Help with adjusting contrast

Starting out in microscopy? Post images and ask questions relating to the microscope and get answers from our more advanced users on the subject.

Moderators: rjlittlefield, ChrisR, Chris S., Pau

dragonblade
Posts: 200
Joined: Sat Oct 18, 2014 11:16 pm

Help with adjusting contrast

Post by dragonblade »

For as long as Ive had my cheap Indian compound microscope, Ive had trouble determining how much to close down the diaphragm to adjust contrast. A few times, I have heard some advice here on this forum which says to close down the diaphragm slightly until the image darkens just a little bit. However, this advice doesn't seem to correspond to my particular scope. If I close down my diaphragm slightly, I cannot see any change in the image at all. In order for the image to darken a little bit, I have to close down the diaphragm quite considerably. And I mean a lot. I can't remember how much but it could something like quarter of a turn or perhaps even half a turn.

Though the thing is - if I do close down the diaphragm slightly, I can see the blades closing via the mirror. So that shows that it's doing it's thing - the amount of light is being reduced and theoretically, there should be a bit of an increase in contrast. Yet the image being viewed through the eyepiece appears the same to me.

So which option should I choose?

1. Close the diaphragm slightly (even though I can't see any visual change with the image.)

2. Close the diaphragm considerably so that the image darkens a little bit.

Of course I don't want an image with insufficient / weak contrast though on the other hand, I don't to lose lots of resolution.

Pau
Site Admin
Posts: 6038
Joined: Wed Jan 20, 2010 8:57 am
Location: Valencia, Spain

Re: Help with adjusting contrast

Post by Pau »

Your #2 option is the right one.

The adequate diameter of the condenser diaphragm depends o several factors:
1-condenser height: try to place it at its ideal position
2- kind of condenser, here I assume a typical Abbe two lens condenser
3- objective magnification and NA: wider with higher NA objectives

To fine tune 1 and 2,
- after focusing the sample remove one eyepiece and look through the tube hole
- close the condenser diaphragm pretty much until it becomes clearly visible
- focus the diaphragm with the condenser focus knob and then open or close it to 70-80% of the field
- If not well centered, center it with its centering knobs and repeat the former step.

A very useful source is the Zeiss booklet "Microscopy from the very beginning" , google for its pdf
Pau

dragonblade
Posts: 200
Joined: Sat Oct 18, 2014 11:16 pm

Re: Help with adjusting contrast

Post by dragonblade »

Thank you for the informative reply. After using my microscope today, I'm actually hesitant about using option 2. It seems that I have to close the diaphragm almost all the way to darken the image a little. And with such a small diaphragm opening, I'm guessing that I would lose a fair bit of resolution.

rjlittlefield
Site Admin
Posts: 23543
Joined: Tue Aug 01, 2006 8:34 am
Location: Richland, Washington State, USA
Contact:

Re: Help with adjusting contrast

Post by rjlittlefield »

As quick explanation... When you stop down the diaphragm, you are narrowing the cone of light that it produces. When the cone of light coming out of the condenser is still wide, it fills the entire aperture of the objective. In that regime, changes to the diaphragm have little or no effect on the image. As the diaphragm continues to close, at some point the cone of light becomes narrow enough that it does not fill the entire aperture of the objective. At that point, several things start to happen: (1) the image gets darker, (2) contrast increases, (3) small details start to blur out, (4) depth of field increases. The more you stop down, the stronger each of these effects becomes. This presents tradeoffs that have to be balanced, and the "best" setting is whatever the observer happens to prefer for the subject, the task, and the microscope at hand.
It seems that I have to close the diaphragm almost all the way to darken the image a little
This is completely normal. It occurs whenever you're working with a low magnification objective, which can accept only a narrow cone of light. Most of the range of the diaphragm control is taken up with wider cones that are not entirely accepted by the objective anyway.

The point at which darkening starts to occur -- regardless of where that is in the range of the diaphragm control -- is the point at which the other effects will start to kick in also. From wherever that happens, continuing to close the diaphragm will give you a darker image with more contrast, less resolution, and more depth of field.

The best setting is whatever works best for you in the moment. You should play with the setting to understand the tradeoffs. Do not worry about how close you are to the end of the diaphragm's range.

--Rik

dragonblade
Posts: 200
Joined: Sat Oct 18, 2014 11:16 pm

Re: Help with adjusting contrast

Post by dragonblade »

Thank you for your detailed reply. Very informative.
rjlittlefield wrote:
Thu Nov 05, 2020 11:35 am
When you stop down the diaphragm, you are narrowing the cone of light that it produces.

rjlittlefield wrote:
Thu Nov 05, 2020 11:35 am
This is completely normal. It occurs whenever you're working with a low magnification objective, which can accept only a narrow cone of light. Most of the range of the diaphragm control is taken up with wider cones that are not entirely accepted by the objective anyway.
In my case, I was using a 40x objective. And I first noted a slight darkening of the image with the 40x when the diaphragm was close to it's smallest opening. So I guess this may be a bit unusual in that it happened with a high magnification objective?
rjlittlefield wrote:
Thu Nov 05, 2020 11:35 am
The best setting is whatever works best for you in the moment. You should play with the setting to understand the tradeoffs. Do not worry about how close you are to the end of the diaphragm's range.

--Rik
Yes good advice. I played around a bit with the condenser and diaphragm settings yesterday. I believe it was a microscopy tutorial video that I saw on youtube which recommended only closing down the diaphragm a little bit. In actual fact, when I have the diaphragm close to wide open, the contrast looks pretty good to me as it is. Plus if I do photography / video through the scope, I will generally increase the contrast in the images a little bit with post processing software.

By the way, when doing darkfield, do many people bother with increasing the contrast with smaller diaphragm settings? I'm guessing that it's usually not required since darkfield is naturally quite contrasty anyway.

rjlittlefield
Site Admin
Posts: 23543
Joined: Tue Aug 01, 2006 8:34 am
Location: Richland, Washington State, USA
Contact:

Re: Help with adjusting contrast

Post by rjlittlefield »

dragonblade wrote:
Fri Nov 06, 2020 6:00 am
In my case, I was using a 40x objective. And I first noted a slight darkening of the image with the 40x when the diaphragm was close to it's smallest opening. So I guess this may be a bit unusual in that it happened with a high magnification objective?
Yes, this is a little unusual. Still, all it says is that the upper end of the diaphragm control doesn't have much effect on the width of the light cone. This depends on exactly how the condenser is designed and manufactured. I have no experience with Radical scopes, so I'm clueless about that.

I will say that my experience with other Radical equipment has not been encouraging. It is limited to three sets of objectives, two of which I sent back because (a) they were not what I had ordered, and (b) some of the lens elements were visibly tipped in the barrels, with corresponding effects on the image. It sounds like you're having better results, which is a good thing.

--Rik

Pau
Site Admin
Posts: 6038
Joined: Wed Jan 20, 2010 8:57 am
Location: Valencia, Spain

Re: Help with adjusting contrast

Post by Pau »

Yes, as Rik said this is unusual. He has explained the subject very clearly.
In any case if the condenser diaphragm hasn't any effect in the light amount it has no any other effect (DOF, contrast and resolution)

Did you follow the method removing an eyepiece I explained? After almost 40yrs teaching how to use a microscope, I'm pretty confident on what I say. Maybe you prefer youtubers.

Dark field is another case: you block the center of the condenser light cone and allow to pass only its periphery. Again the diameter of the cone (now the internal diameter) is related with the objective NA. If you close too much the iris diaphragm you have no light
Pau

Post Reply Previous topicNext topic