One of the natural sources...
Another source... Consider a distant point light source, for example, a
star. When it passes through a fixed aperture, the light is defracted. What
is actually passed to the film or sensor through a perfect lens is no longer
a parallel rays of light focused to a point without dimentions. Rather, it's
the light rays diffracted and spread to form a circular pattern. The
intensity decreases from the center out. For mathematicians who may be
curious, the intensitiy follows a second order bessel function. For non
mathematicians, the function appears similar to a sine(x)/x curve. Anyway,
as the intensity of the source light increases, more of the tail of the
function (extending out the radius from the center) becomes visible and the
bright object (say, a star) becomes "larger". Eventualy, the lobes of the
function become visible as rings or halos of diminishing intensity around
the central point.
Extending this, every single point of the image is actually made up of these
patterns - overlapped. Further, the difraction does not just take place at
the aperture but also outside the camera in fron of the lens. A highly
backlit subject will also show this, such as a bright light behind a subject
with the "halo" around it's edges - perhaps your model's head.
Regards,
Bob...
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From: "James Schenken" <jds@xxxxxxxxxxxxxxx>
It seems that 'blooming' in a film camera can occur when the light
intensity in an area is such that the adjacent areas get some of the
photons that are not absorbed by the silver crystals. For this to happen
in a digital camera, the individual pixel sensors would have to have at
least semi-transparent walls that allowed reflected light to pass to
adjacent sensors. To get significant 'blooming', the light intensity
would have to be such that the adjacent sensors would pass it on, so to
speak.
Does anyone know if the side walls on image pixel sensors are at all
transparent or not?
For 'haloing' to occur in a digital camera, the software that does the
interpolation to fill in the missing colors and adjusts the contrast could
easily do this. You get the same effect by over sharpening in the image
processing software. I'm not aware of a process that yields haloing in a
film camera while the film is still in the camera. Help me if someone on
the list does know how this might happen/
Eslon writes:
: I wonder why it seems to be seen as an aberration in this review of the
: Nikon D300:
:
: "The camera was loved for its "brilliant" image quality, its good colour
: balance, low noise and its LCD. However at times the D300 produced some
: minor haloing in high contrast areas."
