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I don't know if you are talking about some specific function in OpenCV, can you give us more details about it? If you are referring to the buildDoGPyramid function (that actually computes a Difference of Gaussians), you don't need to pass any sigma to the function. Take a look to this source file: modules/nonfree/src/sift.cpp.

If I'm not mistaken, the whole theory of scale space is to filter with a Gaussian Filter that has the same standard deviation in the x and y coordinates. Otherwise the procedure will remove more details in one direction than another, which it does not make too much sense.