[Derbasti]

To make the Gaussian blur look better, it needs to be applied to linear lightness, not to gamma-encoded colors, as it is done here. In reality, the blur is induced by the lens, on linear lightness, and only gamma-encoded afterwards.

The effect being that blurred colors mix wrongly, as you can see where the red from the flower mixes with the green. The transition looks wrong, and not how a lens would render it.

[dylan604]

Shallow depth of field blur is not a Gaussian blur though. Attempts at faking shallow depth of field with that filter will not result with true bokeh.

[mdciotti]

True, but Gaussian or not, convolutional filtering should be performed in linear light space!