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Dirk Yes. Equating the square root of the measured intensity (photons/spot) to the structure factor was sloppy nomenclature on my part. One should look at Darwin's formula for the intensity (photons/spot). The adverse term in it is Vxtal/Vcell (ratio between crystal and cell volumes). Regarding the scaling behaviour of the structure factors with respect to Vcell, I think section 2.7 of the Holton and Frankel article gives good explanations (i.e. looking at the issue from more than one point of view). See The minimum crystal size needed for a complete diffraction data set, Acta Cryst. D, 66, 393-408 (2010). http://journals.iucr.org/d/issues/2010/04/00/ba5148/ . In short, the scaling of the (squared) structure factors compensates for the other Vcell term in the Darwin formula leaving one with the remaining adverse Vxtal/Vcell. For the same size crystal doubling each cell edge leads to average spot intensities being reduced by a factor of 8. Thanks! Colin -----Original Message----- From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of Dirk Kostrewa Sent: 11 January 2012 09:15 To: ccp4bb Subject: Re: [ccp4bb] Sub-angstrom resolution Dear Colin, Am 10.01.12 18:08, schrieb Colin Nave: <snip> > 3. The structure factors are lower for large unit cells. This will mean they will be harder to detect, particularly if there is a high background. </snip> But aren't the total structure factors of a unit cell the sum of the atomic structure factors? For a larger unit cell (assuming a similar solvent content), I would then expect larger structure factors. Best regards, Dirk. -- ******************************************************* Dirk Kostrewa Gene Center Munich Department of Biochemistry Ludwig-Maximilians-Universität München Feodor-Lynen-Str. 25 D-81377 Munich Germany Phone: +49-89-2180-76845 Fax: +49-89-2180-76999 E-mail: [log in to unmask] WWW: www.genzentrum.lmu.de *******************************************************