Darkfield stop sizing

Here you can discuss different microscopic techniques and illumination methods, such as Brightfield, Darkfield, Phase Contrast, DIC, Oblique illumination, etc.
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hans
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Darkfield stop sizing

#1 Post by hans » Sat Feb 13, 2021 9:16 pm

Continuing from viewtopic.php?f=28&t=11958:
crb5 wrote:
Sat Feb 13, 2021 2:28 am
Several factors contribute to this calculation and a good discussion for practical implementation is given at https://www.olympus-lifescience.com/en/ ... darkfield/.
A confusing thing about the Olympus page is it suggests an experiment to measure the projected size of the objective aperture in the condenser diaphragm plane:
The diameter of the opaque stop will vary from objective to objective and should be carefully measured by placing a transparent ruler in the substage filter carrier and holding it steady against the bottom of the condenser. Next, determine the opening size by removing an eyepiece and observing the image of the ruler at the back focal plane of the objective using a phase telescope (or by inserting a Bertrand lens). Make certain that both the substage condenser aperture and field diaphragms are opened to their widest position before performing this maneuver. The number of ruler divisions visible in the back focal plane will be equal to the size of the stop necessary to block zeroth order light from entering the objective. Change to the next largest size objective and take another measurement, repeating until stop sizes for all objectives are known.
But then the actual recommended starting points in table 1 vary only weakly with NA, for example 16-18 mm at 10X 0.25 then 20-22 mm at 20X 0.65 is ~20% increase in stop size while the NA more than doubles.

Horseflesh
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Re: Darkfield stop sizing

#2 Post by Horseflesh » Sun Feb 14, 2021 7:27 am

I made darkfield stops from 10mm - 24mm to identify the best size for different views. I'd like to understand the theory better, because my observations don't make any sense when looking at the expected patch sizes for a given objective. I may still be making operation errors, though.

With my 10x objective, I have to go down to a 12mm patch to get a reasonably dark and uniform background.

(Please excuse the exposure, my phone camera is "helping." To the eye, contrast is better and the background is darker.)

10x, 12mm patch: https://i.imgur.com/Q4WWUmR.jpg

If the patch size increases even 1 mm I get a brown shadow in the middle of the view. At 18mm the whole field of view is too dark to see, with a hint of light at the edge. Again, hard to make the picture look like reality but you can see it is uniformly dark except for the edges.

10x, 18mm patch: https://i.imgur.com/hEOxM2H.jpeg

With the 40x objective I can't get a very good view. Patches bigger than 14m don't provide enough light to see. The 14mm patch provides a brown background but contrast in the center is still pretty bad... barely enough light to see the subject.

40x, 14mm patch: https://i.imgur.com/MPeDXDP.jpg

I wonder what I need to change?

hans
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Re: Darkfield stop sizing

#3 Post by hans » Mon Feb 15, 2021 7:22 pm

Horseflesh wrote:
Sun Feb 14, 2021 7:27 am
I'd like to understand the theory better, because my observations don't make any sense when looking at the expected patch sizes for a given objective.
Those results don't make much sense to me either but I have not done much experimenting with stops yet and am not familiar with the microscope. From the Swift web site it looks like there is only a condenser iris, no field iris, and the height of the condenser is fixed? Where exactly are you putting the stops relative to the condenser iris?

Horseflesh
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Re: Darkfield stop sizing

#4 Post by Horseflesh » Mon Feb 15, 2021 8:07 pm

The condenser height is adjustable on the 380T. I have found that by adjusting height I can sometimes improve contrast, but at a cost in sharpness. Leaving the condenser in the uppermost position usually seems best.

The scope only has one iris--in the condenser, just above the filter holder. In this image, the red arrow on the left is the iris and the blue arrow on the right is the swing-out filter holder. (You can see a bit of the lamp bulb just below that.)

https://i.imgur.com/WsEBNgO.png

I may as well finish making a set of patches smaller than 10 mm and see what happens.

