skymarshal

As some have indicated, the round circle is forming between the elements of the lens. It forms as you climb out and the lens is exposed the cold air at altitude, as you come through freefall the lens warms and the circle goes away. You can more than likely duplicate it if you sit near the door on the ride up and hold you lens in the cold air stream coming through the door. After a few minutes of cooling, take a look into the lens and through the view finder, I bet you see the circle. It will more than likely get worse the more jumps you do in a single day as the lens goes through hot and cold cycles. The only way I fixed it, was buying a single element lens.

Since you are already using the single element lens the only place it can be forming is on the video camera lens itself, or in the space between the single element lens and the camera. If you watch the video closely, you'll notice it only forms after you have been in the cold air for awhile at altitude and it shrinks closer to the ground in the warmer air. At landing it's gone. You can test this the next time you're on the way to altitude if you sit by the door and it's reasonably cold at altitude. Hold your camera lens in the cold air stream all the way up. Just before you get all the way up, fire up the camera and see if the spot is there, if so, take the lens off and look in between them, I bet it will be on the camera lens itself. I've used Cat Crap in the past, I'd be surprised if it fixes it. Most of the time, condensation on the outside of the lens forms at the edges and works it's way in, not from the center out.

If the condensation you are experiencing is a small circular floating spot in the middle of the lens, it is more than likely not forming on the outside of the lens but rather on the inside surface of the lens. Does the spot develop after climb out and then slowly dissipate in freefall as you enter warmer air? If so, it is forming from the temperature and humidity change. It will be particularly bad if doing back to back jumps. You can attempt to keep the lens warm on the way up, but the only true way I've found to eliminate it, is to switch to a single element lens, such as the Royal lenses. Since switching over, I've yet to have a problem. Hope this helps.

They are called T-nuts. They can be found at just about any hardware store, just ask the nuts & bolts guy.