Updated : 2021
The lens blur filter for Affinity Photo can be found in the blur category of the blur menu in Affinity Photo.
Filters menu
Blur
Lens blur filter
Set radius
Number of blades
Blade curvature
Bloom threshold
Bloom factor
Bloom color
Apply
The lens blur can be applied once or multiple times. You can apply the lens blur to layers. You can use the lens blur filter on different channels so you can get different effects in red or green etc. You can also use the lens blur filter in Affinity Photo with selection and more