Most microscopes have at least one aperture as part of the condenser. Some microscope also include a Field Apeture as part of the light source.
The following images demonstrate the effects of stopping / closing down the aperture of the condenser and the light source. The photos show the enhanced contrast and resolution when the apertures are stopped down.
There may be a more technical way of describing how to close the apertures, but I just close the aperture until I see a noticeable darkening at the corners of the image and then back off a bit.
In both cases the apertures are used for adjusting the shape of the light, not to adjust the intensity of the light.
It should be stated the Field Aperture cannot or rather should not be used for objectives greater than 5x. For objectives greater than 5x the Field Aperure is used for Kohler Illumination.
The photos were taken with a 4x obj. and are a crop of about 30% of the full frame. The forum images are lower quality to keep the size manageable and are linked to higher quality versions. The swing out top lens of the condenser was out.
Photo 1 is the base image with both apertures open.
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Photo 2 is with the Field Aperture at 50%.
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Photo 3 is with the Condenser Aperture at .28
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Details:
Canon 5D camera, Meiji 2,5x eyepiece, Labomed LB-592 microscope.
Condenser: Swing out N.A 0.9/0.25
Lighting: 100w Halogen with Field Diaphragm
What a difference an aperture makes
Re: What a difference an aperture makes
Why not use the field diaphragm and Kohler if your 'scope has it? It makes
a huge difference in resolution when used. The swing-out lens is only for the
lowest power (often x4) objective - without it there is not enough light to fill
the x4 FOV.
As you change objectives simply re-set the field diaphragm to just beyond
the FOV of whatever lens you are using - takes about 5 seconds and ensures
optimum Kohler.
JB
a huge difference in resolution when used. The swing-out lens is only for the
lowest power (often x4) objective - without it there is not enough light to fill
the x4 FOV.
As you change objectives simply re-set the field diaphragm to just beyond
the FOV of whatever lens you are using - takes about 5 seconds and ensures
optimum Kohler.
JB
John B
Re: What a difference an aperture makes
I have tried to demonstrate the difference the field aperture make as opposed to the condenser aperture.
If a microscope is equipped with both a field aperture and a condenser aperture,
then the field aperture is used for enhanced depth of field for obj <=5x as the condenser aperture is not all that effective with obj. <= 5x . Kohler illumination is not effective with low power obj.
For obj. >5x only the condenser aperture is effective to enhance the depth of field because the field aperture was used to set the Kohler illumination.
Please note, just adjusting the field aperture is NOT setting the Kohler illumination. The condenser height is also adjusted when setting Kohler illumination.
The two adjustments are different for each obj 10x and greater.
Try this experiment with a 10x or greater objective. Swing the top condenser lens in and out and observe the difference. There are time when doing things wrong may improve your image.
If a microscope is equipped with both a field aperture and a condenser aperture,
then the field aperture is used for enhanced depth of field for obj <=5x as the condenser aperture is not all that effective with obj. <= 5x . Kohler illumination is not effective with low power obj.
For obj. >5x only the condenser aperture is effective to enhance the depth of field because the field aperture was used to set the Kohler illumination.
Please note, just adjusting the field aperture is NOT setting the Kohler illumination. The condenser height is also adjusted when setting Kohler illumination.
The two adjustments are different for each obj 10x and greater.
Try this experiment with a 10x or greater objective. Swing the top condenser lens in and out and observe the difference. There are time when doing things wrong may improve your image.
Re: What a difference an aperture makes
The QCC wrote:I have tried to demonstrate the difference the field aperture make as opposed to the condenser aperture.
If a microscope is equipped with both a field aperture and a condenser aperture,
then the field aperture is used for enhanced depth of field for obj <=5x as the condenser aperture is not all that effective with obj. <= 5x . Kohler illumination is not effective with low power obj.
For obj. >5x only the condenser aperture is effective to enhance the depth of field because the field aperture was used to set the Kohler illumination.
Please note, just adjusting the field aperture is NOT setting the Kohler illumination. The condenser height is also adjusted when setting Kohler illumination.
The two adjustments are different for each obj 10x and greater.
Try this experiment with a 10x or greater objective. Swing the top condenser lens in and out and observe the difference. There are time when doing things wrong may improve your image.
Once the field iris is focused in the image plane via condenser height the only factor my 'scope needs to have altered is the diameter of the field iris - oh and I usually need to slightly center via condenser adjusters too. Of course the lowest power objective is used for finding the subject and isn't involved in Kohler at all. My understanding is that the condenser iris when set at about 70% optimizes contrast and of course closing it will have the same effect as with a camera, i.e. smaller apeture = greater depth of field.
Maintaining Kohler (once set) does for me anyway amount to altering the field iris to just beyond the FOV as I move through objectives (excluding of course the x4 or x5)....
John B
Re: What a difference an aperture makes
Correct, if you mean 70% of the objective's aperture, so both the condenser iris and the field diaphragm need to be readjusted when you change objectives. Also, as with a camera lens, when you stop down too far, diffraction effects begin to show up, with the attendant loss of resolution.mrsonchus wrote:My understanding is that the condenser iris when set at about 70% optimizes contrast and of course closing it will have the same effect as with a camera
Re: What a difference an aperture makes
Very true - it's always a bit of a compromise or balancing act between brightness, resolution and depth of field! It's one of thosegekko wrote:Correct, if you mean 70% of the objective's aperture, so both the condenser iris and the field diaphragm need to be readjusted when you change objectives. Also, as with a camera lens, when you stop down too far, diffraction effects begin to show up, with the attendant loss of resolution.mrsonchus wrote:My understanding is that the condenser iris when set at about 70% optimizes contrast and of course closing it will have the same effect as with a camera
aspects of microscopy that when it goes well is very satisfying when a superb image is achieved (I don't always achieve this though!).
regards
JB
John B
Re: What a difference an aperture makes
Edited post. The original posted images are a bit messy. I have posted larger un-cropped images on my web server.
To illustrate Gekko's points, the following photos are 1:1 crops of 5600x3800 pixel photos of the mineral Gneiss as photographed by a Canon 5D MkII using a Nikon PL 2.5x eyepiece, Labomed LB-592 microscope and a 10x Plan, Pol, Infinity objective.
The forum limits width to 1024 pixels so the crops represent a significant magnification factor. The JPEG format does not help.
The new posted images are here.
Condenser aperture effects.
Photo 12 is a TIFF version of the original RAW image with Kohler illumination set.
Photo 08 has the condenser aperture set for optimum contrast and detail.
Photo 09 the condenser aperture is stopped down to far.
Photo 10 has the top condenser swung out, looking similar to a stopped down aperture.
To illustrate Gekko's points, the following photos are 1:1 crops of 5600x3800 pixel photos of the mineral Gneiss as photographed by a Canon 5D MkII using a Nikon PL 2.5x eyepiece, Labomed LB-592 microscope and a 10x Plan, Pol, Infinity objective.
The forum limits width to 1024 pixels so the crops represent a significant magnification factor. The JPEG format does not help.
The new posted images are here.
Condenser aperture effects.
Photo 12 is a TIFF version of the original RAW image with Kohler illumination set.
Photo 08 has the condenser aperture set for optimum contrast and detail.
Photo 09 the condenser aperture is stopped down to far.
Photo 10 has the top condenser swung out, looking similar to a stopped down aperture.