“…Laplacian-based operators measure the focus by calculating the second derivative of an image, [15]- [19]. Wavelet-based operators consider the wavelet transform to describe an image's spatial and frequency contents, [15], [20], [21], whereas, statisticalbased operators take into account different statistics of an image to measure the focus, [22], [23], [19], [16]. Discrete cosine transform (DCTE) based operators consider DCT coefficients from the frequency content of the image to calculate the focus level, [24], [25], and finally, many other operators, which do not lie in any of the aforementioned categories, are grouped into a category named as miscellaneous operators, [26], [27].…”