Exponential shadow maps sampling with PCF for sampler2DShadow instead of sampler2D
PCF does N shadow tests, where each test returns 1 (in light) or 0 (in shadow), and returns the average of the results. sampler2DShadow is a special sampler where PCF is applied to the nearest texels to a sample, based on the depth coordinate provided. So you can't use it with ESM.