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Re: Darkfield stop sizing

#5 Post by LouiseScot » Tue Feb 16, 2021 12:48 am

Hi

I've had difficulty obtaining decent darkfield illumination on my 380T. I think the problem is that at higher magnifications the stop has to have such a large diameter that there's not enough peripheral light passing through for the illumination. The 380T only has a 1W LED which is more than adequate for most things but maybe not for darkfield. I don't know if there's any possibility of upgrading the LED. When I get a minute I might try using a small mirror and a 5W LED lamp to see if that works better. I'll be sure to post if it does.

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

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Re: Darkfield stop sizing

#6 Post by Horseflesh » Tue Feb 16, 2021 1:16 am

I too wondered if more light might help. If you try that let us know what you find out.

I also wonder if a replacement, superior condenser might be possible.

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Re: Darkfield stop sizing

#7 Post by LouiseScot » Tue Feb 16, 2021 1:29 am

Horseflesh wrote:
Tue Feb 16, 2021 1:16 am
I too wondered if more light might help. If you try that let us know what you find out.

I also wonder if a replacement, superior condenser might be possible.
Doubtful on the 380T as I've never seen any accessories or upgrades for it - my 380T only cost £216 so for a proper darkfield condenser might as well buy another scope with one fitted! Still, I'll give the bright lamp a try.

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

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Re: Darkfield stop sizing

#8 Post by apochronaut » Tue Feb 16, 2021 2:28 am

This forum gets a steady stream of threads started by people trying to make a silk purse out of a sow's ear. It's mostly that they blew 300 or 350 bucks on a cheap Chinese microscope and then want to save a few pennies by diy'ing DF stops for the cheap condenser, rather than buying a proper DF condenser. The guy who runs this forum apparently posted a video praising the Swift 380T. Am I correct in that? I have only watched a few minutes of a couple of his videos and they seemed way off base. I couldn't take any more. Is there some sort of run on these Chinese hobby grade microscopes because of some internet recommendations?
It is a bit of a tragedy, if so.

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Re: Darkfield stop sizing

#9 Post by LouiseScot » Tue Feb 16, 2021 2:42 am

apochronaut wrote:
Tue Feb 16, 2021 2:28 am
This forum gets a steady stream of threads started by people trying to make a silk purse out of a sow's ear. It's mostly that they blew 300 or 350 bucks on a cheap Chinese microscope and then want to save a few pennies by diy'ing DF stops for the cheap condenser, rather than buying a proper DF condenser. The guy who runs this forum apparently posted a video praising the Swift 380T. Am I correct in that? I have only watched a few minutes of a couple of his videos and they seemed way off base. I couldn't take any more. Is there some sort of run on these Chinese hobby grade microscopes because of some internet recommendations?
It is a bit of a tragedy, if so.
I really like my Swift 380T - it's excellent value for money! There's no option to fit a DF condenser - it's a basic finite microscope without upgrades. It's not meant to compete with anything more professional or sophisticated - it is what it is. As I said, if one really needs a DF condenser then one should get another microscope. But there's no harm fiddling with what you have and trying to get the best out of it! For me, fiddling about is half the fun! Maybe more than half :)

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

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Re: Darkfield stop sizing

#10 Post by Horseflesh » Tue Feb 16, 2021 3:18 am

It's easy to buy a cheap Chinese microscope and it's very easy to buy an expensive Japanese or European microscope. It is not at all easy to find good information on something in the middle. I spent a while looking at used listings and couldn't find anything good.

So I bought a Sow's Ear 380T, and I am making the best of it. Sorry to mess up your nice forum!

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Re: Darkfield stop sizing

#11 Post by Roldorf » Tue Feb 16, 2021 6:38 am

Hi Horsflesh and Louise,
take no notice of apochronaut he is an ex technician who seems to think that:-

1. It has to have been made by one of the top 3 microscope manufacturers otherwise it's rubbish, even if his perfect scope has been used and abused for the last 60 years.
2. You need to spend most of your time taking apart your scope (see 1 for maker) and searching the internet for spare parts to replace the old worn out parts.

