What are the measurable factors of image sharpness?
Some references on image sharpness metrics:


Encoding Visual Sensitivity by MaxPol Convolution Filters for Image Sharpness Assessment, IEEE Transactions on Image Processing, 2019
A Fast Approach for No-Reference Image Sharpness Assessment Based on Maximum Local Variation, IEEE Signal Processing Letters, 2014
Image Sharpness Assessment Based on Local Phase Coherence, IEEE Transactions on Image Processing, 2013
$S_3$: A Spectral and Spatial Measure of Local Perceived Sharpness in Natural Images,  IEEE Transactions on Image Processing, 2012