The K&F CONCEPT wide angle lens filter for DJI Action 4 and 5 comes with a cleaning cloth and a silicone case.
The lens consists of a metal frame and multi-coated glass. The workmanship is very good and of high quality.
Installing the lens is a bit difficult because the original lens is screwed very firmly if you haven't unscrewed it before.
It definitely won't work with your fingers.
There are many different tips on the web, ranging from pliers, rubbers, 3D printing clips, etc.
We simply took a zip tie and attached it around the lens and tightened it. After moving around a bit (counterclockwise), the lens came loose and
the wide-angle lens works as expected. You get a noticeably larger viewing area, which is particularly useful when shooting action, as it allows you to see more of the landscape or the movements or filmed objects. It's also really cool when taking selfies, of course.
You can notice a slight curvature of the wide angle, but it is not strong and, depending on the wide angle or degree, it is also normal.
The images are qualitatively identical to the original lens.
Overall, a very good accessory, but it also comes at a price.