Since my new 200mm fast lens and other similar vintage, especially non APO
"analog" lenses, suffer from purple fringing , I got interested in why, it is
so much worse with digital cameras and why some seem worse than others. I had
noticed that hazy/lightly clouded skies seem to make it worse.
>From Wikipedia below, it seems that one of the main culprits is higher
>sensitivity of digital sensors to blue end of spectrum and UV. So a
>traditional UV filter could help here and what is interesting, is that it may
>be glass dependent. In most common optical lens glasses they do not transmit
>UV that well som vendors lenses may act better as UV filters than others.
>Also a large amount of glass in a more complex lens may reduce UV more, for
>that reason. Simlarly the comment below about UV flare varying with
>anti-reflecive coatings. The Canon lenses had generally good AR coatings and
>all vendors upped their game in that department over time. It would be
>interesting to know how much variation that causes.
In theory a sensor vendor could add a UV filter to the IR filter they already
add to reduce the very high sensitivity of silicon sensors to IR. This would
likely help reduce flare too.
The QE of Silicon does drop off substantially with decreasing wavelength,so
that maybe why they tend not to bother?
This also explains why Flare and Blue fringes from directly imaging LED lamps
is such a problem. White LEDS produce quite short wavelength blue light ,that
is then converted to green and red by phosphors,but there is still a very large
blue component,often larger than the Green and Red ,especially with High color
temperature,high efficiency LEDs.
========================================
Here is Wikipedia:
Purple fringing
>From Wikipedia, the free encyclopedia
In photography, and particularly in digital photography, purple fringing
(sometimes called PF) is the term for an out-of-focus purple or magenta "ghost"
image on a
photograph. This defect is generally most visible as a coloring and
lightening of dark edges adjacent to bright areas of broad-spectrum
illumination, such as daylight or various types of gas discharge lamps.
Lenses in general exhibit axial chromatic aberration in which different colors
of light do not focus in the same plane.
Normally, lens designs are optimized so that two or more (at least three for
apochromatic lenses) wavelengths of light in the visible range focus at the
same
plane. Wavelengths very different from those optimized in the design
process may be severely out-of-focus when the reference colors are in
focus; this axial chromatic aberration is usually severe at short
(violet) wavelengths. Lens performance may be poor for such wavelengths
in other ways too, including an increase in flare due to anti-reflective
coatings also being optimized for the expected wavelengths.
Most film has relatively low sensitivity to colors outside the visible range,
so light spread in the near ultraviolet (UV) or near infrared (IR) rarely has a
significant impact on the image recorded. However,
sensors used in digital cameras commonly are sensitive to a wider range
of wavelengths. Although the lens glass itself filters out much of the
UV light, and all digital cameras designed for color photography
incorporate filters to reduce red and IR sensitivity, the chromatic
aberration can be sufficient for bright out-of-focus violet light to
tint nearby dark regions of the image. Bright cloudy or hazy skies are
strong sources of scattered violet and UV light, so tend to provoke the
problem.
The term purple fringe used to describe one aspect of chromatic aberration
dates back to at least 1833.[1] However, Brewster's description with a purple
fringe on one edge and a green fringe on the other is a lateral chromatic
aberration. A general defocus of the shortest wavelengths
resulting in a purple fringe on all sides of a bright object is the
result of an axial or longitudinal chromatic aberration.
Quite often these effects are mixed in an image. Axial chromatic
aberration is more subject to reduction by stopping down the lens than
lateral chromatic aberration is, so the purple fringing can be very
dependent on f-number: a larger f-number (smaller aperture) reduces axial
aberration.
Other explanations
Purple fringing is usually attributed to chromatic aberration as described
above, although it is not clear that all purple fringing
can be explained this way. Other attributed causes of purple fringing in
digital photography include many hypothesized sensor effects:
* Digital noise in dark areas
* Image processing and interpolation artifacts (almost all CCDs and
CMOS require considerable processing)
* Stray ultraviolet and/or infrared light
* Image bloom from overexposure of CCD sensor (not applied to CMOS)
________________________________
From: ChrisB <ftog@xxxxxxxxxxxxxx>
To: Olympus Camera Discussion <olympus@xxxxxxxxxxxxxxxxx>
Sent: Thursday, December 18, 2014 10:09 PM
Subject: Re: [OM] Cuba?
Amen!
Chris
> On 19 Dec 14, at 02:51, Chris Trask <christrask@xxxxxxxxxxxxx> wrote:
>
> In all, I have nothing but good experiences with Cuban nationals.
>
> I'm compelled to get one of their cigars and enjoy seeing the last icon
> of the Cold War come to an end.
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