Martin Schneider correctly reminded me that the frequency response per IEC 60268-4 is actualy given for a plane sound field, that is theoretically at an infinite distance, rather than at a distance of 1 m as per the 'old' standards. Considering the frequencies (wavelength) generated by a classical guitar this has negligible consequences at the recording distances beyond 1 m ...

While at 45 deg of sound incidence to the axis the proximity effect is 3 dB less than on axis (20 log (cos 45 deg)) , at an angle of 22.5 deg it is not even 1 dB (20 log (22.5 deg)) less than on axis.

In addition, Martin pointed out the link to the booklet by Gerhard Bore and Stephan Peus, simply titled 'Microphones', which does include among many other interesting topics a good explanation of the proximity effect with diagrams and formulas:

http://www.neumann.com/download.php?down load=docu0002.PDF