He is fortunately wrong that 'cheap Chinese microscopes' don't have darkfield condensers or the suggestion that they are so 'crappy' that they are useless for the more serious 'microscopist'. https://www.bresser.de/en/Microscopes-M ... ondensers/ as an example. Prices for the scopes these fit start at 249 euro.

His post here is insulting, maybe he got out of the 'wrong side of the bed', but I don't think so. I have seen him make similar comments in other posts.

For the best results in your experiments with darkfield patch stops:-

1. Make sure that the iris in your condenser is fully open.
2. Set the condenser height as close as possible to the slide.

Another interesting effect is oblique illumination which is more easy to attain especially with a swing out filter holder.
Location: Northern Germany

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hans
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Re: Darkfield stop sizing

#12 Post by hans » Tue Feb 16, 2021 7:56 am

Horseflesh wrote:
Mon Feb 15, 2021 8:07 pm
The condenser height is adjustable on the 380T. I have found that by adjusting height I can sometimes improve contrast, but at a cost in sharpness. Leaving the condenser in the uppermost position usually seems best.

The scope only has one iris--in the condenser, just above the filter holder. In this image, the red arrow on the left is the iris and the blue arrow on the right is the swing-out filter holder. (You can see a bit of the lamp bulb just below that.)
Sounds reasonable. You may be interested to try viewing the objective rear aperture looking in one tube with the eyepiece removed. Should be able to see a sharply-defined, uniform disk of light corresponding to the opening in the condenser iris, or the objective aperture once the condenser iris is opened beyond the NA of the objective. (The iris/aperture is small when viewed directly through the eye tube, magnifying the view with a phase telescope makes it easier to see what is going on but not strictly necessary.) Checking to see whether intensity is falling off already towards the edge of the 40X aperture (or even 100X) might help narrow down whether the problem is something specific to your stops vs. a more general limitation of the illumination system.

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Re: Darkfield stop sizing

#13 Post by LouiseScot » Tue Feb 16, 2021 12:20 pm

One could retrofit an Amscope DF condenser for around £109 (dry) or £150 (oil). This would require some 3D printing but it's doable. However, I am actually slightly tempted to splurge out on one of their T720QC scopes:
T720_quint_dark.JPG
T720_quint_dark.JPG (43.29 KiB) Viewed 9588 times
The cost of shipping and taxes grates me though it's apparently half the normal price for the scope itself, so maybe overall still a good price for a budget infinity df scope with a quintuple nosepiece and 3W LED :)

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

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Re: Darkfield stop sizing

#14 Post by apochronaut » Tue Feb 16, 2021 1:52 pm

Roldorf wrote:
Tue Feb 16, 2021 6:38 am
Hi Horsflesh and Louise,
take no notice of apochronaut he is an ex technician who seems to think that:-

1. It has to have been made by one of the top 3 microscope manufacturers otherwise it's rubbish, even if his perfect scope has been used and abused for the last 60 years.
2. You need to spend most of your time taking apart your scope (see 1 for maker) and searching the internet for spare parts to replace the old worn out parts.

He is fortunately wrong that 'cheap Chinese microscopes' don't have darkfield condensers or the suggestion that they are so 'crappy' that they are useless for the more serious 'microscopist'. https://www.bresser.de/en/Microscopes-M ... ondensers/ as an example. Prices for the scopes these fit start at 249 euro.

His post here is insulting, maybe he got out of the 'wrong side of the bed', but I don't think so. I have seen him make similar comments in other posts.

For the best results in your experiments with darkfield patch stops:-

1. Make sure that the iris in your condenser is fully open.
2. Set the condenser height as close as possible to the slide.

