NavList:

   

Reply

I've changed the threadname to make it more relevant.

Earlier, I had suggested- "It would indeed be possible to calibrate the
transfer function of a lens system and remap images on a corrected grid,
though this would call for a lot of work if a zoom lens is being used."

and Marcel answered-
"The "lot of work" may really depend on what you aim at."

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.

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..

My suggested method (which hasn't been tried by me) calls for
precisely-timed shots of the Sun as it crosses the frame, preferably
corner-to-corner diagonally, in such a way as to pass close to its centre.
A star might be better still. The method needs no horizon, just a very
firmly-clamped camera. It presumes (as Marcel's method doesn't) that any
distortion is centred on the mid-point of the frame, and is uniform in all
directions from that point. It results in a radial distortion factor that
can be used to correct the position of any point in the frame. Any motion
of the camera over the period of observation (max: 2.5 hours or so) would
damage accuracy, so it might be worthwhile to position the camera in such a
way that a neighbour's chimney-pot (say) appeared in a corner of the frame
as a check. It employs the fact that the Sun's motion in its path is always
very close to 1 degree in every 4 minutes of time, varying very slightly
but predictably as a result of the same factors as give rise to the
equation of time, and being slightly affected by refraction, so requiring a
rough knowledge of observer's position.

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.

Marcel went on-

"When you wrote a "lot of work" you may have thought to calibrate for being
able to measure any (oblique) distance within the x/y-plane. Would this
really be necessary for CN applications? "

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.

and concluded-

"I could however imagine that there exist somewhere professional programs
which would help to do also this sort of calibration and analysis of the
measurements."

I can only agree. I'm pretty sure that he and I must be retracing
well-trodden ground, if only we knew where to look.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

   

Reply