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Rear-standard focusing with infinity objectives?
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Lou Jost



Joined: 04 Sep 2015
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Location: Ecuador

PostPosted: Thu Nov 19, 2015 7:39 pm    Post subject: Reply with quote

Sounds interesting Rik. So, what would count as a limiting aperture? Does it have to be smaller than any other constriction in the optical set-up? A too-big iris won't do anything.
Haven't read the chapter yet, will get on it now....
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rjlittlefield
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PostPosted: Thu Nov 19, 2015 11:21 pm    Post subject: Reply with quote

Lou Jost wrote:
So, what would count as a limiting aperture?

It has to be small enough to restrict the cone of light that gets through the whole setup, for every point in the field of view. For a microscope objective, that can still be a pretty big hole -- a sizable fraction of the front element.

--Rik
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Lou Jost



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PostPosted: Fri Nov 20, 2015 4:30 am    Post subject: Reply with quote

Does it imply paying a significant diffraction penalty?

I suppose we can afford to pay that price when using the objective with a lower-power tube lens. That is when the perspective issue is most important anyway. When used at low m, the microscope objective far out-resolves my APS-C sensor, so a slight increase in diffraction will not be observable.
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Lou Jost



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PostPosted: Fri Nov 20, 2015 5:25 am    Post subject: Reply with quote

Do you have any thoughts on where the iris should be? Maybe one focal length in front of the objective when it is focused on the front of the subject? Then move everything except the iris when stacking?
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rjlittlefield
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PostPosted: Fri Nov 20, 2015 10:43 am    Post subject: Reply with quote

The diffraction penalty depends entirely on how far you have to stop down, in order to make the added aperture be the limiting aperture for all points in the field. That's determined in part by where you place the iris. Offhand I imagine that it's best to put it as close as possible to the front lens element. That's why I initially wrote "positioning a non-moving iris just in front of the lens when the lens is at its farthest forward."

--Rik
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