On Wed, 25 Jul 2001 21:08:09 GMT, "Alexander Ibrahim"
Ummmm....;-)
>Well, I thought it was odd too before I thought about it.
>
>You do know that these cameras are sensitive to light in the near UV range
>right ?
Yes, that is possible (though they are much more sensitive
at the IR end of the spectrum, I suspect), but the
multi-element lenses used on them do not pass very much
UV...
>It is natural that some vignetting occurs in that light range,
If there is no vignetting in the visible range, there could be
only miniscule vignetting in the range just outside that range,
at worst, I would think... And that vignetting, if any, if made
visible, would be added to the (non-vignetted)
visible-light image, reducing most of its effect (which
would be VERY slight to begin with, if present at all).
>and that the
>camera makes it visible in NTSC colors.
This would happen only if the lens passed enough UV to be
made visible by the CCDs - but comparing the image with and
without a UV filter used, on the most UV-light-rich day
I can find, shows no change in the image...
>This is most likely in a sky scene
>when you have the most UV light. UV light is more energetic than visible
>light so it will create some optical effects somewhat more easily than
>visible light. How ?
>
>Some lenses will not transmit UV light properly registered. When you know
>how hard optics designers have to work to deal with registration it is easy
>to see why they might ignore it in spectral ranges they consider
>unimportant. The net effect is that you have optical distortions in certain
>spectra that are not necessarily present in other spectra.
The UV is not "corrected" for the same plane of focus as the
visible spectrum, since it is not seen. If it were (as with old
blue-sensitive films [which were also sensitive to UV],
shooting with simple 3-4 element lenses), its effect would
be a "haloing" around shadowed edges with UV-rich areas
behind, as with rooftops and trees against skies. This is
due to the UV image being somewhat out of focus compared
with the visible-light image. This is not seen in video (at
least with clean optics...;-), with or without a UV filter
on the lens...
>They don't make UV filters for no reason. I mean they could just make clear
>filters so why bother with the UV spectrum if they don't have to ? In this
>case, an optical distortion present in UV causes vignetting that is barely
>visible, a UV filter can reduce this.
UV filters are made probably more for traditional reasons
than anything else at this point, since they rarely can have
any positive effect on the image (or negative, either,
with good-quality filters...;-) - they serve only as
front-element protectors, and some companies now offer a
"clear" alternative... As for the original poster, it is
not uncommon to see illumination roll-off toward the
corners (most evident with smooth-toned subjects evenly
lit) with zooms at one or both FL extremes (or throughout),
possibly worst near minimum or maximum focus distance,
near the widest stop - lens designs are compromises, and with
zooms, one of the first things to go in the design is even
illumination at wide stops...