Ideally, you'd want to see the exact same image for framing as you would of what's being shot by the camera. Unfortunately, a heads-up display that can superimpose the camera image onto your goggles is just not an economically practical solution. So, some sort of sight, not actually bound to the camera image must be used. Because the sight is not through the lens, parallax comes into play. For my camera and for the purposes of this discussion, the parallax starts with the lens and my eye being about 9 inches apart. A larger distance will create a larger parallax error, while a smaller distance will create a smaller parallax error.
Before we get too far into parallax issues, let's look at a few types of sights available.
Camera flyers can use a number of types of sights. From the least accurate and least expensive to the most accurate and most expensive they are;
What makes matter worse, is that since the goggles are so close to the eye, that even a very small positioning error creates a very large angle of misalignment. For instance, assume the surface of your goggles is 1 inch away from your eye. A misalignment of your goggles on your face of just 0.0875 of an inch would create a misalignment of 5 degrees. At 10 feet, that 5-degree angle is a misalignment of about 19.5 inches -- over a foot and a half. At 40 feet, that misalignment is about 41.8 inches. That said, if you're shooting very close-up freefly with a very wide angle lens, an angular offset of 5 degrees is probably not going to be all that noticeable.
The key advantage of Newton Cross and Concentric Ring sights is that they do not require exact repositioning of the helmet in order to work correctly. The axis of the sight optics will maintain their orientation all by themselves. All that is required is that they be aimed properly and then fixed in place. The optics will then present the eye with an aim point that represents the optics axis of orientation. The Newton Cross gives a somewhat vague aim point, the Concentric Ring gives a very well defined and precise aim point.
Since the difference between the position of the lens and the position of the eye is about 9 inches, the very best you could ever hope for in terms of accuracy at all distances would be a parallel offset of about 9 inches. What I mean by this is that even if you had a perfect setup with perfectly parallel lens and eye alignment at every distance from close up to infinity the alignment would be about 9 inches off. Now, at very large distances a 9 inch parallax is no big deal, but as you get very close to the lens of the camera, say a shooting tandem distance of maybe 3 to 5 feet, it could be the difference between having the tandem master completely in the shot or having his head cut off by the top of the frame. Even with a perfect alignment you must take the parallax into account for very close distances.
Some camera flyers align their sights at fairly close distances in order to attempt to correct for the parallax issue, however, in my view this is just flat out wrong. While it may be correctly aligned for a specific distance, at all other distances, it would be misaligned.
I shoot 4-way. When it comes to camera flying, that's my bread and butter. My ideal distance and position from the team is about 10 feet above with the lens of the camera right over the center of the formation. By aligning the camera and sight specifically for that point the angular offset would be 4 degrees. That could be a perfect setup specifically for shooting 10 feet away, but if I were to then shoot a 20-way at a distance of say, 40 feet, then that 4 degrees translates to a misalignment of 26.9 inches -- maybe an arm length.
So, the upshot of this is that when I try to align my cameras, I take all of this into consideration and attempt to align them not to a specific distance, but so that the camera and sight are as parallel as possible. I do this by aiming at a point as far away as possible. I know I'll still have to deal with the 9-inch parallax offset when it comes to stuff that's close-up, but for everything else, when I put my sight on a target, it should be accurate to about 9 inches.