Another interesting effect is oblique illumination which is more easy to attain especially with a swing out filter holder.
Very little of what you wrote here is in fact correct, Roldorf. I am willing to overlook your ignorance, however.
I am well aware of what is good bad and indifferent in the world of microscopes, Chinese or otherwise. I am in the process of buying a Chinese microscope at this very moment and as soon as the Lunar New Year festivities are over, it will be shipped. I don't stratify instruments according to brand. I strictly look at what an instrument is, what it's potential for growth is and what it's cost is. The Swift 380T is a pig in a poke .
My comments were directed at the Swift 380T primarily but it is relevant to other over marketed cheap hobby scopes and the on line marketers such as Amscope, who routinely take advantage of naiive buyers. The number of posts about the 380T microscope have blossomed since it was promoted on the Microbe Hunter website. Since there are numerous posts about how to make it a better microscope, one would assume that all is not well in bargainville.

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Re: Darkfield stop sizing

#15 Post by LouiseScot » Tue Feb 16, 2021 2:09 pm

[/quote]
Very little of what you wrote here is in fact correct, Roldorf. I am willing to overlook your ignorance, however.
I am well aware of what is good bad and indifferent in the world of microscopes, Chinese or otherwise. I am in the process of buying a Chinese microscope at this very moment and as soon as the Lunar New Year festivities are over, it will be shipped. I don't stratify instruments according to brand. I strictly look at what an instrument is, what it's potential for growth is and what it's cost is. The Swift 380T is a pig in a poke .
My comments were directed at the Swift 380T primarily but it is relevant to other over marketed cheap hobby scopes and the on line marketers such as Amscope, who routinely take advantage of naiive buyers. The number of posts about the 380T microscope have blossomed since it was promoted on the Microbe Hunter website. Since there are numerous posts about how to make it a better microscope, one would assume that all is not well in bargainville.
[/quote]

But as I said before - 'it is what it is' - a basic finite, trinocular microscope for £216/$300 - nothing much to lose by having one. Anyone who feels the need to have a better, more sophisticated, more expandable instrument can fork out the extra and get one! I don't see any reason to be critical of something which is generally enjoyable to use and mostly does the job. What's the problem? One could easily spend many times the cost of a Swift on just a new objective or accessory for an alternate instrument.
Still, would be interested to know which 'Chinese' microscope you have ordered? :)

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

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Re: Darkfield stop sizing

#16 Post by Hobbyst46 » Tue Feb 16, 2021 3:15 pm

The above shown advertisement for the 720T Amscope microscope, IMHO, doess not look like a special bargain (pricewise, that is).
The Bresser catalogue shows equivalent ( according to the specs) microscopes, that along with their OIL darkfield condenser (costing 249 E) could perhaps be less expensive, and are technically supported, and nearer to the UK.
apochronaut wrote:...I am in the process of buying a Chinese microscope at this very moment and as soon as the Lunar New Year festivities are over, it will be shipped. I don't stratify instruments according to brand. I strictly look at what an instrument is, what it's potential for growth is and what it's cost is.
Great ! thanks in advance for info ! It has been several years since this forum has seen an extensive review of a Chinese hobby microscope (last one was "Bestscope"), although many folks bought them.

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Re: Darkfield stop sizing

#17 Post by LouiseScot » Tue Feb 16, 2021 4:40 pm

Hobbyst46 wrote:
Tue Feb 16, 2021 3:15 pm
The above shown advertisement for the 720T Amscope microscope, IMHO, doess not look like a special bargain (pricewise, that is).
The Bresser catalogue shows equivalent ( according to the specs) microscopes, that along with their OIL darkfield condenser (costing 249 E) could perhaps be less expensive, and are technically supported, and nearer to the UK.
There is an AmScope UK (but limited stock catalogue) though I'm not sure what technical support one might need? Couldn't see anything on the Bresser web page that was quite as good specwise as the 720T-Quint-DF with 100x iris objective? Not cheap or easy to get things from the EU now either :(
Anyway, I think I was perhaps just being a bit wishful - I'm not going to get another scope and I don't really have the money for one anyway - or anywhere to put one!

