I think you are going to be surprised by the quality of the results you get with that simple camera. The lens is plastic, but IIRC remember blowing up some b/w shots made with one which were really pretty good. Was there a 3 1/4 x 4 1/4 pos neg film made? I think that is what I used.
As stated, the knob is for lighten and darken. You set the film speed on the top with that sliding switch.
I sold that camera for about 40 bucks retail in the earkly 1970s.
Anscojohn, Mount Vernon, Virginia USA