Good afternoon,

At 10.40 fm 99-10-27 +0200, you wrote:
>Dear confocalists,
>
>I would like to know if iris implementation in high N.A. objectives affects
>in any way image quality and resolution.

If I remember correctly, the iris in high N.A. objectives originally was
implemented for dark-field observation (where you have to make sure
that the 2*(N.A./n[imm]) of the objective is smaller than the angle of the
dark cone of the dark field condensor).
Using the adjustment ring on the microscope objective you can
change the numerical aperture within certain limits and the respective
values are printed on the objectives.
When the iris is fully opened, then the objective should have an N.A.
value which matches the max. N.A. value indicated on the objective.
For instance, if you have a lens "40X/0.75-1.25 Oil Iris", at iris fully
closed, the N.A. should be 0.75, at iris fully opened, the N.A. should be
1.25.

However, I have now and then seen that users did not know about
this feature and did not care about the setting of the iris, i.e. numerical
aperture, on the objective.
In order to have max. resolution and max. light,  the iris must be fully 
opened.

As what concerns the second part of the question:
When changing the N.A. of an objective, you are going to change
depth discrimination / axial resolution as well as lateral resolution and
light transmission.
One can adjust for depth discrimination in another, and possibly
preferable, way by adjusting the pinhole size. Many
commercial CSLM systems allow to adjust pinhole size
"keyboardwise" or "mousewise".

Best regards

Johannes Helm


>Is it true that if both objectives (with and without iris) can attain
>identical N.A. values, image quality does not suffer?
>
>Also, at the same N.A., is it justified to go for objectives with iris to
>allow the "extra" (field depth control)?
>
>Thanks.
>
>
>F Javier Diez-Guerra
>Centro de Biologia Molecular Severo Ochoa
>Facultad de Ciencias
>Universidad Autonoma
>Cantoblanco, 28049 Madrid
>Spain
>
>phone:  34-913978051
>Fax:    34-913978087
>e-mail: [log in to unmask]

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Paul Johannes Helm

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