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

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Re: Darkfield stop sizing

#18 Post by Roldorf » Wed Feb 17, 2021 8:22 am

Since we have lots of posts here showing support for Amscope, here is a low price alternative from Bresser and the darkfield condenser is Dry.
https://www.bresser.de/en/Microscopes-M ... am=5102000
https://www.bresser.de/en/Microscopes-M ... r-dry.html

428 Euro for both.

I have no idea if the scope is any good, all I want to point out is that there are low priced units on the market that do have darkfield condensers as an addon.
Location: Northern Germany

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Re: Darkfield stop sizing

#19 Post by LouiseScot » Wed Feb 17, 2021 10:47 am

Roldorf wrote:
Wed Feb 17, 2021 8:22 am
Since we have lots of posts here showing support for Amscope, here is a low price alternative from Bresser and the darkfield condenser is Dry.
https://www.bresser.de/en/Microscopes-M ... am=5102000
https://www.bresser.de/en/Microscopes-M ... r-dry.html

428 Euro for both.

I have no idea if the scope is any good, all I want to point out is that there are low priced units on the market that do have darkfield condensers as an addon.
I reckon, as a basic microscope, the Swift 380T is much better value for money!

https://www.amazon.co.uk/Magnification- ... ics&sr=1-1

Unfortunately, it's increased in price by about £60 since we left the EU :( However, I guess the price in Euros from Swift EU would be less than the £280 it's being sold for here. Maybe you could check that?

True, it doesn't have an official darkfield condenser addon but I think, with a little ingenuity, a third-party one could be retrofitted without too much trouble :) I think I might actually have a go! :)

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

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Re: Darkfield stop sizing

#20 Post by LouiseScot » Wed Feb 17, 2021 3:55 pm

On closer inspection of my Swift 380T - well, taking off the condenser! I found that the factory-fitted brightfield one appears to be just an 'industry standard' 37mm condenser. It has this helical up-down fitting but the condenser itself is just held in place by an M2 screw. The latter goes through the knurled plastic ring and holds the condenser in place when tightened. I've ordered an M3 10mm thumb screw to replace it. I'll just have to re-tap the hole. I should then be able to swap over with a darkfield condenser when needed. Hopefully that will work ok - fingers crossed! I've ordered an AmScope dry darkfield condenser from 'AmScope UK' which I don't think is actually in the UK - they price in Euros and ship from the Netherlands. It should be about £95 - or whatever Euros 108.99 works out at. Of course, there are still holdups between the EU and UK so I'll have to be patient! I'm quite excited - I've wanted to do some darkfield for a long time. I won't know until I try as to whether the 1W LED will be bright enough. If not, I might have to attempt an upgrade... :geek:

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

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Re: Darkfield stop sizing

#21 Post by Horseflesh » Wed Feb 17, 2021 4:39 pm

Outstanding Louise! Looking forward to your findings. Thanks for being our test subject.

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Re: Darkfield stop sizing

#22 Post by LouiseScot » Wed Feb 17, 2021 4:43 pm

Horseflesh wrote:
Wed Feb 17, 2021 4:39 pm
Outstanding Louise! Looking forward to your findings. Thanks for being our test subject.
I'm a compulsive fiddler! It started when I was very young and always wanted to take things apart to see how they worked :)
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

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Re: Darkfield stop sizing

#23 Post by Hobbyst46 » Wed Feb 17, 2021 5:13 pm

LouiseScot wrote:
Wed Feb 17, 2021 3:55 pm
I should then be able to swap over with a darkfield condenser when needed. Hopefully that will work ok - fingers crossed! I've ordered an AmScope dry darkfield condenser from 'AmScope UK' which I don't think is actually in the UK - they price in Euros and ship from the Netherlands. It should be about £95 - or whatever Euros 108.99 works out at. Of course, there are still holdups between the EU and UK so I'll have to be patient! I'm quite excited - I've wanted to do some darkfield for a long time. I won't know ntil I try as to whether the 1W LED will be bright enough. If not, ...
Very good idea. Very likely it will work and serve much better than the trial-and-error tinkering with stops. At least, it will be reproducible.

