(meteorobs) Fisheye lens misunderstanding and photography update
clp at alumni.caltech.edu
Mon Dec 7 15:32:01 EST 2009
It is true aliasing, resulting from having higher spatial frequency
components in the image than the spatial sampling frequency (the pixel
spacing). It will show up in any optical system producing a small spot size
compared with the pixel size, regardless of how linear the meteor path is on
the image. The "spiral" effect may be a combination of the aliasing and the
curved path- that's related to the Moiré effect referenced by Leo. But it's
still fundamentally caused by aliasing. A meteor image crossing multiple
pixels also makes it easier to see the limiting spatial resolution of the
image- that is, the fact that the distance between pixels in the final image
is larger than the resolution of our eye. We essentially have a pixelated
image because of that.
The only condition under which the effect would be hidden (although the
aliasing still exists) is when the meteor so exactly follows a single row or
column of pixels that no other pixels are exposed. Not a likely event.
Chris L Peterson
----- Original Message -----
From: "Peter Hirons" <peter at galley.ie>
To: "'Global Meteor Observing Forum'" <meteorobs at meteorobs.org>
Sent: Monday, December 07, 2009 1:20 PM
Subject: Re: (meteorobs) Fisheye lens misunderstanding and photography
>I wouldn't call this aliasing - it's just the effect of the fish-eye lens
> in turning straight lines into curves. Try taking a photo of a building
> in daylight.
> You can get software to correct this, but I'd stick to a linear lens as
> wide as you can get. You probably only want to be looking towards the
> radiant most of the time anyway.
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