I always felt something was a bit off about UE4 motion blur. The default amount of 0.5 should represent the average, commonly used 180°/half-frame shutter, but MB in Unreal Engine always felt a lot more smeary than 180° motion blur look I am used to see when rendering using offline renderers.
In one of the recent version, new setting was introduced in motion blur settings, called “Target FPS” with a tooltip saying:
“Defines the target FPS for motion blur. Makes motion blur independent of actual frame rate and relative to the specified target FPS instead. Higher target FPS results in shorter frames, which means shorter shutter times and less motion blur. Lower FPS means more motion blur. A value of zero makes the motion blur dependent on the actual frame rate.”
What caught me by a huge surprise was realization that the default value is 30 (Motion blur length adequate to half frame shutter at 30FPS), but when I toggle the Post Process volume override for this setting off, the motion blur remains exactly the same. Enabling it and raising the value to 120 radically shortened the motion blur length, making it exactly what I’d expect half frame shutter motion blur length to look like at any framerate. Setting the target FPS value to 0 as the tooltip says has identical effect.
The main point here is that pretty much everyone is getting wrong, much smearier motion blur than what the correct default of half frame shutter should be due to default target FPS for motion blur being 30, and it requires actively overriding the value to 0 so that your motion blur becomes correctly frame rate dependent. No wonder many people dislike Unreal’s out of the box motion blur, given all this.
By default, no matter what framerate you are running at, you are getting motion blur length for 30FPS, so if you are at 120FPS, you are getting 4x the motion blur size it should be in reality.