same label.

"Required Pixel Size" shown in the applet at the top of that page is exactly half of the "Required Pixel Size" shown in Table 1 in the middle of the page!

For example:
Image

What I think is going on is this... [this explanation is wrong -- see followup posts]

The value shown in the applet is pixel size = resolution limit / 4. This is a fine sampling that will not miss or degrade any pattern very much, no matter how it happens to be positioned.

The value shown in Table 1 is pixel size = resolution limit / 2. This is the coarsest sampling that could possibly capture a repeating pattern of features at the resolution limit, if they happen to be ideally positioned on the pixels. Patterns that are not ideally positioned may be severely degraded or lost altogether.

Again, both of these statements are referring to feature sizes at the resolution limit of the optics, where their contrast is guaranteed to have been degraded by 10X or so depending on the nature of the pattern.

--Rik

Edit: to properly state "What I think is going on"
Last edited by rjlittlefield on Mon Sep 21, 2009 6:52 am, edited 2 times in total.

Charles Krebs
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Post by Charles Krebs »

Rik,

I think the applet and the chart are consistent. The lower chart seems to be using a "relay magnification" of 1, while in the screen clip of the applet you've posted shows a "video coupler magnification" of 0.5X. If you slide that to "1" the numbers will match.

What puzzles me is why they have calculated these numbers using 450nm as the wavelength of light. It's more typical to use 550nm for these calculations, and they mention the 550nm value several times in the text.

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Post by rjlittlefield »

Ah, very good, thanks for the correction! So indeed, the numbers are consistent.

But in that case, I am bothered by their claim of "determining the minimum pixel density necessary to adequately capture all of the optical data from the microscope".

As you have mentioned, and has been shown elsewhere, 2 pixels per element at the resolution limit will lose or degrade detail if it happens to be positioned badly. Actually it will degrade detail --- as in reduce contrast -- even in the best case.

So what puzzles me is why they use the label "Optimum CCD Array Size".

It would be more accurately labeled as "Minimum CCD Array Size", where "minimum" is understood as only losing patterns that are badly positioned.

As to 450 vs 550, I think they're using 550 but with their formula #1 that r = lambda/(2*NA). At least the numbers seem to work out properly under that assumption.

--Rik

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Post by Charles Krebs »

That's it. Wish the charts and graphs were annotated better.

When I was working on the spreadsheet I mentioned earlier I looked at many references and r = 0.61 × λ/NA seems to be more widely used and accepted, so that's sort of burned in my head. But the difference is not great.

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