Thanks but what I can't make out is why this comes up as an answer to a thread
on Cuba (cf infra)
Puzzled Philippe
Le 19 déc. 2014 à 11:54, Hughes <timhughes@xxxxxxxx> a écrit :
> 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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