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Determining number of shots for a stack?
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bvalente



Joined: 18 Jan 2010
Posts: 69
Location: Los Angeles, CA

PostPosted: Mon Mar 01, 2010 10:59 am    Post subject: Reply with quote

Hi Andrew

Thanks - regarding NA is it then correct to say objectives with higher NA is preferable (i.e., 'faster' lenses)?


Cheers

Brian

EDIT: answered my own question - apparently this is true, though it seems the NA of objectives is somewhat tied to the magnification.


http://www.microscopyu.com/articles/optics/objectiveproperties.html

I guess that makes sense that the narrower the AOV the more the optics can be focused on gathering light.
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Ulf W



Joined: 30 Jul 2010
Posts: 13

PostPosted: Wed Aug 25, 2010 4:50 am    Post subject: Reply with quote

Hi everybody,

DOF and suitable step length for stacking have been discussed repeatedly here.

I have found a way that works for me and would like to share it.

My stacking setup is not close by my computer, and I find it practical to use tabulated data on a few sheets of paper.
I select suitable optics from the size of the subject, change the extension to get a good framing and find the magnification
by a test exposure of a ruler ( 1/10mm ) The magnification gives the step from my cheat-sheets.

I have set up two Excel sheets, one for microscope objectives and one for other macro objectives.
At the camera I'm using the printouts, folded and laminated as cheat-sheets to quickly find suitable step lengths.

The Excel files should be configured to suit the user and some of the parameters like e-factor and overlap should be
entered by the user, preferably after some experiments.
The math in the Excel files is nothing new and has been mentioned here in several posts and used by web calculators.
I have tried to add references and formulas in a readable form to the files and hope that there are no mistakes in how
they are used in the files. It would be nice if this was verified to avoid confusion if anyone else would like to use these tools.

There is a problem with the printout and viewing formats as I am using the (European?) A4 sheet format.
Also, I have never ever seen a MS-Office file where the format survived when opened with a program of different version.
I expect interesting things will happen if the files are opened in any other version than my Office 2008 on Mac OS-X.
I made some pdfs to show how I hoped the printout should look, but they look quite different from the Excel page.

The files are at a file hosting site I just found. I have never used such sites before and hope this one is OK.
The files can be downloaded at
http://www.mediafire.com/?ov5c8e8tr6qrq

There is a lot to be said about my thoughts behind these files, but this post is long enough for now. I have to come back to that later.

I'm looking forward to any comments to this.

Ulf
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ChrisLilley



Joined: 01 May 2010
Posts: 680
Location: Nice, France (I'm British)

PostPosted: Fri Aug 27, 2010 10:25 am    Post subject: Reply with quote

Ulf W wrote:
Hi everybody,
The Excel files should be configured to suit the user and some of the parameters like e-factor and overlap should be
entered by the user, preferably after some experiments.


The two excell files downloaded fine for me an opened ok (in OpenOffice 3).

e factor?
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Ulf W



Joined: 30 Jul 2010
Posts: 13

PostPosted: Sat Aug 28, 2010 3:56 am    Post subject: Reply with quote

Hi Chris,

Thank you for looking! I hope for confirmation that I have made this correctly, but I tend to
make mistakes and those are difficult to find by myself.

I'm glad to see that non MS programs handle those files OK!



The e-factor is something I found in a calculator by one of the member in this forum.
I don't remember who just now. It might have been Mr. Krebs.

It is ment to define how many pixels that are needed to describe a feature in the
image or the minimal possible circle of confusion.

The size depends of both the properties of the low pass filter in front of the sensor
and how efficient the RAW-conversion routines are.

Usually, a color camera sensor (except for the Foveon sensor) is arranged in groups of
four pixels, two sensitive for green and one each for blue and red. In front of the sensor,
there is a low-pass filter to suppress moire effects.

The filters have different cutoff behavior, and the software routines are more or less
efficient to extract details from the sensor information. This is valid both for the cameras
internal conversion to jpeg and external raw converters.


Until a good e-factor value is detemined by experiments a value between 1.5 and 3 is OK
to start with. Combined with the overlap the choise is not very sensitive.


Last edited by Ulf W on Sat Aug 28, 2010 11:22 pm; edited 1 time in total
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Charles Krebs



Joined: 01 Aug 2006
Posts: 5713
Location: Issaquah, WA USA

PostPosted: Sat Aug 28, 2010 11:07 am    Post subject: Reply with quote

Yes, the "e" value was mentioned on the spreadsheet I use ( found here).

I was using a relationship obtained on this page:
http://www.microscopyu.com/tutorials/java/depthoffield/index.html
Quote:
The variable e is the smallest distance that can be resolved by a detector that is placed in the image plane of the microscope objective


So there is room for some "discussion" as to what might be most appropriate, but at the minimum it would be 2*pixel size. (For stacking purposes you're probably safe using a value in the 2 to 3 range)
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