http://confocal-microscopy-list.275.s1.nabble.com/Condenser-lens-choice-for-a-given-objective-tp7584901p7584910.html
This is a little bit oversimplified. The Rayleigh criterion does not apply to a widefield image, but does apply in fluorescence. The condenser NA normally IS the objective NA, since they are usually one and the same thing, but condenser NA only affects brightness, not resolution.
The Abbe criterion r = 0.5 lambda / NA applies in transmitted light, but ONLY if the condenser aperture equals or exceeds the objective NA. Reducing the condenser NA does not have the same effect as reducing the objective NA. Reducing the condenser NA to 0 (parallel illumination) worsens the resolution to r = lambda/NA - ie 50% of what the objective should give.
In a transmitted-light brightfield image, the Rayleigh criterion includes both the objective NA and the condenser NA
(1.22 lambda/(NA_obj +NA_cond)) . The makes sense because even a very small NA objective can receive light scattered at a large angle if the NA of the condenser is large. (This is how dark-field works).
So it would appear that in principle, you benefit from having a condenser with as large as NA as possible (although you may not have much contrast on that brightfield image).
BTW, you can have a "poor" man's dark field scope by using a low-NA objective with a phase ring made for a higher NA objective.
For example, in my teaching lab, the students get very nice darkfield images using our 4x/NA=0.1 objective with the ph2 phase ring.
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>
> Thanks for the feedback, Barbara. It is very helpful.
>
> I have heard the advice before about the condenser NA needing to be
> greater than or equal to the objective NA. Can you offer some physical
> explanation or intuition for why this is?
>
> One admittedly incomplete explanation I can think of for the
> recommendation goes like this: the light collected by the objective
> consists of two parts. One part is the transmitted light that is not
> scattered by the sample. The other part is the light scattered by the
> sample. If the condenser's working NA is smaller than the objective's,
> then the unscattered, transmitted light fills only a portion of the
> objective's back focal plane. However, the light scattered by the
> sample will probably be dispersed across the entire back focal plane
> because it will encode all the spatial frequencies of the sample.
>
> I wonder if it's the inhomogeneous distribution of light from the two
> components in the objective's back focal plane that leads to the
> matched NA requirements of the condenser and objective. Does this make
> sense?
>
> Thanks!
> Kyle
>
> On 03/17/2016 08:22 PM, Barbara Foster wrote:
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>>
>> Hi, Kyle
>>
>> The general rule of thumb is that the NA on the condenser should meet
>> or exceed that of the objective.
>>
>> If you are using oil immersion objectives, ideally, to achieve that
>> goal, you should use an oil immersion condenser, otherwise you are
>> limited to an NA of 0.9.
>>
>> Also, remember that the aperture iris in the condenser adjusts the
>> condenser's WORKING numerical aperture. Just because the condenser
>> is marked 1.4 NA doesn't mean that, in a practical experiment, it
>> will be operating at 1.4. I follow the guidelines set down by Frits
>> Zernicke (inventor of Phase contrast): gently close the aperture
>> iris to the "Oomph" position: that delicate balance between
>> sufficient edge definition and optimum resolution (Yes, the condenser
>> does contribute to resolution).
>>
>> As for planning for growth:
>> You might want to invest in a turret condenser early on. That will
>> give you the option to add those other contrast techniques as you
>> grow into them.
>>
>> And just one more reminder, specifically regarding DIC:
>> If you are going to use plastic vessels (petri dishes, multi-well
>> plates, growth flasks), use Hoffman Modulation Contrast instead of
>> DIC. DIC uses polarized light. The plastic will affectt the shear
>> and cause effects that will be hard to interpret. Some HMC set-ups
>> do use pol to control the width of the slit in the condenser, but all
>> of that is on the incoming side of the sample and will not be
>> affected by plastic containers.
>>
>> Good hunting!
>> Barbara Foster, President & Chief Consultant Microscopy/Microscopy
>> Education ... "Education, not Training"
>> 7101 Royal Glen Trail, Suite A - McKinney, TX 75070 - P:
>> 972-924-5310 www.MicroscopyEducation.com
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>>
>> NEW! Getting involved in Raman or FTIR?
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>> microscopists!
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>> course on nearly any topic, from fluorescence to confocal to image
>> analysis to SEM/TEM.
>> Call today for a free training evaluation.
>>
>>
>>
>>
>> At 08:36 AM 3/17/2016, Kyle Michael Douglass wrote:
>>> *****
>>> To join, leave or search the confocal microscopy listserv, go to:
>>>
http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy>>> Post images on
http://www.imgur.com and include the link in your
>>> posting.
>>> *****
>>>
>>> Hello listers,
>>>
>>>
>>> I have a couple of questions about condensers for you. I'd like to
>>> do some transmitted light imaging in an inverted microscope using
>>> high magnification, oil-immersion objectives. For the moment, I
>>> don't need to do anything other than brightfield with a high power
>>> LED light source. It might be nice to do phase contrast or DIC in
>>> the future, but I don't need it now.
>>>
>>>
>>> My questions are:
>>>
>>>
>>> 1) What are the rules of thumb for matching a brightfield condenser
>>> to an objective? I won't be using anything but oil immersion
>>> objectives with NA's greater than 1.4.
>>>
>>>
>>> 2) If I do want to do phase contrast or DIC in the future, should I
>>> put special consideration into the condenser lens selection now? I
>>> imagine the condenser NA will determine what phase contrast rings I
>>> can use, but does it impact DIC?
>>>
>>>
>>> Thanks!
>>>
>>> Kyle
>>>
>>>
>>> Dr. Kyle M. Douglass
>>> Post-doctoral Researcher
>>> EPFL - The Laboratory of Experimental Biophysics
http://leb.epfl.ch/
>>>
http://kmdouglass.github.io>