NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Camera distortion of sky images. was Re: NG's "Midnight Fun"
From: Marcel Tschudin
Date: 2010 Jun 15, 17:10 +0300
From: Marcel Tschudin
Date: 2010 Jun 15, 17:10 +0300
George, you wrote: > Well, all I was thinking of, really, was that if a zoom lens was being > used, that has a whole range of possible expansion factors, and presumably, > a somewhat different lens-distortion for each one, so the "lot of work" was > just to measure calibration curves at all these zoom settings. But I'm > aware that it's difficult, if not impossible, to return to exactly the same > known zoom factor as before, in the absence of a precisely marked > scale-of-zoom. So I expect that the only way to use a zoom lens for such a > purpose in practice is probably to use it at either maximum or minimum > zoom, not in-between, and hope it returns to exactly the same value when > doing so. Greg attaches to his camera different lenses, he doesn't use a zoom. For my observations I however do use a zoom. I always set it at the stop of maximal zoom. The results I obtain (StdDev better than 1.5% of sun diameter) is sufficient for my purposes which are not CN. > I see now that Marcel's approach to calibration is very different from > mine. He measures altitudes on a central axis crossing the frame > vertically, presumably in portrait mode, up from a horizon point placed > near the frame's centre, and compares those altitudes with predictions.. There were actually two different procedures used for the two types of calibration. In both cases it was decided to use the camera always that the broader side of the image is vertical and the shorter side horizontal. The coordinates along the broader side were called the x-coordinates and those along the shorter side the y-coordinates. We started actually with the complicated one where the sun's altitude is found by integration: For obtaining the differential scales, Greg shot at 5 different y-values a series of ten sun photos (only) over the range of different x-values. These photos had to be made with the broad side being horizontal since only the horizontal sun diameter can be used as a reference measure. Greg suggested then that it might be easier to calibrate only the centre line by using photos from the sun above the apparent horizon and by comparing then the measured altitudes in pixels to the calculated moa-values for the given location and time. The graphs which have been sent earlier show this type of calibration. > For Marcel's biggest-angle observations, presumably made with a "50mm" > lens, I would be very interested in the relationship between object > altitude above the horizon, in degrees, and image separation, in pixels, to > see how well it corresponds to the Tan A relationship that I've predicted. Well it's actually Greg who made the observations. Sorry, George, I'm not sure what you mean here with image separation. Could it be that you think of the midnight-fun photo which has been done with multiple exposures? Our adopted procedure didn't require multiple exposures. Each photo is one measurement at a given time. George, you suggested then > Well, I'm thinking about it mainly in polar coordinates, not (x, y), and > presuming only a radial variation, in which case it becomes reasonably > simple. I strongly doubt that a calibration in polar coordinates would be easier than in x/y-coordinates. But may be you would use a different technique for calibration which I'm not aware of. Marcel