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Re: Darkfield stop sizing

#24 Post by LouiseScot » Wed Feb 17, 2021 5:23 pm

Hobbyst46 wrote:
Wed Feb 17, 2021 5:13 pm
Very good idea. Very likely it will work and serve much better than the trial-and-error tinkering with stops. At least, it will be reproducible.
We'll see - fingers crossed!
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

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Re: Darkfield stop sizing

#25 Post by Plasmid » Wed Feb 17, 2021 6:13 pm

This may sound naive, but does the 100X and 40X annulus on a wheel PH condenser work on the same principle as a patch stop? I get excellent quality with my 100X annulus up until the 20X objective after that no effect.

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Re: Darkfield stop sizing

#26 Post by LouiseScot » Wed Feb 17, 2021 7:06 pm

Plasmid wrote:
Wed Feb 17, 2021 6:13 pm
This may sound naive, but does the 100X and 40X annulus on a wheel PH condenser work on the same principle as a patch stop? I get excellent quality with my 100X annulus up until the 20X objective after that no effect.
I don't know a lot about phase contrast (have never had the equipment or done any) but there might be some similarities. Have a look here https://www.microscopyu.com/techniques/ ... figuration

Louise
A Nikon CF plan 20x; A Swift 380T; A DIY infinity corrected focus rail system with a 40x/0.65 Olympus Plan, a 10x/0.30 Amscope Plan Fluor, and a 20x/0.75 Nikon Plan Apo

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Re: Darkfield stop sizing

#27 Post by Hobbyst46 » Wed Feb 17, 2021 7:42 pm

Plasmid wrote:
Wed Feb 17, 2021 6:13 pm
This may sound naive, but does the 100X and 40X annulus on a wheel PH condenser work on the same principle as a patch stop? I get excellent quality with my 100X annulus up until the 20X objective after that no effect.
Yes, the 100X (or Ph3) position on a turret phase contrast condenser yields nice darkfield with the 4X, 10X and 16X (actually, with objectives of NA up to about 0.3-0.4).

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Re: Darkfield stop sizing

#28 Post by crb5 » Wed Feb 17, 2021 8:22 pm

Wow, quite a lot of discussion here. To get back to the first point, it seems the calculations of stop diameters to match different NA objectives might need an adjustment for the field-of-view. For 40x 0.65NA objective the field-of-view is small relative to the calculated stop diameter, but it becomes significant for lower power, lower NA objectives. Table 1 in the Olympus article https://www.olympus-lifescience.com/en/ ... darkfield/ does show a large jump up between the 1x and 2x objectives. I have made dark field stops for a stereo scope which require fairly large stops to match the large field of view but here condenser mechanism was different and so the comparison is not direct https://youtu.be/TIXAr5cCDOc.

I made dark field stops for my Amscope 120 compound scope from transparent plastic food containers and black electrical tape. They work well with the 10x 0.25NA objective and are OK with the 40X 0.65NA but the latter is more difficult to achieve. This scope was binocular, and after a year’s use it became apparent that a trinocular scope would be more useful. The primary driving force was triggered by Covid and the need the transmit live video images over Zoom to a class of community college students who were monitoring plankton. The cheapest option was to buy a Swift 350T scope which shares some parts with the Amscope. I prefer the condenser on the Amscope 120 which has a rack and pinion drive, rather than the helical twist lever on the Swift 350 (which is like the 380, I believe). Optically they are very similar and probably come from the same factory. I attach a dark field image made with the Swift 350T with 40x objective, a 21 mm diameter stop and a 1.3 MP USB camera which sees the central 1/5 of the field cf. the eyepieces. The height of the condenser is very critical – almost but not quite at the top position with a 1 mm thick slide. I was looking at live organisms from my pond. There seems to be quite a lot of chromatic aberration in the images which was not so obvious by eye (may be the camera color balance is off when operating with low light intensities). The color fringes were not apparent with the 10x objective, so they may also arise from the condenser operating with a full aperture. Also, it is important to look at rather sparse samples – if there is too much scattering material nearby you can get a high background fog in dark field.
it's a basic finite microscope without upgrades. It's not meant to compete with anything more professional or sophisticated - it is what it is. As I said, if one really needs a DF condenser then one should get another microscope. But there's no harm fiddling with what you have and trying to get the best out of it! For me, fiddling about is half the fun!
As far as buying economy scopes, I agree with Louise on this point. My first scope was a $30,O00 Zeiss Axiovert with fluorescence and DIC. It was a great scope for routine biology but insufficient for our purposes of looking at single fluorophores The Zeiss emission filters were not good enough and we ended up on spending double that amount of grant money on additions (lasers, emCCD camera etc.) and upgrades. When I retired, I took up plankton sampling as a hobby and bought a $200 Amscope. I considered this good value for money and certainly good enough for purpose of IDing live plankton at low magnification. I became involved with a local community college and joined students in their weekly tows in the Monterey Bay. The college has just bought 6 Swift 350T’s, partly based on my recommendation, so some students can explore their own plankton and show to the rest of the class in Zoom sessions. I have never been with such an enthusiastic bunch of students who are eager to learn the basics of microscopy. And if a few screws come loose and they have to take the scopes apart, an even better education!
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Horseflesh
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Re: Darkfield stop sizing

