help fspecial 即可明白:
H = FSPECIAL('gaussian',HSIZE,SIGMA) returns a rotationally
symmetric Gaussian lowpass filter of size HSIZE with standard
deviation SIGMA (positive). HSIZE can be a vector specifying the
number of rows and columns in H or a scalar, in which case H is a
square matrix.
The default HSIZE is [3 3], the default SIGMA is 0.5.
即高斯低通滤波,其标准偏差为SIGMA,其实其意义已经由其字母表达了,学过概率统计的人就会明白。
你说的情况是有区别的,只是不明显。