Good lord, people don't half expect a lot from a dirt cheap camera these days
It's an absolute miracle anything this good and this complicated can be made and sold at a profit for the price they are.
I've been a pro photographer for 35 years. My mind boggles at the thought of a 95degree (horizontal coverage) f1.8 lens for a manufactured cost of less than 10 grand. Obviously there have to be a few compromises here
but some things are just optics...
Vignetting is inevitable because of the proximity to the focal plane. Rays near the edge are spread out more by striking the sensor at an angle, which means less light per unit area, which makes the image darker toward the edge of the field of coverage. That's just simple geometry (same as why we have winter and summer) and there is no way to escape it without fancy retro-focus designs (that extend the back focus), and/or building in axial ND filters to the lens, and/or using software to correct exposure across the field of view. High-end digital cameras do deal with this vignetting in image processing, and many raw converter softwares will do it too. You won't find any of that in a $5 lens and a £100 camera.
All lenses produce focus on a hemispherical plane, not a flat plane like a sensor or film is, and with wide-angles this is pronounced. I'll try and explain this...
All rays pass through the optical node (centre) of the lens. If the lens is sharply focussed on an object 10m away, the back focus distance to the sensor is x. If the object is on the lens axis, and the sensor is x distance away on axis, we have a sharp image in the centre of the sensor. But if the object is 10m away and near the edge of the image, the back focus is still x, but the distance to the sensor is a lot more than x (= x/sin (angle of ray from the optical axis)).
This can be largely corrected by using multi-element construction and fancy glasses with different refractive properties so that the focal point varies across the frame. That's why a lens that doesn't show the effect much will cost plenty. A simple, cheap lens will have to rely on depth of focus hiding most of the off-axis softness. And that's what I see in the Viofo. In bright light the lens is stopped down and the edge is almost as crisp as the centre. At night, though, the lens is at or near full aperture and the images edges are relatively soft. Moreover, the lens is fixed at the hyperfocal distance and wider apertures make for less depth of field. Things that are closer or further away will look softer at wider apertures (at night).
Then there's decentering. Some of the faults described hereabouts are due to assembly errors. With a tiny, multi-element lens, any slight defect in element position will have consequences. The precision required is extreme. This could well explain the infamous 'unsharp left side of image' problems often seen with the 119. Or the lens may not be correctly aligned to the sensor, eg the optical axis is not dead-on perpendicular to the sensor. For all I know, the difficulty of getting lenses manufactured with enough precision might explain the change of lens in the S.
And while we are at it, there is no inherent engineering deficiency with using plastic as a lens barrel material. Canon and Nikon have been doing it for decades. Some plastics like acrylic are exceptionally thermally stable, better than metals, and capable of precision manufacture and of course cheap. What plastics are not much good for in lenses is high-wear, high stress moving components like focus helicoids and mounts - but our little dashcam lenses don't have those.
Frankly, 30 years ago these cameras would have looked like magic.