#29 Post by Horseflesh » Wed Feb 17, 2021 8:45 pm

Great post crb5, thanks. I'll try the 21mm stop again and fiddle with the condenser height. I think the 350/380 have the same overall optical path.

I too have noticed chromatic aberration that shows up much more dramatically in a photo than to the eye.

quantum
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Re: Darkfield stop sizing

#30 Post by quantum » Thu Feb 18, 2021 3:08 am

I want to chime in with my two cents on relatively affordable Chinese scopes based on my experience.

First, I love vintage scopes, and have some good AO Series 10 scopes from the 1960s or so, brightfield and phase.

I also have what I consider a rather nice Chinese scope. I think it is Chinese. The model number is Omax M828D5. You can google for it and find it for $495 but you do not find it promoted on eBay. I know I paid less than $495 but I don't remember exactly how much--probably around $300 or $400 about ten years ago.

That scope has quintuple nosepiece, five brightfield PLAN objectives, Kohler illumination with a variable-intensity halogen bulb and proper field diaphragm, solid metal construction, a binocular viewing body with diopter and inter-pupillary adjustment, wide-field (10x) and a 16x which seems useless but might be projection eyepieces, 1.25NA condenser (non-centerable). It is a 160mm tube length system that takes 45mm DIN objectives.

I feel that I am picky about image quality and can tell when a scope is not working properly, and this particular Chinese scope is every bit as good a a fully-working and well-performaning AO Series 10 in brightfield. The AO10 would cost less, but would require hobby time and knowledge to get to the endpoint, unless you are lucky.

I have used a good AO10, an Olympus CH2, and the Chinese scope side-by-side and the Chinese scope is every bit as good doing brightfield.

Now, what about upgradability? Probably not so much. Suppose I want to add phase contrast to it. I can buy a $600 phase kit, maybe it is good maybe it is not, but that is a lot of money. It is not a "cheap scope" anymore.

What about home-brew extensions and playing around with optical things? Well, again not so much. But the AO 10 is not particularly friendly for these things either. If you want to fool around with Reinberg illumination and similar, you will be equally frustrated because neither platform is an experimenters platform. (I am investigating the AO2/AO4 platform precisely for that intended purpose.)

The AO10 does have mainstream accessories for it--things like phase contrast equipment, dedicated darkfield, etc. These things are not particularly cheap to get to the endpoint, and you will go through some junk getting there too. It is great if that is something you enjoy.

But to keep it simple and just talk about brightfield, I say that you can get a good Chinese scope for a reasonable amount of money. The problem is that it is much easier to get a bad one, for a little less money, and it might not be obvious which is which. Here is a rule-of-thumb, though. If it promotes the fact that it does 2000x magnification or more, it is probably a piece of junk. In my opinion.

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