This is a set of 4 x Neutral Density optical glass filters for the DJI OSMO series of action cameras.They are packed in a sturdy and impact resistant mattress plastic case with individual cut outs for each filter. The filters all look the same in their case so once you have determined which is which I would endeavour to be disciplined about putting them back in the same hole to save time later. They are all labelled on the outside rim, but you cannot see that from the front.To install the filter you simply push it on over the original clear OSMO lens cover. It feels a bit weird the first time but once yo get the hang of it it is very quick.Optically these seem to be great, I didn't notice any additional flaring and they resist greasy finger marks. I have posted a pair of photos to show a possible use. Both images are the same scene except the darker one is the ND64 or -5 stops filter. As you can see it effectively acted as an iris shutting down the exposure to leave a moody scene. So these can be used indoors and outdoors to control what the camera sees. A very useful and creative tool to have.