The depth of field blur filter for Affinity Photo can be found in the blur category in the filters menu. The tool is a destructive effect and can be used once or multiple times to great effect in the application.
Set center / focus can be set interactively / the transition area (inner and outer) along with Radius and Vibrance and Clarity and then click Apply. Set the radius to a large value without modifying the rings and you will see the outer part modified. The default setting is elliptical but you can change that to tilt. You can extend the values of the sliders beyond the limits just by typing 400px etc or 700px into the radius or clarity. The vibrance only goes up to 100%
Reduce the inner circle and you will see more of the effect towards the origin point. You can interactively reposition the origin point just by dragging up or down or left or right. If you place the inner and outer ring very close together with then the transition between the effect and the normal image is a lot less. You can also use the interactive controls to drag out the effect from a circular to elliptical effect. Clarity and vibrance are subtle effects even when applied to the maximum. If you have a large gap between the inner and outer circles then you will see the transition of the image and the blurred effect even more clearly.
With tilt, instead of circles, you see lines. In this case it is parallel lines. Outside of the lines it is the full effect, in between the other lines you see the transition and you can tweak them independently so one side can be very close and another side, further away. You can see the transition from the normal (inside those lines) to the outside lines and full effect. You can also rotate the lines by clicking on the blue points and rotating
You can use it across different color channels so it can be used in the red with different settings to the green channel. You can use the elliptical or tilt as well as both by using the color channels and combine them to create unusual color effects.
You can also use it with selections. This will limit the range of the effect even more along with the changing of the distances / lines in the elliptical and tilt effect.
You can use it with shapes as well as text to create all kinds of amazing designs.
The result of the repeat can result in the effect applied to most of the image but not the core central part and that can, after a few repeats, stand out like a sore thumb.
You can also use this to great effect with the fade found in the layer menu. Apply the obscuring effect and then go to the layer menu and use the fade with blending mode and opacity to change the result such as hard mix, darken, multiply, linear burn etc.