Diameter of black disc for darkfield?

dragonblade · Post by **dragonblade** » Sat Mar 17, 2018 12:33 am

I want to start experimenting with darkfield on the cheap. Ive seen some really nice results from someone who simply cut out a circular piece of cardboard and placed it on his microscope's light source. So just wondering what size diameter circle would I need for my objectives? I have an SP4 / 0.10 160 / 0.17, an SP10 / 0.25 160 / 0.17 and an SP40 / 0.65 160 / 0.17. Ive got a filter holder which I'll use for holding the clear plastic which the black cardboard disc will be attached to.

Pau · Post by **Pau** » Sat Mar 17, 2018 4:18 am

It depends on the condenser construction, stop placement and objective NA.

With 4X and 10X it's very easy, with a 40X 0.65 and a not well corrected condenser, it could be doable but difficult and plagued with chromatic aberrations because you only use the external, less corrected part of the condenser.
In principle the best position is as close as possible to the condenser diaphragm, in classic old style Abbe condensers at the filter, try under the diaphragm.

If you show us your condenser....

dragonblade · Post by **dragonblade** » Sat Mar 17, 2018 6:04 am

Oh that's unfortunate. Most of the time I use the 40x objective. It's hard to find aquatic organisms that are large enough to look decent with the 10x objective (except for rotifers.) Daphnia and hydra would be good candidates but Ive never come across them in the wild. I guess I could buy some prepared samples.

I'll take a photo of the condenser tomorrow.

dragonblade · Post by **dragonblade** » Tue Apr 03, 2018 2:45 am

Here's a photo of the condenser.

https://imgur.com/a/71dzE

I think I'll just settle for a disc which is the right diameter for the 4x and 10x objectives.

phil m · Post by **phil m** » Tue Apr 03, 2018 2:51 pm

There are devices made called Df rosettes. They are a reverse iris diaphragm, with a group of leaves that expand as you move the lever.They fit into the filter carrier under the condenser . I have an older one but recently I saw a new one for sale, somwhere and was surprised that they still make them.....if I could only remember where?

GaryB · Post by **GaryB** » Tue Apr 03, 2018 5:11 pm

I searched high and low for these and only ever found reference to then on somewhere like micrscopyU. I'd love to find one of them. If it ever comes back to you where you saw it, there are a few of us who'd love to know.

dragonblade · Post by **dragonblade** » Tue Apr 03, 2018 9:27 pm

DF Rosettes sounds like an interesting concept. In the meantime, I have some black card that I can cut up into a circle. I can use a coin as a stencil. Since I live in Australia, I have the following coins and their respective diameters for tracing.

5c - 19mm
10c - 23mm
20c - 28mm
$1 - 24mm
$2 - 20mm

Out of those, which do you think would be the most suitable size (or sizes) for use with 4x and 10x objectives?

GaryB · Post by **GaryB** » Tue Apr 03, 2018 9:33 pm

If you have a suitable sized filter that fits the tray, or even a piece of clear plastic (CD case) you can lay the coins directly onto that to test for size. Sticky putty like blutak helps hold things in position. It's nearly always trial and error on stop size and condenser height.

dragonblade · Post by **dragonblade** » Tue Apr 03, 2018 9:49 pm

Ah thanks Gary. I thougt bluetac might come in handy! Yea luckily my microscope has a filter tray which will make things easier.

GaryB · Post by **GaryB** » Tue Apr 03, 2018 10:00 pm

blutac always comes in handy

JohnyM · Post by **JohnyM** » Wed Apr 04, 2018 1:59 am

Is your condenser swing-out / screw - top ?

dragonblade · Post by **dragonblade** » Wed Apr 04, 2018 4:52 am

JohnyM wrote:Is your condenser swing-out / screw - top ?

That's a good question. I actually don't know the answer as Ive never opened it up. I guess it's not possible to tell from the photo I posted above?

phil m · Post by **phil m** » Fri Apr 06, 2018 6:12 am

GaryB wrote:I searched high and low for these and only ever found reference to then on somewhere like micrscopyU. I'd love to find one of them. If it ever comes back to you where you saw it, there are a few of us who'd love to know.

also called a variable dark field stop. the maker , I'm pretty sure was in India but I have lost the specific source. the one i have , came from old Leitz stock at the distributor. perhaps contacting a few dealers or distributors might turn one up.

Smokedaddy · Post by **Smokedaddy** » Fri Apr 06, 2018 10:14 am

dragonblade,

Check this link out.

https://www.olympus-lifescience.com/es/ ... darkfield/

-JW:

GaryB · Post by **GaryB** » Fri Apr 06, 2018 2:52 pm

Another blutak way to play with if you want to mess around is to get a slide, roll a ball of blutak and squish it into the middle of the slide until it's a round disk, get two more smaller pieces and use them to tack the slide to the bottom of the condenser. You can use Smokedaddy's post as a rough starting point for appropriate sizes.

If you remove an eyepiece and look down the tube, you'll see the blob and it's size so you can adjust it's position to center it, then look through the eyepiece to see how it looks, move the condenser up and down to see if you get decent darkfield. Being pliable, you can reshape it as much as you want until it's the right size for each objective. Measure what works and make the permanent ones from that.

Also, you can use the same technique for 'COL' or Circular Oblique Lighting. This technique gives a kind of phase contrast look and shows very high contrast and amazing detail. To do this, the blobs are sized so you see a thin ring of light when looking down the eyetube and the effect is varied by moving the condenser up and down a little which adjusts how thick the ring of light appears.

The following explains it well, it's in 4 parts.
http://www.microscopy-uk.org.uk/mag/ind ... pjcol.html

Have fun!