**PURPOSE**- Compute the variance of a 3D reconstruction in Fourier space as a function of frequency.

**SEE ALSO**

**USAGE**- .OPERATION: VA F

- .FIRST TEMPLATE FOR 2-D IMAGE NAME: IMG***

[This is normally a set of projections of a 3D reconstruction, see notes.].FILE NUMBERS OR SELECTION DOC. FILE NAME: 1-700

[Enter projection file numbers or the name of the document file containing projection file numbers in the first column.].SECOND TEMPLATE FOR 2-D IMAGE NAME: DATA***

[This is a set of 2D images used to calculate 3D reconstruction for which the variance in Fourier space is to be evaluated.].MASK FILE: MAS999

[Enter the name of mask image for masking each image in the set.].RING WIDTH: 0.5

[Enter ring width of each frequency radius for the variance computation.].DOCUMENT FILE: DOC001

[Enter name of resulting document file.]

**NOTES**

- If you do not want to mask the images, you still have to create
a mask image with constant value 1 by using command 'MO'.
- The variance is calculated according to:
VA(I) = SUM[(P(kx,ky) - D(kx,ky))^2]/(N-1)

Sum over Fourier units in each ring of radius I - Reference:

P.A. Penczek. Variance in three-dimensional reconstructions from projections. In preparation.

**SUBROUTINES**- VARF, VARF1

**CALLER**- UTIL4

; Calculation of Fourier space variance BP 3F stack@***** 1-799 angles001 * vol001 ; Get the variance PJ 3Q vol001 (35) 1-799 angles001 prjn@*** RF VAR dala@**** 1-799 prjn@*** 1-799 circle (1) variance EN

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